Bedangadas Mohanty

Dear Students,

This week I bring to you a new STAR paper: “First observation of deuteron–Λ correlations at RHIC”, a pioneering measurement that reveals how a Λ-hyperon interacts with a deuteron in heavy-ion collisions.

https://arxiv.org/pdf/2511.15493

Message for all:

Imagine smashing two heavy nuclei together at near the speed of light, producing a dense, hot “soup” of nuclear fragments and strange particles. Among these fragments, sometimes a deuteron (a bound proton–neutron pair) and a Λ (a strange baryon) fly out together. This study looks at how often such pairs come out close to each other, and whether their final separation shows evidence of an attractive “force” between them.

The result: deuteron–Λ pairs come out significantly more often at low relative momentum, a clear sign they “feel” each other even after the collision. That means in simple terms: strange particles and light nuclei can weakly bind together, even in the chaotic aftermath of a nuclear smash-up.

This finding helps us understand exotic forms of “strange‐nuclei,” and might even tell us about the “glue” that could exist inside ultra-dense objects like neutron stars.

What was done (scientific summary)

(a) Collision system: Au+Au at √sNN = 3 GeV (fixed-target mode at STAR).

(b) Reconstruction: deuterons via TPC + TOF, Λ via its weak decay (Λ → p + π⁻).

(c) Built the correlation function C(k*) from same-event and mixed-event pairs; corrected for background, feed-down, and hypernuclear decay contamination.

(d) Used a femtoscopic framework (Lednický–Lyuboshitz) with Bayesian inference to extract scattering parameters (scattering length, effective range) for the d–Λ system.

Key Findings

A large enhancement in C(k*) at small relative momentum → strong final-state interaction between deuteron and Λ.

Extracted spin-dependent scattering parameters for the first time:

Spin state Scattering length f₀ (fm) Effective range d₀ (fm)

Doublet –26.1 ± 5.6 (fit limit) ≈ 8 fm

Quartet +18.7 ± 2.8 6.5 ± 1.8 fm

Negative f₀ in the doublet channel suggests the possibility of a bound Λ–d (hypertriton-like) system.

Using these parameters, the authors estimated the Λ-separation energy in a hypothetical ³ΛH as BΛ ≈ 0.04⁺⁰·¹²₋₀·₀₃ MeV (95% CL) → confirming an extremely weakly-bound, halo-like hypernucleus.

The source-size (emission region radius) shows centrality dependence (roughly 2.1 fm → 2.7 fm from peripheral to central), deviations from transport-model expectations indicate room for improved collision dynamics modeling.

Why this matters

(a) Opens a new experimental handle to study hyperon–nucleus and hypernuclear interactions directly, rather than relying only on theory or scarce hypernuclear data.

(b) Provides the first spin-dependent constraints on the d–Λ interaction — crucial for understanding the structure of hypernuclei (like hypertriton) and, by extension, for modeling matter under extreme density (as in neutron stars).

(c) Confirms that hypertriton (if viewed as Λ + deuteron) is extremely loosely bound and has a large spatial size — insights relevant for nuclear structure, hadronization dynamics, and the fate of strange matter in dense astrophysical objects.

(d) Demonstrates the power of femtoscopic correlation techniques + Bayesian analysis even for rare hyperon–nucleus pairs in heavy-ion collisions — a promising tool for future studies (other hyperon–nucleon/nucleus pairs, hypernuclei, etc.).

Enjoy reading the paper at your leisure.

Thanks, With Best Wishes, Bedanga

Dear Students,

ALICE released several papers in arXiv this week. I will bring to you one of them. Hope you will enjoy reading leisurely during Sunday.

Paper titled:Strangeness enhancement at its extremes: multiple (multi-)strange hadron production in pp collisions at √s = 5.02 TeV https://arxiv.org/abs/2511.10413

Simple summary: When protons collide at the LHC, they briefly create a hot medium and a dense spray of particles. Some of these particles contain strange quarks, which are a bit harder to make. This study doesn’t just ask “how many strange particles on average?” --- it asks “how often do we get 0, 1, 2, 3… 7 of them in a single event?” That new perspective shows that very strange-rich events happen more often in busy (high-multiplicity) collisions and gives a powerful way to check our computer models of how quarks become hadrons. The current models capture the general trend but struggle with the most extreme cases, which will help theorists improve the modeling of how quarks connect and turn into particles.

What was measured: Instead of just asking “how many strange hadrons on average?”, ALICE measured the full probability distribution P(nS)=probability of finding "n" strange hadrons in a single event. ALICE studied KS0, Λ, Ξ, and Ω within ∣y∣<0.5. Some collisions had as many as 7 KS0, 5 Λ, 4 Ξ, or 2 Ω at midrapidity. The probability to see events with many strange hadrons rises steeply with the overall activity of the event — the busier the collision, the more likely it is to produce several strange quarks.

Data set & event selection: (a) System/energy: pp at √s = 5.02 TeV (Run-2, 2017).

(b) ≈ 1.1×10⁹ minimum-bias events; L_int ≈ 2.3 nb⁻¹; INEL>0 selection (≥1 charged particle in ∣η∣<1).

(c) Multiplicity classes: defined with V0M forward scintillators; results also shown multiplicity-integrated (INEL>0)

Detectors used (key sub-systems):

(a) Central barrel: ITS + TPC (tracking & PID via dE/dx), TOF (PID).

(b) Forward: V0A/V0C (trigger + multiplicity estimator).

(c) Standard V⁰/cascade topological reconstruction for KS0, Λ(Λˉ), Ξ, Ω.

What the data look like:

(a) The measured P(nS) shapes are well described by Negative Binomial Distributions (NBD) — the same statistical form used for total charged-particle multiplicities in pp collisions.

(b) This suggests that the fluctuations in strange-particle production follow similar underlying dynamics as the overall event activity.

From distributions to “multiplets”: From these P(nS), ALICE computed the average number of strange-hadron pairs, triplets, etc. — denoted as ⟨Y₂⟩, ⟨Y₃⟩, and so on. These quantities test whether strange hadrons are produced independently or in correlated groups: If production were random, the probability of getting two or three in one event would simply follow Poisson statistics from ⟨Y₁⟩. If the measured ⟨Y₂⟩ or ⟨Y₃⟩ are larger, it means producing one strange particle makes it more likely to produce others in the same collision. If smaller, it means suppression.

ALICE finds that the data are closer to independent production than most Monte Carlo models predict — i.e. models tend to underestimate how often multiple strange hadrons appear together.

Multiplicity bias (important detail):

Events containing several strange hadrons are not typical: they also tend to have more total particles overall. That means when you select “strange-rich” events, you are automatically biasing toward higher-multiplicity collisions. This is called strangeness-induced multiplicity bias. For example, events with at least one Ω have about 50 % higher charged-particle multiplicity than unbiased events. Hence, the often-quoted Ω/π enhancement versus multiplicity partly reflects this built-in bias.

Why this measurement matters: This is the first time that per-event strangeness probability distributions P(nS) have been measured in any system. They provide much more information than average yields and help test hadronization models like PYTHIA 8 Ropes and EPOS-LHC. The new observables — especially ⟨Yₙ⟩ ratios and the quantified multiplicity bias — give a sharper probe of how color strings interact and how quarks combine into hadrons in high-density pp events.

Model sensitivity:

(a) PYTHIA8 QCD-CR+Ropes and EPOS-LHC generally agree better with mean yields than PYTHIA8 Monash; however, all models face non-trivial tension once you look at multiplet ratios and the tail of P(nS).

(b) Models tend to underpredict high-n multipliclets relative to an “independent production” baseline; data are closer to independence within uncertainties.

(c) Benchmark: The new P(nS) and ⟨YSn⟩ observables provide sharper constraints on color reconnection, rope hadronization, and “core–corona” pictures than average-yield systematics alone.

enjoy reading the paper.

Thanks, With Best Wishes, Bedanga

Dear Students,

Hope you all had a nice festival season vacation, one more related to Diwali is coming soon. However, we need to always keep in touch with our passion - doing science.

Today I bring to you another paper from STAR Collaboration this time - Charge Separation Measurements in Au+Au Collisions at √sNN = 7.7–200 GeV in Search of the Chiral Magnetic Effect (arXiv:2506.00275) https://arxiv.org/pdf/2506.00275

Physics Goal and Importance

The study aims to search for the Chiral Magnetic Effect (CME) , a fundamental QCD phenomenon in which a net chirality imbalance (μ₅ ≠ 0) in a deconfined quark-gluon plasma (QGP) generates an electric current parallel to the magnetic field, leading to a charge separation along that direction. This effect is theoretically linked to local parity (P) and CP violation in QCD, arising from topological transitions in the QCD vacuum.

If conclusively observed, CME would be a landmark discovery, providing:

(i) Experimental evidence for QCD topology and chiral anomaly effects.

(ii) Insights into baryogenesis-like mechanisms from the early Universe.

(iii) A deeper understanding of the magnetic field–induced phenomena in strongly interacting matter.

Historical Context

The CME concept was proposed in the late 1990s and has been confirmed in condensed matter systems (Weyl and Dirac semimetals). However, its observation in heavy-ion collisions has been difficult because the expected signal is small and overwhelmed by background correlations, mainly from collective flow and charge conservation. Previous measurements at RHIC and LHC energies (Au+Au, Pb+Pb) showed positive charge-dependent correlations but could not isolate a CME contribution. The 2018 STAR isobar run (Ru+Ru vs Zr+Zr) provided null results, suggesting backgrounds dominate at √sNN = 200 GeV. The current study revisits this question using high-statistics Beam Energy Scan II (BES-II) data, where magnetic fields are stronger and persist longer.

Dataset and Collision Systems

The analysis uses high-statistics Au+Au collision data from √sₙₙ = 7.7 to 200 GeV, collected during the BES-II program (2018–2021) and earlier top-energy runs. The energy range covers regimes from below deconfinement onset to top RHIC energy, with particular focus on the 10–20 GeV region where CME is theoretically expected to be most visible.

Detectors Used

The measurement relies on several key STAR detector subsystems:

(i) Time Projection Chamber (TPC): Tracking, vertexing, and dE/dx particle identification.

(ii) Vertex Position Detector (VPD): Timing and vertex constraint at high energy.

(iii) Time-of-Flight (TOF): Background rejection and pile-up control.

(iv) Event Plane Detector (EPD): Reconstruction of the spectator event plane at BES energies.

(v) Zero Degree Calorimeter Shower Maximum Detector (ZDC-SMD): Spectator plane determination at 200 GeV.

Analysis Technique

The CME signal is probed through the charge-dependent three-particle correlator:

γ112=⟨cos(ϕα+ϕβ−2ΨRP)⟩

where ϕα,ϕβ are azimuthal angles of charged particles and ΨRP is the reaction plane angle.

The CME contribution to Δγ112=γ112OS−γ112SS is expected to be positive. A novel Event Shape Selection (ESS) technique is used to suppress the dominant flow-related background by extrapolating observables to the zero-elliptic-flow limit (v₂ → 0).

This is complemented by:

(i) Use of the spectator plane for event plane reconstruction, which is better correlated with the magnetic field direction and less sensitive to nonflow effects.

(ii) Exclusion of (anti)protons from the analysis to reduce bias from baryon flow differences.

(iii) Additionally, a background-sensitive correlator, γ₁₃₂, is measured and used to cross-check the effectiveness of background suppression.

Key Observables

Δγ₁₁₂: CME-sensitive charge-dependent correlation.

Δγ₁₃₂: Background indicator (expected to be zero if non-CME backgrounds are removed).

NₚₐᵣₜΔγ₁₁₂: Centrality-scaled version of the CME signal.

Δγ₁₁₂ᴱˢˢ: CME-sensitive signal after ESS background suppression.

Results

(1) The background indicator γ₁₃₂ᴱˢˢ is consistent with zero across all energies, confirming effective background removal.

(2) A significant positive charge separation signal (Δγ₁₁₂ᴱˢˢ > 0) is observed in mid-central Au+Au collisions (20–50% centrality) at intermediate energies:

√sNN = 11.5 GeV: ~2.6σ

√sNN = 14.6 GeV: ~3.1

σ√sNN = 19.6 GeV: ~3.3σ

Combining results between 10 and 20 GeV yields a 5.5σ overall significance.

(3) No significant signal is seen at √sₙₙ = 200 GeV or below 10 GeV.

(4) Approximately 80% of the raw Δγ₁₁₂ signal is attributable to flow-related backgrounds, implying a CME fraction of ~20% in the sensitive energy range.

Conclusions

The analysis provides the most sensitive CME search to date at RHIC. The results reveal clear evidence of charge separation consistent with CME expectations at intermediate collision energies (≈10–20 GeV), while showing null results at both lower and higher energies. The findings suggest that:

(a) Magnetic field lifetime and strength and QCD chiral symmetry restoration conditions are most favorable for CME in this intermediate energy regime.

(b) The absence of signal at 200 GeV and LHC energies constrains theoretical models of magnetic field evolution and CME dynamics

(c) .Future work should focus on theoretical interpretation of the observed energy dependence, possible links to the QCD critical region, and refined measurements with upcoming facilities.

Layman Physics Highlights

(i) The experiment searched for a tiny imbalance in how positive and negative charges are emitted in heavy-ion collisions — a subtle fingerprint of deep QCD vacuum structure.

(ii) This imbalance is predicted if quarks in the plasma behave asymmetrically in strong magnetic fields, a phenomenon known as the Chiral Magnetic Effect.

(iii) STAR found evidence of this effect only in a narrow energy “sweet spot” (10–20 GeV), where the right conditions — strong magnetic fields and restored chiral symmetry — occur simultaneously.

(iv) This is a significant step toward observing one of the most elusive quantum effects in the strong interaction and could illuminate how matter-antimatter asymmetry arose in the early universe.

Enjoy reading the paper.

Thanks, With Best Regards, Bedanga

Dear Students,

Warm Sharadiya greetings to you and your family.

I am sure many of you have taken a break to enjoy the festival season, however, as QGP physics is also our passion, we should still be in touch with interesting physics results that come out. In this spirit, I have come today with a new STAR paper tiled - Observation of Charmonium Sequential Suppression in Heavy-Ion Collisions at RHIC (STAR) - https://arxiv.org/pdf/2509.12842

When you get time during these puja holidays, have a look. As usual below is a summary of this paper for you.

Physics Goal and Importance:

The aim is to study how different charmonium states—bound states of a charm quark and antiquark—are modified inside the Quark-Gluon Plasma (QGP).

J/ψ (ground state) and ψ(2S) (excited state) serve as probes.

QCD predicts that loosely bound, larger states like ψ(2S) should “melt” more easily in the hot QGP.

This expected hierarchy, known as sequential suppression, provides insight into color screening and QGP temperature.

Historically, Matsui and Satz (Phys. Lett. B 178, 416 (1986)) proposed charmonium suppression as a direct QGP signature, making this a long-standing physics goal.

Then in the paper S. Digal, P. Petreczky, H. Satz, Phys. Rev. D 64, 094015 (2001), they introduced the concept of sequential suppression: Different charmonium states have different sizes and binding energies. ψ(2S), χc melt at lower QGP temperatures, while J/ψ survives up to higher T. Predicts a hierarchy of melting → thermometer of QGP.

Historical Context:

NA50 (CERN SPS, 17.3 GeV) and ALICE/LHC (5.02 TeV) earlier observed stronger suppression of ψ(2S) relative to J/ψ.

However, the energy gap between SPS and LHC left RHIC (200 GeV) unexplored. This STAR result fills the gap and provides crucial information on the energy dependence of sequential suppression.

Dataset and Collision Systems:

Data: ~4 billion minimum-bias events.

Collisions: Ru+Ru and Zr+Zr at √sₙₙ = 200 GeV.

Year: 2018 run.

Provides high-statistics measurements with isobar systems, complementing Au+Au and p+Au results.

Detectors Used:

Tracking and electron ID: Time Projection Chamber (TPC), Time of Flight (TOF), Barrel Electromagnetic Calorimeter (BEMC).

Trigger: Vertex Position Detectors (VPD).

Signal extraction: machine-learning (XGBoost-based BDT) optimized electron-pair selection.

Analysis Technique:

Reconstructed J/ψ and ψ(2S) via dielectron channel at mid-rapidity (|y|<1).

Calculated yield ratios ψ(2S)/J/ψ vs. pₜ and centrality.

Defined a double ratio: (ψ(2S)/J/ψ)A+A / (ψ(2S)/J/ψ)p+p to cancel systematic effects.

Compared with interpolated p+p baseline and cold nuclear matter (CNM) effects estimated from p+Au/d+Au.

Key Observables:

Inclusive ψ(2S)/J/ψ yield ratios. Centrality dependence of the double ratio. Transverse momentum dependence of suppression.

Results:

Double ratio (0–80% centrality): Measured value = 0.41 ± 0.10 (stat) ± 0.03 (syst) ± 0.02 (ref). Below unity by 5.6σ, clear evidence that ψ(2S) is more suppressed than J/ψ at RHIC.

Centrality trend:

Hints of stronger ψ(2S) suppression from peripheral → central collisions. Similar to SPS (17.3 GeV), unlike the flat centrality dependence at LHC (5.02 TeV).

pₜ dependence:

Stronger suppression at low pₜ (0.2–2 GeV/c) than at higher pₜ. Difference from p+p reference is significant (4.1σ at low pₜ).

CNM baseline (p+Au):

The observed suppression goes beyond CNM effects, confirming hot medium effects (true QGP signature).

Conclusions

First direct evidence at RHIC for sequential suppression of charmonium. Confirms ψ(2S) melts more easily than J/ψ in the QGP. Provides a missing energy-scale bridge between SPS and LHC.

Results constrain QGP models:

Transport models (continuous suppression & regeneration) show consistency with data. Statistical Hadronization Models also describe global trends.

Layman Physics Highlight

In these experiments, two “siblings” of the same family of particles === J/ψ and ψ(2S) == are placed inside a tiny fireball of primordial matter (QGP).

The tightly bound J/ψ survives more often. The loosely bound ψ(2S) gets dissolved more easily.

This difference acts like a thermometer for the early universe, showing that the QGP formed at RHIC is indeed hot and dense enough to melt the weaker bonds of matter.

From a theoretical idea in 1986 to experimental confirmation in 2025, the story of quarkonium suppression has unfolded step by step:

First the concept (Matsui–Satz), Then the detailed hierarchy prediction (Digal–Petreczky–Satz), Now direct evidence across energies (SPS → RHIC → LHC).

This is a textbook case of theory-to-experiment validation in QGP physics.

Enjoy reading the paper.

With best wishes, Bedanga

Dear Students,

How are you doing? Finally the small system collision (OO and Ne-Ne) first data papers are out from ATLAS and ALICE. CMS has a public note on nuclear modification factor measurements. Today I will bring to you the summary of the ALICE paper. Spend some time reading it over the weekend. Then you will enjoy the LHC expt + machine seminars next week.

Paper: Evidence of nuclear geometry–driven anisotropic flow in OO and Ne–Ne collisions at √sNN = 5.36 TeV -- https://arxiv.org/abs/2509.06428

(A) Physics Motivation

Central QCD question: Do light-ion collisions (smaller than Pb–Pb) create quark–gluon plasma (QGP) droplets that exhibit collective flow?

Light ions (¹⁶O, ²⁰Ne) are ideal because their nuclear structures are well studied and exhibit distinct intrinsic geometries (tetrahedral α–clustered ¹⁶O, “bowling-pin” α+¹⁶O structure for ²⁰Ne).

Hypothesis: Flow in such systems is strongly influenced by nuclear geometry, not just by fluctuations as in pp/p–Pb.

(B) Data Set

Collisions: ¹⁶O–¹⁶O and ²⁰Ne–²⁰Ne at √sNN = 5.36 TeV.

Statistics: ~3 billion OO events and ~400 million Ne–Ne events (collected in July 2025).

(C) Detectors Used

Tracking: Inner Tracking System (ITS), Time Projection Chamber (TPC).

Centrality estimation: Fast Interaction Trigger detector (FT0C, –3.3 < η < –2.1).

Standard vertex, pileup, and track-quality selections applied.

(D)Analysis Technique

Anisotropic flow quantified via Fourier coefficients vn. Measured v₂ (elliptic flow) and v₃ (triangular flow).

Methods:

Two-particle cumulants with pseudorapidity gap |Δη| > 1.4 to suppress nonflow. Four-particle cumulants v₂{4} to confirm collectivity. Systematic checks via subevent method and dihadron correlation templates. Ratios of Ne–Ne to OO results used to cancel final-state effects and isolate nuclear geometry influence.

(E) Key Results

v₂{2} and v₂{4}:

Both nonzero, showing clear collective flow. Centrality dependence follows initial eccentricity trends. v₂{4} increases toward peripheral collisions, unlike in pp/p–Pb

v₃{2}: Decreases with centrality, consistent with fluctuation-driven origin.

Ne–Ne vs OO: v₂ larger in central Ne–Ne collisions than OO → signature of nuclear shape effects (quadrupole deformation in ²⁰Ne vs tetrahedral ¹⁶O). Ratios v₂(Ne–Ne/OO) peak at ~1.08 in ultracentral collisions.

Model Comparisons:

Hydrodynamic calculations (Trajectum framework with NLEFT and PGCM nuclear structure inputs) reproduce the data reasonably well. NLEFT matches magnitudes better; PGCM closer for system ratios. IP-Glasma + MUSIC + UrQMD describes system ratios more accurately, pointing to sensitivity to subnucleon size (wq ≈ 0.1–0.2 fm preferred).

(F) Interpretation

Collective anisotropic flow is clearly present in OO and Ne–Ne collisions → strong evidence for QGP droplets in light-ion systems. Nuclear geometry (α-clustered shapes of O and Ne) directly imprints onto final-state flow. System ratios highlight deficiencies in current modeling of initial conditions and pre-equilibrium dynamics, especially subnucleon fluctuations.

(G) Take-home Message

First direct evidence that nuclear geometry drives collective flow in light-ion collisions at the LHC. Hydrodynamics with realistic nuclear structure successfully explains the data, bridging the gap between heavy-ion (Pb–Pb, Xe–Xe) and small systems (pp, p–Pb). Results constrain nucleon and subnucleon structure (favoring smaller partonic widths ~0.1 fm).

Opens a new avenue: use relativistic collisions to “image” α–cluster structures in nuclei like ¹⁶O and ²⁰Ne.

Enjoy reading the paper. Enjoy listening to LHC experiment presentations next week.

Stay happy.

best wishes, bedanga

Dear Students,

Today as you wait to see a total lunar eclipse (commonly known as a blood moon), I bring to you another experimental paper which you may find interesting to read.

STAR PRL: Onset of Constituent Quark Number Scaling in Heavy-Ion Collisions at RHIC

https://journals.aps.org/prl/pdf/10.1103/2qhx-cp79

This is from the STAR detector operating in Fixed target mode, not the nominal collider mode in which it operates.

Main Scientific Idea

(a) Partonic (quark and gluon) collectivity is a key signal for the formation of a Quark–Gluon Plasma (QGP).

(b) A hallmark of this is Number of Constituent Quark (NCQ) scaling of elliptic flow (v₂), meaning that hadron v₂ divided by the number of quarks (nq) follows a universal trend when plotted vs transverse kinetic energy per quark.

(c) This paper investigates the energy threshold where NCQ scaling emerges in Au+Au collisions at RHIC.

(d) Aim: To identify the onset of partonic degrees of freedom as the collision energy increases from hadron-dominated to parton-dominated dynamics.

Data Analysis

(a) Data sample: Collected in the STAR Fixed-Target (FXT) program during 2019–2020.

(b) Energies studied: √sNN = 3.2, 3.5, 3.9, 4.5 GeV (with ~100–220 million events per energy).

(c) Centrality: 10–40% events.

(d) Event selection ensured high-quality vertices, removal of pileup, and stable run conditions.

(e) v₂ measured using the three-sub-event plane method with resolution corrections.

Detectors Used

(a) STAR Time Projection Chamber (TPC): tracking + dE/dx particle ID.

(b) Time-of-Flight (TOF): complementary particle ID.

(c) Endcap TOF (eTOF): enhanced acceptance at forward rapidity (in collaboration with colleagues from CBM).

(d) Inner TPC upgrade: improved tracking.

(e) Event Plane Detector (EPD): centrality determination & event plane reconstruction.

Techniques / Observables

(a) Elliptic flow v₂ of identified hadrons: π±, K±, K0S, p, and Λ.

(b) Scaling variables:

(i) v₂/nq vs (mT − m0)/nq (transverse kinetic energy per quark).

(ii) v₂/nq vs pT/nq (alternative scaling variable).

(c) Systematic uncertainties assessed by varying PID cuts, track quality, and event-plane resolution.

Results

(a) At centre of mass energy = 3.0 and 3.2 GeV:

(i) v₂ is negative or small, dominated by spectator shadowing.

(ii) NCQ scaling breaks down → hadronic interactions dominate.

(b) At center of mass energy = 3.9 and 4.5 GeV:

(i) v₂ becomes positive and stronger.

(ii) NCQ scaling gradually emerges, especially clear at 4.5 GeV.

(c) Transport model comparison:

(i) Hadronic models (JAM, SMASH, AMPT-HC) reproduce low-energy data but underpredict v₂ at 4.5 GeV.

(ii) Partonic model (AMPT-SM) describes 4.5 GeV data better, indicating importance of parton interactions.

(d) Integrated v₂: changes sign from negative to positive around 3.2 GeV, consistent with the onset of partonic effects.

Interpretation & Significance

(a) The energy dependence of NCQ scaling shows a transition:

(i) Below ~3.2 GeV → hadronic interactions dominate, no partonic collectivity.

(ii) By 4.5 GeV → partonic interactions dominate, NCQ scaling holds.

(b) This provides direct experimental evidence for the onset of partonic collectivity in RHIC’s Beam Energy Scan (BES-II).

(c) Establishes a critical energy threshold for QGP formation in the high baryon-density regime.

Summary

(a) Key result: First clear observation of the onset of NCQ scaling at √sNN ≈ 4.5 GeV.

(b) Implication: Strong evidence for partonic collectivity in heavy-ion collisions at lower energies than previously established.

Enjoy reading papers and particularly this one on a leisurely Sunday.

Thanks. best wishes, bedanga

Dear Students,

Hope you are enjoying the summary of the papers from ALICE and STAR. Today I bring to you a fascinating measurement from STAR at RHIC proton-proton collisions - Probing the Quantum Vacuum: Hyperon Spin Correlations at RHIC.

Good luck to STAR in the referee process in Nature. Request to you all do not do away with the habit of reading papers.

Paper title: Probing QCD Confinement with Spin Entanglement

Details: https://arxiv.org/pdf/2506.05499

Physics Motivation: Chiral Condensate, Confinement, and the Origin of Mass

(a) The QCD vacuum isn’t empty, it hosts a condensate of virtual quark–antiquark pairs (including strange quarks) responsible for spontaneous chiral symmetry breaking, a key mechanism generating >99% of hadronic mass.

(b) However, the detailed mechanism connecting this vacuum structure to quark confinement remains an unsolved puzzle.

(c) A fundamental question: Can the spin-entangled nature of these vacuum quark pairs be detected experimentally after they hadronize into observable particles?

Experimental master stroke: Tracing Spin Entanglement from Quarks to Hyperons

The STAR experiment at RHIC measures spin correlations in ΛΛbar hyperon pairs, using them as proxies for the spin state of their parent strange quark–antiquark (ss̄) pairs.

Key Idea:

(a) ss̄ pairs from the vacuum are spin-aligned (triplet state) due to the vacuum’s quantum numbers (Jᴾᶜ = 0⁺⁺).

(b) If hadronization preserves this correlation, the resulting ΛΛ̄ pairs should also exhibit spin alignment.

(c) Detection of this spin correlation thus probes the nonperturbative QCD dynamics linking vacuum structure to hadron formation.

Detector and Dataset

(a) Detector: STAR at RHIC, featuring a Time Projection Chamber (TPC) for tracking, charge ID, and particle identification (via dE/dx).

(b) Data: ~600 million minimum-bias p+p collisions at √s = 200 GeV (from 2012).

Λ Reconstruction

(a) Reconstructed via weak decay channels: Λ → p π⁻ and Λ̄ → p̄ π⁺.

(b) Careful topology cuts (e.g., decay length, DCA, pointing angle) and PID ensure a clean sample.

(c) ~11% of ΛΛ̄ pairs are primary; the rest arise from feed-down (e.g., Σ⁰ → Λγ), modeled via PYTHIA 8.2 + STAR GEANT simulations.

Data Analysis: Measuring Spin Correlation

Experimental Observable:

The angular distribution of decay protons in the Λ rest frame follows:

dN/dcosθ* =1/2(1+α1α2PΛ1Λ2cosθ∗)

PΛΛ̄: The spin correlation parameter.

α1, α2: Known weak decay parameters.

Key Steps:

(a) Construct ΛΛbar, ΛΛ, and ΛbarΛbar candidate pairs.

(b) Subtract background using like-sign combinations and fit the invariant mass with a 2D Gaussian.

(c) Correct angular distributions using the mixed-event (ME) technique to account for acceptance.

(d) Extract PΛΛ̄ via fits to the angular distributions.

Results and Interpretation

Key Discovery:

(a) Positive spin correlation observed for short-range ΛΛ̄ pairs (Δy < 0.5 and Δϕ < π/3):

PΛΛˉ=0.181±0.035stat±0.022sysP

Significance: 4.4σ ==== the first experimental evidence of spin correlations in hyperon pairs in high-energy p+p collisions.

No correlation found in (null hypothesis or tests):

ΛΛ or ΛbarΛbar pairs. Long-range ΛΛ̄ pairs.

Spin correlation decreases with pair separation (ΔR): Consistent with quantum decoherence as pairs interact more with the QCD medium.

Theory Comparisons

(a) SU(6) model: Predicts spin correlation of ~9.6% including feed-down — consistent with data.

(b) Burkardt–Jaffe model: Predicts smaller correlations (~1.5%) === disfavoured.

(c) PYTHIA 8.3 baseline gives zero spin correlation (no polarization module) === confirms non-trivial origin of the observed effect.

What Does This Teach Us?

QCD Vacuum Structure: Confirms the spin-aligned nature of strange quark pairs in the vacuum, strengthening the idea of a chiral condensate.

Confinement and Hadronization: Demonstrates that nonperturbative QCD processes preserve spin correlations across the hadronization boundary. Spin Decomposition and the Proton Spin Puzzle: Suggests that strange quark spin dominates Λ spin, unlike the proton spin case.

Quantum Entanglement and Decoherence:ΛΛ̄barpairs appear to emerge from maximally entangled quark pairs (Bell states). Loss of correlation with ΔR is interpreted as quantum decoherence, with possible links to Bell inequality tests in QCD.

Applications to Chiral Symmetry Restoration: At high temperatures (e.g., in the QGP), the quark condensate melts. ΛΛbar spin correlations may become a novel QGP probe.

Takeaway for PhD Students

This paper is a milestone in the experimental exploration of nonperturbative QCD. As a PhD student:

(a) Appreciate how quantum correlations survive across complex processes like hadronization.

(b) Learn about advanced analysis techniques: mixed-event corrections, 2D invariant mass fits, spin correlation extraction.

(c) Understand the importance of model comparisons and systematic treatment.

(d) This is also a great case study in balancing scientific ambition with interpretative caution, especially when linking to profound concepts like confinement and entanglement.

Enjoy reading the paper on the weekend.

Thanks. With best wishes, bedanga

Dear Students,

I come to you with a summary of another interesting experimental paper from ALICE.

The paper title is - First direct access to the ρ0-p interaction via correlation studies at the LHC, from ALICE experiment. The details can be found at - https://arxiv.org/abs/2508.09867 and https://alice-publications.web.cern.ch/node/11822

Hope you will get time to read the paper this weekend.

What problem is being solved?

We want a direct, data-driven handle on the ρ–nucleon interaction in vacuum. That interaction feeds into how the ρ spectral function changes in matter (a key ingredient behind dilepton measurements and chiral-symmetry-restoration claims in heavy-ion collisions). Until now it was inferred mostly from photoproduction/VMD or low-energy Partial Wave Analysis (PWAs); this paper measures it directly via correlations.

Core idea

Use two-particle femtoscopy in high-multiplicity pp at 13 TeV to measure the ρ⁰–p correlation function C(k*), isolate the final-state interaction (FSI), and fit it with a coupled-channel χEFT/UChPT model to extract scattering parameters.

What the analysis actually does using pp collision data at 13 TeV

(1) Reconstruct particles

Protons: standard TPC/TOF PID with high purity. ρ⁰ → π⁺π⁻: build π⁺π⁻ pairs; fit the m(ππ) spectrum; select a signal window (~0.70–0.85 GeV/c²) and sidebands to estimate background; require pₜ(ρ⁰) > 1.8 GeV/c to improve purity.

(2) Build the correlation function

Compute C(k*) = N_same(k*)/N_mixed(k*) in the pair rest frame; normalize to unity at high k* (600–800 MeV/c). Combine ρ⁰–p and ρ⁰–p̄ (consistent within uncertainties).

(3) Remove out non-FSI backgrounds

Correlated “minijet” background (jet-like structures) is modeled data-driven from ρ sidebands. Mis-ID/flat pieces are tiny after selections. Use a λ-parameter decomposition (from single-particle purity/primary fractions) and a derived formula to recover the genuine ρ⁰–p correlation C_ρp(k*). This sideband subtraction method is validated with MC closure.

(4) Source model (where pairs are emitted from)

Use the common source picture established in ALICE pp femtoscopy: a Gaussian “core” radius that scales with m_T plus resonance feed-down tails. For the ρ⁰–p pair the inferred core radius ≈ 0.8 fm (with uncertainties).

(5) Physics extraction with coupled channels

The ρ⁰p system couples to channels like ρ⁺n, ωp, ϕp, K*Λ, K*Σ … so the fit uses unitarized χPT (hidden-gauge), solving a coupled-channel Bethe–Salpeter equation. Production weights for each channel and the source per channel are taken from data/systematics; ϕ–p femtoscopy is fit simultaneously to better constrain the model.

What shows up in the data?

The genuine C_ρp(k*) is suppressed below ~200 MeV/c by ~4σ, a clear FSI signal. At larger k* it’s ~1. This suppression points to strong inelasticity/absorption from coupled channels and the ρ width.

Key numbers/findings

(a) Scattering length (s-wave):

a₍ρ⁰p₎ = (−0.46 ± 0.04) + i (0.20 ± 0.04) fm (real part ≠ 0 → interaction strength; imag part > 0 encodes inelasticity/finite ρ width).

(b) Effective range:

r₍ρ⁰p₎ = (−1.31 ± 0.17) + i (97.94 ± 0.05) fm (the large imaginary r₀ is a built-in consequence of the short ρ lifetime).

(c) The fit finds two poles consistent with N(1700) and N(1958) in the coupled-channel T-matrix—evidence for the dynamically generated resonance picture seen in similar systems.

Why this matters for heavy-ion physics (this results is from pp collisions)

Provides a vacuum baseline for the ρ–N interaction used in in-medium ρ self-energy Σ_ρN calculations that drive ρ spectral-function broadening and hence dilepton excess interpretations (CERES/NA60/PHENIX/STAR/ALICE). Now you can replace model priors with direct constraints from collider data.

Practical takeaways for a student analyst

(A) If you’re in ALICE (pp/hadronic femtoscopy):

Follow the sideband-driven minijet subtraction literally, it’s the heart of the analysis. Control plots validating sideband symmetry and closure are essential.

ρ⁰ purity is low (signal a few percent). The pₜ cut and mass-window choice matter a lot; quantify their effect on λ-parameters and on C(k*).

Use the common-source m_T scaling from published pp femtoscopy to set priors on the Gaussian core; propagate resonance feed-down properly.

When fitting, include coupled channels and, if possible, constrain the model with another correlation (e.g., ϕ–p) in a joint fit to reduce degeneracies.

(B) If you’re in STAR (RHIC energies):

The method is portable to pp, p+A, and even peripheral A+A if you can reconstruct a clean ρ⁰ and control minijet backgrounds.

At RHIC, m_T and source sizes differ; use HBT/femtoscopy at 200 GeV for source priors (or extract a STAR-specific m_T scaling).

Channel weights (ρ⁺n, ωp, ϕp, K*Λ/Σ) will shift with √s and acceptance; re-measure them for STAR kinematics.

The ρ width and inelasticity dominate; expect a suppression-type C(k*) at low k* too—check sensitivity studies with toy models before unblinding.

Enjoy reading the paper.

Thanks. With best regards, bedanga

Dear Students,

This week I will bring to you another paper from ALICE, which you may find interesting. This topic is of special interest to me, as I was involved in the first experimental ALICE paper on spin alignment measurements and glad to see this being extended in different directions.

The paper titled "First measurement of D*⁺ vector meson spin alignment in Pb–Pb collisions at 5.02 TeV" by the ALICE Collaboration details of which can be found at - https://arxiv.org/abs/2504.00714 reports the first-ever measurement of the spin alignment of prompt D*⁺ mesons in relativistic heavy-ion collisions.

Scientific Context

Quark-Gluon Plasma (QGP) is expected to form in heavy-ion collisions and exhibits properties like low shear viscosity and strong collectivity. In non-central collisions, a large angular momentum (~10⁷ ℏ) and a strong but short-lived magnetic field (~10¹⁵ T) are generated perpendicular to the reaction plane. These fields may polarize quarks, leading to spin alignment in the hadrons they form, notably vector mesons.

Objective

To measure the spin alignment (quantified by the spin density matrix element ρ₀₀) of prompt D*⁺ mesons produced in Pb–Pb collisions at 5.02 TeV, and examine its dependence on:

Transverse momentum (pT) Rapidity (y) Collision centrality

Key Methodology (this time I am not discussing the detectors and event selection as those are standard as before in my emails)

Spin alignment is inferred from angular distributions of decay products using:

dcosθ∗dN∝[1−ρ00+(3ρ00−1)cos2θ∗], where ρ₀₀ = 1/3 implies no alignment.

D*⁺ reconstructed via the decay D*⁺ → D⁰(→ K⁻π⁺)π⁺. Separation of prompt vs non-prompt mesons using Boosted Decision Trees (BDT) trained on simulated data (PYTHIA+HIJING). Corrections applied for detector acceptance, efficiency, and event-plane resolution.

Main Results

ρ₀₀ > 1/3 (signifying spin alignment) is observed for pT > 15 GeV/c in the 0.3 < |y| < 0.8 rapidity range in 30–50% centrality events with 3.1σ significance.

No significant alignment (ρ₀₀ ≈ 1/3) in central (0–10%) events or in |y| < 0.3 across all pT.

A comparison with inclusive J/ψ meson spin alignment at forward rapidity (2.5 < y < 4) suggests a similar increasing trend with pT, although more theoretical input is needed.

Interpretation and Theoretical Relevance

High-pT D*⁺ mesons likely retain early-stage charm-quark polarization induced by magnetic fields.

Existing models (e.g., holography, ϕ-meson field fluctuations, glasma-induced correlations) partially explain the trend but: Are mainly developed for flavourless mesons. Do not fully incorporate fragmentation-based hadronization, dominant at high pT. New charm-sector specific theoretical models are needed.

Conclusion and Outlook

This measurement opens a new probe of QGP dynamics via heavy-flavor spin alignment. Anticipated Run 3 ALICE upgrades will enable higher precision and extended studies (e.g., Λc spin alignment).

The findings offer essential constraints for models of early-time QGP magnetic and vortical fields.

Hope you will enjoy reading the paper on this relaxing Sunday.

BTW: Subsequent to the data coming out a theory paper from came out from NISER theory group on this - https://arxiv.org/pdf/2502.20352

With best wishes, bedanga

Dear Students,

Today I am discussing a new ALICE paper (came a few months back in arXiv). Hope you will get a chance to take a look at it. This paper titled - Evidence for J/ ψ suppression in incoherent photonuclear production can be found in arXiv: https://arxiv.org/pdf/2503.18708

While reading this do not get confused by J/Psi suppression due to QGP.

J/ψ suppression can arise from two fundamentally different mechanisms. In high-energy heavy-ion collisions, suppression due to the Quark-Gluon Plasma (QGP) occurs as a final-state effect, where the hot, deconfined medium screens the color force and causes the J/ψ production to be suppressed relative to pp collisions. In contrast, suppression due to gluon saturation in the Color Glass Condensate (CGC) framework is an initial-state effect, where the high density of gluons in a nucleus at small Bjorken-x leads to nonlinear QCD dynamics that limit J/ψ production, especially at small transverse distances (high momentum)

In case you are wondering what is gluon saturation: Gluon saturation is a phenomenon predicted by Quantum Chromodynamics, where the density of gluons inside a hadron or nucleus becomes so high at small Bjorken-x(i.e., high energies or small longitudinal momentum fractions) that their growth slows down due to nonlinear interactions. A self-limiting process that tames the infinite growth predicted by linear QCD.

This study presents the first measurement of the energy and momentum transfer (∣t∣) dependence of incoherent J/ψ photoproduction off lead (Pb) nuclei in ultra-peripheral Pb–Pb collisions at the LHC. It provides direct experimental evidence for suppression of J/ψ production at large ∣t∣, suggesting that gluon saturation effects, a key feature of high-energy Quantum Chromodynamics (QCD), play a significant role in small spatial-scale gluon configurations.

Experimental & Analysis Method:

Collision System: Ultra-peripheral Pb–Pb collisions at 5.02 TeV.

Measured Process: Incoherent photonuclear production of J/ψ mesons, reconstructed via their decay to dimuon pairs (μ+μ−).

Rapidity coverage: Both forward (via muon spectrometer) and midrapidity (via central barrel).

Photon source: Quasi-real photons from the electromagnetic fields of fast-moving Pb nuclei (Weizsäcker-Williams approximation).

Key observables: WγPb,n: Photon–nucleus center-of-mass energy. And ∣t∣ =pT2: Square of the momentum transferred, sensitive to the spatial resolution of the gluon distribution.

Event Selection:

Muon pairs in the J/ψ mass region.

Zero-degree calorimeters (ZDC) detect neutrons from target dissociation (confirming incoherence).

Vetoes on activity in other detectors (V0, AD) ensure ultra-peripheral nature.

Analysis method:

Statistical extraction of J/ψ signal via fits to invariant mass spectra.

Feed-down and coherent contributions subtracted using MC templates.

Cross sections calculated as a function of WγPb,n in three ∣t∣ bins:

Low: 0.09<∣t∣<0.36 GeV2

Medium: 0.36<∣t∣<0.81 GeV²

High: 0.81<∣t∣<1.44 GeV²

Detectors Used

Muon Spectrometer (for forward rapidity dimuons)

Central Barrel (ITS, TPC) (for midrapidity dimuons)

Zero Degree Calorimeters (ZDC) (for neutron tagging)

V0 and AD detectors (for rapidity gap vetoes)

Key Result

At large ∣t∣, the J/ψ production cross section rises more slowly with energy and flattens at high energies (up to WγPb,n =633 GeV).

The ratio of large-∣t∣ to small-∣t∣ cross sections drops significantly, e.g., to ~0.52 ± 0.13 at the highest energy.

Importance of the result

First-ever differential measurement of W and |t| in incoherent J/ψ production.

Provides new data to discriminate between QCD saturation and shadowing models.

Saturation models better describe the observed suppression at high ∣t∣, while leading-twist shadowing fails to account for the large-∣t∣ suppression.

Supports the notion that small-size gluon configurations saturate faster, an effect central to the Color Glass Condensate framework and future EIC physics.

BTW: Shadowing is also a nuclear effect observed in high-energy processes (like deep inelastic scattering or photoproduction), where the parton (especially gluon) densities in a nucleus are reduced compared to what you'd expect if you simply added up the parton densities of individual nucleons.

Inside atomic nuclei, gluons, carriers of the strong force, multiply with energy. But nature "tames" this growth to prevent runaway behavior, a process called gluon saturation. By observing how the J/ψ particle is produced via light interacting with lead nuclei, scientists see less growth at finer resolutions, revealing this taming mechanism in action.

Please often quote this example: It's like shining a flashlight on a crowded dance floor, when zoomed in enough, everyone starts to look the same. That "sameness" signals saturation.

In summary: The ALICE experiment has provided quantitative evidence that at small spatial scales, gluons inside nuclei stop increasing with energy as quickly as they do at larger scales. This effect, seen through suppressed incoherent J/ψ production, marks a crucial step toward directly observing gluon saturation, validating key predictions of high-energy QCD.

Enjoy reading this paper.

Thanks. With best wishes, Bedanga

Dear Students,

Over the weekend you may want to look at this nice paper from ALICE which recently appeared in the arXiv: "Study of ⟨pₜ⟩ and its higher moments, and extraction of the speed of sound in Pb–Pb collisions with ALICE" (arXiv:2506.10394v1)

Imagine striking two bells — one made of brass, the other of iron. Even if they are the same size and shape, they sound distinctly different. Why? It’s because the speed of sound inside each material is different, which changes how vibrations travel and how the bell resonates.

In brass, sound waves travel faster, producing a clearer, higher-pitched tone.

In iron, the waves move slower, resulting in a duller, heavier sound.

This difference arises from the intrinsic properties of the material — its elasticity and density — and reflects how it responds to deformation.

Now Think of the QGP Fireball

In heavy-ion collisions, the QGP is like a “bell” made of strongly interacting matter. When it's created, pressure gradients act like the striker — they generate expansion waves inside the medium. The speed of sound in the QGP determines how fast those pressure waves propagate — i.e., how efficiently the fireball converts initial energy into collective flow.

A low speed of sound (like soft iron) implies a sluggish, diffusive expansion.

A high speed of sound (like stiff brass) leads to a strong, rapid expansion.

This is why measuring the speed of sound is so important: It tells us what "material" the early universe was made of — soft and compressible, or stiff and reactive — and helps map the QCD equation of state.

Objective of the paper

To study event-by-event fluctuations in mean transverse momentum (⟨pₜ⟩) and its higher moments in ultracentral Pb–Pb collisions at √sₙₙ = 5.02 TeV with the ALICE detector, and use these measurements to experimentally extract the speed of sound (cₛ²) in the quark–gluon plasma (QGP).

Key Concepts

QGP Thermodynamics: In ultracentral collisions, the volume remains almost constant while entropy (related to charged-particle multiplicity, Nₓₕ) fluctuates due to quantum effects.

Speed of Sound: Defined as cs2=dln/dlnNch , under the assumption that ⟨pₜ⟩ scales with temperature and Nch with entropy.

Centrality Selection Bias: Multiple centrality estimators are used to understand the influence of pseudorapidity gaps and detector-specific effects on the measurements.

Detectors Used in the Study: V0 Detector, Inner Tracking System (ITS); Time Projection Chamber (TPC) and Zero Degree Calorimeters (ZDC)

Analysis Technique: Pb–Pb collisions at 5.02 TeV (2018 LHC Run 2) ~193 million minimum bias events analyzed; Vertex selection: Vz < 10 cm. Primary charged particles: Excludes secondaries and weak decays and used some quality cuts:

Centrality Estimators

Based on: TPC: Nch and ET

SPD: Tracklets

V0: Forward multiplicity

Key feature: Use of pseudorapidity gap to reduce autocorrelation biases between centrality determination and measurement region

Observable Extraction

⟨pₜ⟩ and ⟨dNch/dη⟩: Calculated from corrected pₜ spectra (0.15–10 GeV/c), extrapolated to full pₜ using blast-wave fits

[pₜ] Distribution Analysis: Cumulants up to 4th order (variance, skewness, kurtosis) Corrected using particle weights and bootstrapping for uncertainties

Speed of Sound Extraction

Derived from:

cs2=dln/dl(dNch/dη)

Fitting ⟨pₜ⟩normvs ⟨dNch/dη⟩norm using a parametric model

Corrections and systematic uncertainties from: Extrapolation model (Blast-Wave, Lévy-Tsallis, Hagedorn) Uncertainties in knee parameters (mean, width of multiplicity distribution)

Key Results

Speed of Sound (cₛ²) varies significantly with centrality estimator:

Lowest: 0.1146 ± 0.0028 (stat.) ± 0.0065 (syst.) using V0 estimator (largest pseudorapidity gap).

Highest: 0.4374 ± 0.0006 (stat.) ± 0.0184 (syst.) using ET estimator with no pseudorapidity gap.

Trend: cₛ² decreases with increasing pseudorapidity gap, reducing biases from jet fragmentation and autocorrelations.

Non-Gaussian features in [pₜ] fluctuations:

Variance (k₂) decreases sharply with centrality.

Skewness (k₃) peaks near the knee of the multiplicity distribution and then falls—attributed to vanishing impact-parameter fluctuations in the most central collisions.

Radial Flow Effects: Enhancement in intermediate pₜ and depletion in low pₜ observed in spectra for most central events, consistent with hydrodynamic expansion.

Comparison with Models: HIJING fails to describe ⟨pₜ⟩-Nₓₕ correlation.

Trajectum (hydro + hadronic cascade with variable EoS) reproduces the data trends well, including the minimum in ⟨pₜ⟩norm.

Conclusions

The ALICE experiment provides the first experimental extraction of the speed of sound in QGP from ultracentral events, establishing a novel link between final-state ⟨pₜ⟩ fluctuations and the underlying QCD thermodynamics.

The results highlight the importance of minimizing centrality selection biases (e.g., by using large pseudorapidity gaps).

Future studies with better acceptance and finer binning can further constrain QGP properties such as compressibility and early-time fluctuations.

Remember: Measuring the speed of sound (𝑐ₛ) in heavy-ion collisions is crucial because it encodes fundamental information about the Equation of State (EoS) of strongly interacting matter — i.e., how pressure builds up in response to energy density in the Quark-Gluon Plasma (QGP).

Happy weekend reading …

Best wishes, bedanga

Dear Students,

I am back with a new paper which you may find very interesting to read. Actually, the weekend is a good time to read this.

Here is a short write up of the findings from the ALICE Collaboration paper titled “Revealing the microscopic mechanism of deuteron formation at the LHC” (CERN-EP-2025-081, arXiv:2504.02393):

Observable: Two-particle femtoscopic correlation functions between charged pions (π±) and (anti)deuterons (d or dbar) as a function of the relative momentum k∗k∗ in the pair rest frame.

How it is measured:

Correlation function C(k∗)=N[Nsame(k∗)/Nmixed(k∗)]

Nsame: Relative momentum distribution of pairs from the same event.

Nmixed: Reference distribution from mixed events.

Measurements are done using the ALICE detector in high-multiplicity pp collisions at s=13 TeV.

Particle identification (PID) of pions and (anti)deuterons with TPC + TOF, with high purity (≥99%).

Key Findings

(A) Discovery of Dominant Production Mechanism: ~80% of (anti)deuterons are formed via nuclear fusion involving nucleons produced from short-lived resonances, primarily the Δ(1232), in the presence of a meson (pion). The pion carries away the excess energy, enabling the fusion to form a bound state, e.g., p+n+π→d+π

(B) Evidence from Femtoscopic Correlation: The π±–d correlation functions show a prominent peak at k∗∼200 MeV/c, matching the expected signature from Δ resonance decay. This peak is absent if deuterons are assumed to be produced thermally or without resonance involvement.

(C) Model-Independent Signal: The observed correlation peak is robust and cannot be explained without including the Δ resonance decay process. The resonance contribution is isolated using data-driven modeling of π–p correlations and coalescence modeling using EPOS 3 + afterburners.

Quantitative Results: Measured fraction of deuterons formed via Δ decay: 60.6 ± 4.1%. Total estimated fraction from all resonances (including extrapolated states): 78.4 ± 5.5%.

Novel Physics Revealed: This is the first direct experimental evidence showing that light nuclei like the deuteron are not directly produced as thermal states but instead emerge via coalescence/fusion after resonance decays, a three-body process catalyzed by mesons.

It claims to resolve a long-standing puzzle: How loosely bound states (~2 MeV binding energy) can form in environments with thermal energies >100 MeV (e.g., in high-energy collisions).

This insight is crucial for: (a) Modelling cosmic-ray antinuclei, relevant for indirect dark matter searches. (b) Understanding QCD hadronization and nucleosynthesis under extreme conditions.

Conclusion: This work from ALICE redefines the understanding of light nucleus formation in high-energy hadronic collisions by demonstrating a dominant resonance-assisted fusion mechanism. It provides strong experimental support to microscopic models that include meson-catalyzed coalescence and offers a reliable framework for both accelerator-based and astrophysical nucleosynthesis studies. No wonder it has been submitted to a high impact journal.

Enjoy reading the paper.

Thanks. best wishes, bedanga

Dear Students,

Today I bring to you an ALICE paper which is making a lot of news in social media saying ALICE produces Gold from Pb ion collisions. It is to do with ALICE Results on Electromagnetic Dissociation in Ultraperipheral Pb–Pb Collisions (√sₙₙ = 5.02 TeV) and the paper is titled - Proton emission in ultraperipheral Pb-Pb collisions at 5.02 TeV

Details can be found at

https://doi.org/10.1103/PhysRevC.111.054906

or

https://arxiv.org/abs/2411.07058

When you get time, have a look. This is what I have understood reading the paper.

What is the Goal?

To study how lead-208 nuclei (Pb-208) dissociate in ultra peripheral collisions (UPCs) — where they don’t touch, but their electromagnetic fields interact. This causes electromagnetic dissociation (EMD), breaking up nuclei via photon absorption.

Key Physics Mechanism

Virtual Photon Exchange: A fast-moving Pb-208 nucleus emits a virtual photon (γ*). The other Pb-208 nucleus absorbs this photon → becomes excited.

Nuclear Excitation and Decay: The excited nucleus emits neutrons and protons. The residual nucleus changes element (e.g., Au, Hg, Tl).

What Did ALICE Measure?

ALICE measured the cross sections (probabilities) for events with 0, 1, 2, or 3 protons, each accompanied by at least one neutron:

Emitted Particles

Residual Nucleus

Measured Cross Section

0p + ≥1n Pb isotopes 157.5 ± 4.7 mb

1p + ≥1n Tl (Z = 81) 40.4 ± 1.6 mb

2p + ≥1n Hg (Z = 80) 16.8 ± 3.7 mb

3p + ≥1n Au (Z = 79) 6.8 ± 2.2 mb

How Were These Events Detected?

ALICE used specialized detectors called Zero Degree Calorimeters (ZDCs) placed at very forward angles (close to the beam direction): ZNA / ZNC (Zero Degree Neutron Calorimeters): Detect neutrons emitted along the beamline. These neutrons are uncharged and travel straight through magnetic fields. ZPA / ZPC (Zero Degree Proton Calorimeters): Detect protons, which are charged and are bent slightly off-axis by LHC magnets. Protons reach these calorimeters due to their energy and charge.

Key Features: High granularity and timing resolution. Allow counting of individual protons and neutrons per event. Enable event classification by multiplicity: 0p, 1p, 2p, 3p, etc.

These measurements required coincidence detection: Events were selected only when ≥1 neutron was detected, And protons were separately counted to determine event category.

What Do These Numbers Mean?

Neutron-only (0p) events are most common. As more protons are emitted, the event type becomes progressively rarer. The cross section has this hierarchy

σ0p>σ1p>σ2p>σ3p

How Rare is the 3p + ≥1n Channel?

Cross section = 6.8 mb

Total EMD cross section ≈ 187,000 mb

Probability:

P≈6.8/187,000≈3.6×10−5

So on average: 1 in ~27,000 EMD events results in 3p + ≥1n

Physics Insight: Why Mostly Neutrons?

Neutrons are easier to emit (no Coulomb barrier). Protons require more excitation energy due to electrostatic repulsion. Therefore, neutron emission dominates in low-energy EMD.

Why Is This Interesting?

Validates photonuclear reaction models (like RELDIS).

Helps design better forward detectors (ZDC-like) at LHC and future colliders.

Provides benchmark data for modeling beam-induced radiation and losses.

Demonstrates how even without contact, nuclear transformations can occur!

Enjoy reading this paper and yes do not forget to mark a LIKE in social media for the ALICE results.

Thanks, With Best Regards, Bedanga

Dear Students,

Sometimes, when you have a very good detector and bright ideas, you can extend the physics reach of an experiment. The paper I am discussing below is one such case of ALICE contributing to cosmic ray science. When you get time, look at the ALICE paper submitted to the Journal of Cosmology and Astroparticle Physics - Multimuons in cosmic-ray events as seen in ALICE at the LHC

https://arxiv.org/abs/2410.17771

Why is it interesting, and how are they detected:

The origin of cosmic rays and their composition has been a long-standing open science problem since its discovery in 1911-1912. The cosmic ray particles are now known to consist primarily of protons, helium, carbon, nitrogen and other heavy ions up to Iron. The source is from within the solar system and beyond. Above about 10^5 eV, their rate is less than one particle per square meter per year, and direct observation in the upper layers of the atmosphere is inefficient. An efficient way is to do experiments that exploit the atmosphere as a giant calorimeter. The incident cosmic radiation interacts with the atomic nuclei of air molecules and produces air showers which spread out over large areas. The size of an extensive air shower (EAS) at sea level depends on the primary energy and arrival direction. Many such ground-based experiments are on these principles. In May 1989 and October 1991, cosmic ray detectors recorded an event of energy ~ 10^20 eV. Their energy corresponds to a center of mass energy of the order of 700 TeV almost 50 times the energy of the Large Hadron Collider (LHC).

What has ALICE measured:

From EAS, because ALICE is more than 52 meters underground, only muons and neutrinos reach the detector. It has 28 m of overburdened rock and 1 m of iron magnet yoke above it, corresponding to about 80 m water equivalent. Simulations show that muons with an energy of 16 GeV or above can reach the ALICE detector. The number of muons produced in an EAS depends on the energy of the primary cosmic ray and its mass. Therefore, this number is an observable that gives information on the atomic mass number of cosmic rays and allows one to get insight into the energy spectrum of the primaries. ALICE has measured events with muon multiplicity greater than four and as high as 100-300. The effective running time accumulated was 62.5 days for a total number of 15702 multi muon events for which the number of reconstructed muons is greater than 4. The effective detector area exposed to such an experiment is 17 m^2. As the average number of muon multiplicity in an event is proportional to the primary cosmic ray particle energy (see Figure 5 of the paper), ALICE has detected events with average primary cosmic ray energy greater than 10^16 eV.

How does ALICE detect them?

A dedicated trigger using the Time-of-Flight (TOF) detector is used. The TOF is a modular cylindrical RPC-based detector system. It has 18 sectors (20 degrees each), 9 on the top and 9 below. The TOF trigger for cosmic events requires a signal in one of the 9 upper sectors and another signal in the opposite lower sector with respect to the central axis of the detector. The trigger rate was 70 Hz.

Once there is a back-to-back coincidence, the ALICE Time Projection Chamber comes into play to reconstruct the tracks in the event. The accelerator is not on during this period. TPC has an inner radius of 0.8 m, an outer radius of 2.8 m and a total length of 5.0 m along the LHC beam direction. That makes the area exposed to muons of 25 m^2. However, good quality track selection criteria reduces the effective area exposed to cosmic muons to 17 m^2. Cosmic muons are typically reconstructed as two individual tracks in the upper and lower halves of the TPC. An algorithm is applied to match each up track with its corresponding down track to reconstruct the muons' full trajectory and remove double counting. Events with four or more such tracks are of interest.

Findings:

Figure 3 shows ALICE cosmic-ray muons detected for 62.5 days of data taking. The highest cosmic muon multiplicity event recorded by ALICE is 287 muons in an event. See the beautiful event displayed in Fig. 7. Comparison to models shows that cosmic rays with only protons as primary particles cannot explain the measurements. It also requires cosmic rays to have heavier elements like up to Iron to match the data. The average primary particle energy is estimated to be between 10^16 and 10^18 eV. ALICE also measures the rate of high multiplicity muon events, which provides big constraints for models in the field. See the results in Fig. 8 of the paper.

Enjoy reading this paper over the week.

Thanks. With best wishes, Bedanga

Dear Students,

Recently, I visited a collaborating institute, and students there said they liked the interesting paper summary series. Motivated by that, I am continuing. Zubayer-da had asked me to discuss topics via online meeting, but it is, for the moment, difficult due to several meetings.

When you get time, look at the ALICE paper submitted to Nature Physics - Observation of partonic flow in proton-proton and proton-nucleus collisions

https://arxiv.org/pdf/2411.09323

Why is it interesting:

Proton-proton or proton-nucleus collisions were traditionally used as baseline measurements to infer QGP formation in nucleus-nucleus collisions. However, at LHC energies, such small system collisions started showing features we thought exclusively belonged to heavy-ion collisions. For example,

(a) Ridge behaviour – observation of long-range (in eta) near side (phi difference close to zero) angular correlations in pp collisions,

(b) Mass dependence of elliptic flow at low transverse momentum, and (c) enhanced production of multi-strange hadrons.

People attributed the observed collective behaviour to QGP formation in small system collisions. As usual, this motivated non-QGP models to come into play in explaining the features. These include

(a) Color Glass Condensates attributed the observed flow patterns to come from the initial gluon momentum correlations in the colliding hadrons.

(b) PYTHIA 8 event generator attributed these to Collectivity without plasma in hadronic collisions through the generation of QCD strings that are transversely extended, and results in a transverse pressure and expansion, similar to the flow in a deconfined plasma.

What new does this paper bring to the table?

This paper measures elliptic flow in pp and p-Pb collisions for pion, kaon, proton and lambda using 2-particle correlations. The measurements suppress non-flow correlations to the level of 6% at low pT and 1% at high pT (estimates from using similar analysis as data in Pythia). It observes that baryons and mesons v2 split at intermediate pT at five sigma level. The baryons and mesons themselves separately group together within one sigma level.

This is an exciting result, as it demonstrates a feature seen in heavy-ion collisions, which so far has been attributed to flow developed at the partonic phase and remained unchanged by subsequent hadronization via recombination mechanism. It has been claimed as evidence of Partonic Collectivity (first time used in the STAR experiment paper, which had phi-meson elliptic flow; there was a big scientific fight with the referee to use this word in the PRL paper) and hence the existence of QGP.

Models using hydrodynamic evolution of a quark–gluon plasma with a partonic equation of state, followed by hadronization via coalescence along with jet-medium interactions at intermediate pT and then uses a linear Boltzmann transport (LBT) model for high pT with hadronization via quark fragmentation can explain the measurements. Partonic coalescence is essential to explain the data; the same model, if switched off this part is unable to explain the data.

These measurements, for the first time in high multiplicity small system collisions, show statistically significant characteristic grouping and splitting of v2 for mesons and baryons intermediate pT, similar to measurements in heavy-ion collisions. A critical piece of result in support of likely QGP formation in high multiplicity pp and p-Pb collisions. That there will be a stage in the system created in p–Pb and pp collisions exhibiting Collectivity at the partonic level was never thought of 10 years back.

Note that the claim is not that there are partonic degrees of freedom before hadronization (which we know to be there in the jet shower or multi-particle interactions) but that these partons are a medium with collective flow (implying strong final state interactions).

Enjoy reading this paper over the week.

Thanks. With best wishes, Bedanga

Dear Students,

I am coming back to this topic of interesting recent experimental papers after quite some time, it has been a busy time.

When you get time, look at the ALICE paper submitted to Physical Letters B - Direct-photon production in inelastic and high-multiplicity proton–proton collisions at √s = 13 TeV

https://arxiv.org/pdf/2411.14366

Why is it interesting:

Direct photons are generally those which are not coming from the decay of hadrons. The slope of the momentum distribution of a class of direct photons called thermal photons gives the idea of temperature and radial flow of the system. This is used to estimate the QGP temperature. A subclass of direct photos which come from initial hard scattering processes are called prompt photons; they are from quark–antiquark annihilation and quark–gluon Compton scattering. The direct photons can be created both in the form of real (massless) photons and in the form of virtual photons with non-zero mass which convert into a lepton pair (dimuon μ+μ− or dielectron e+e−). There is a way to connect the yield of real photons to those associated dielectron pair production.

This paper deals with virtual photons. It tries to see if high multiplicity pp collisions produce direct photons of the class called thermal photons. If so it will be another addition to the growing evidence that high multiplicity pp collisions show QGP like expectations.

So did we see evidence of thermal photons in pp collisions at LHC ?

The events corresponding to the highest V0 multiplicity class is called a High Multiplicity Events. 3.38 ×10^8 high-multiplicity pp events were analysed, they correspond to 0.1% of the total minimum bias events.

Step-by-step the di-leptons coming from (a) real photon conversions in detector material; (b) J/Psi (charm/beauty) decay; (c) related to decays from light flavour hadrons like - π0, η, η′, ρ, ω and φ mesons are removed. Further the knowledge of the inclusive photon is used. All these allows to extract the direct photon spectrum via a detailed and careful analysis.

Once the direct photon transverse momentum spectra in high multiplicity pp events are extracted the measurement is compared to perturbative QCD (pQCD) calculations which is expected to give the contributions from prompt photons. Two different pQCD calculations, which differ by the way they handle the (a) parton distribution function and (b) fragmentation function, are compared to data. Both fail to explain the measurements in the lower pT range (~1-3 GeV/c), although the uncertainties are large. In fact you need to scale pQCD photons by a factor of about 12 to explain the data.

A statistical analysis gives a p-value of 0.1. A p-value is a calculated probability that measures the strength of evidence against a null hypothesis. P-values are important for data analysis and decision-making. A p-value of 0.1 or more is considered insufficient evidence that the model describes the data. A smaller p-value of (say) 0.05 or less is commonly considered statistically significant.

Hence the conclusion in this case is there is a need of an additional source of direct photons and/or an increased yield of prompt pQCD photons to properly describe the high multiplicity data.

If one uses additional thermal photon (from QGP like source) contributions, then the measurements are described better at low transverse momentum in pp collisions at the LHC energies.

The collaboration is not going fully forward claiming to measure photons from QGP, because of two reasons (a) uncertainties are still appreciable and (b) there are hardly any state of the art theory calculations for pp collisions at LHC to make a good comparison to data. Hence the collaboration says:

“The multiplicity range covered in the present analysis is of particular interest in the context of the search for a possible onset of thermal radiation in small systems. The statistics that will be collected during the Run 3 and Run 4 periods of LHC operation will allow one to study the direct photon yield with higher statistical precision and in a more differential way. The results presented in this Letter and the future results from Run 3 data should stimulate the interest of the theoretical community to calculate the yield of direct photons in small systems as a function of charged-particle multiplicity to shed light on the possible presence of a QGP in small systems.”

Enjoy reading this paper over the weekend. Great opportunity for a student to pick up such an analysis using Run 3 data. It will need a lot of hard work though.

Thanks. With best wishes, Bedanga

Dear Students,

When you get time, look at the ALICE paper submitted to Physical Review Letters - Measurements of chemical potentials in Pb–Pb collisions at √sNN = 5.02 TeV.

https://arxiv.org/pdf/2311.13332.pdf

Why is it interesting: during various talks and presentations, we say that the little bangs created in heavy-ion collisions resemble the big bang scenario in the early Universe. The standard cosmological model says that in the early Universe, there were nearly equal abundances of matter and antimatter present, leading to a net-baryon-free and electrically neutral environment. This paper deals with how close the medium formed in Pb-Pb collisions at LHC is to this scenario.

In a typically heavy-ion collision scenario, there can be baryons at mid-rapidity due to various reasons - initial baryon number can be transported to midrapidity via either baryon junction formation or di-quark breaking, this will deviate the system from Early Universe scenarios.

So, how close are we to a net-baryon-free and electrically neutral early universe scenario environment?

The quantities to be measured are baryon chemical potential and charge chemical potential, and see if they are Zero.

The other question is RHIC (Indian group analysis), which revealed that as we go to lower collision energies, the peripheral collisions give a different baryon chemical potential than central collisions at fixed collision energy. What is the scenario at LHC?

To arrive at proper conclusions, the paper makes use of

(a) ALICE measured particle and anti-particle ratios (effect of volume and correlated uncertainties cancel out) involving protons, Ω− baryons, and light (hyper)nuclei for estimating baryon chemical potential, pions used for estimating charge chemical potential.

(b) A statistical thermal model FIST is used with the grand canonical ensemble and chemical freeze-out temperature fixed from earlier studies to be 155 +/- 2 MeV.

Results: Baryon chemical potentials μB = 0.71±0.45 MeV and charge chemical potential μQ = −0.18 ± 0.90 MeV, which are compatible with zero within 1.6σ and 0.2σ , respectively. No centrality dependence is observed. The nuclear transparency regime has been reached, and baryon transport from the colliding ions to the interaction region is negligible.

This ALICE work shows with unprecedented precision that the medium created in heavy-ion collisions at the LHC approaches the early Universe conditions more than any other experimental facility.

Enjoy reading this paper.

Thanks. With best wishes, Bedanga

Dear Students,

This time, I bring to your notice a recent paper from ALICE on Correlation femtoscopy.

Titled - Exploring the strong interaction of three-body systems at the LHC

https://arxiv.org/pdf/2308.16120.pdf

Submitted to Nature Physics.

It deals with measurements of two-particle momentum space correlations between deuterons and kaons or protons.

The correlation function is constructed in a given particle pair momentum difference bins by counting the particles of interest produced in the same collision (correlated pairs) and dividing it by the number of uncorrelated pairs typically obtained by combining particles produced in different collisions.

The correlation functions can be connected to the scattering theory parameters like the phase shift and, hence, the scattering length. Then, they can be connected with the effective range of the interactions. In a simple model, this is done by assuming that the incoming particle is a plane wave and the outgoing wave is a superposition of spherical waves with phase shifts that depend on, among other things, the relative angular momentum (l) of the projectile and target. It is the interactions that determine the phase shift values. For example, l=0, S-wave refers to s-wave scattering, related to scattering length that characterizes interaction at zero energy and is associated with the cross-section measured in scattering experiments.

A repulsive interaction translates to a negative correlation function, and an attractive interaction to a positive correlation function.

ALICE measured kaon-deuteron and proton-deuteron correlation functions to be negative as a function of the relative momentum of the particle pair in proton-proton collisions at the centre of mass energy of 13 teV at LHC. See Figure 1 of the paper.

The Kaon-deuteron correlations are well explained by considering the system to be an effective two-body problem and invoking coulombic interactions + strong interactions. The strong interactions are considered in the model via the scattering parameters: scattering length and effective range.

Surprisingly, the proton-deuteron correlations are no longer described by the treatment of the system as an effective two-body problem. The model yields a positive correlation function while the measurements are negative.

The picture of point-like particles breaking down for the p-d system may be due to Pauli-blocking at work at short distances. Calculations that consider all the relevant two and, most importantly, the system's three-body interactions and possible spin configurations can only explain the ALICE measurements. See Figure 2 of the paper.

In the case of p-d, the inner structure of the deuteron (proton + neutron) has to be considered for a theoretical description of the measurement.

Isn't it very interesting? Enjoy reading the paper.

Thanks. With best regards, Bedanga

Dear Students,

I want to draw your attention to the following new paper from ALICE - Measurement of Non-prompt D0-meson Elliptic Flow in Pb–Pb Collisions at √sNN = 5.02 TeV.

https://arxiv.org/pdf/2307.14084.pdf

Measurement of collectivity through an observable called elliptic flow which depends on the azimuthal distribution of particles produced in heavy-ion collisions has brought important insights about (a) thermalization of the medium formed in heavy-ion collisions, (b) applicability of hydrodynamics to the system formed in the collisions and (c) extraction of transport properties of the medium formed in heavy-ion collisions.

Systematic measurements of elliptic flow for light, strange, and charm valence quark-carrying hadrons at LHC energies have shown that they all participate in the collective expansion of the medium form in Pb-Pb collisions. Measurements have been used to obtain the shear viscosity to entropy ratio and diffusion coefficient.

The question is, what about beauty quark carrying hadrons?

Given the current event statistics and detector configurations, one can address it through the measurements elliptic flow of J/ψ or D-mesons mesons originating from beauty-hadron decays (non-prompt) or elliptic flow of leptons from beauty-hadron decays. The issue with the latter is that small lepton masses lead to a broad correlation between the kinematic variables (pT and direction) of the beauty hadrons and the decay leptons.

Using information like the impact parameter of the D0-meson daughter tracks, the distance between the D0-meson decay vertex and the primary vertex, and the cosine of the pointing angle between the D0-meson candidate line of flight and its momentum vector as inputs to a machine-learning approach with multi-class classification based on Boosted Decision Trees (BDT), ALICE has been able to separate D-mesons from prompt and non-prompt sources.

The measurement of the elliptic flow of non-prompt D0-mesons shows:

It is positive with 2.7 sigma away from zero.

It does not have a transverse momentum dependence.

It is 3.2 sigma smaller than prompt D meson elliptic flow at lower transverse momentum.

All these suggest that beauty quarks have a different degree of participation in the collective motion of the medium and hence a different degree of thermalization compared to charm, strange and light quarks.

Future data samples to be collected with the upgraded ALICE detector will provide a more accurate conclusion and allow for the extraction of the spatial diffusion coefficient of beauty quarks.

Enjoy reading the paper.

Thanks. With best regards, Bedanga

Dear Students,

If you have time, please take a look at the following new ALICE paper –

Charm production and fragmentation fractions at midrapidity in pp collisions at √s = 13 TeV

https://arxiv.org/pdf/2308.04877.pdf

What did we measure and why:

Any invariant yield versus transverse momentum distribution for a hadron in proton-proton collisions can be understood using the convolution of three contributions:

(a) Parton distribution function – probability distribution of quarks and gluons inside a proton with which we are colliding. It cannot be calculated directly and must be obtained from experiments.

(b) partonic cross section - scattering probability calculated as a perturbative series expansion in the strong coupling constant.

(c) fragmentation function – the probability of a parton hadronising into a particular hadron. It cannot be calculated directly, and it has to be obtained from experiments.

Then a concept of factorisation is invoked, for example, Collinear factorisation theorems that isolate long-distance non-perturbative parton distribution functions from perturbatively calculable short-distance matrix elements.

What we have measured in this paper are:

(a) The probability that a charm quark fragments into hadrons: D0, D+, D+s , D∗+, Λ+c , and Ξ0c hadrons, J/ψ mesons and Ξ+c and Σ0,+,++c baryons. It is experimentally obtained for each hadron species by normalising the production cross section with the sum of the pT-integrated production cross sections of the measured production cross sections of D0, D+, D+s , J/ψ, Λ+c , Ξ0c , and Ξ+c etc.

(b) The probability for a charm quark to hadronise along with a strange quark to it hadronising with light flavour quarks u and d. Also referred to as the strangeness suppression factor. This is obtained using, for example, prompt crosssection ratio D+s /(D0 + D+).

(c) The ccbar production cross section at midrapidity. It is just the sum of the production cross sections at midrapidity of the D0, D+, D+s , J/ψ, Λ+c , Ξ0c , and Ξ+c hadrons.

Key findings:

(a) Charm baryon production is enhanced in pp collisions at LHC energies relative to that in e+e- and ep collisions (reported by LEP, HERA and BELLE). Probably caused by different hadronisation mechanisms in the parton-rich environment produced in pp collisions, regardless of the centre-of-mass energy.

From comparison to models and baryon-to-meson ratios measured in ALICE, it seems to require (i) within statistical hadronisation additional excited charm-baryon states, (ii) within PYTHIA it requires an improved colour reconnection mechanisms beyond the leading-colour approximation and (iii) within a Catania model which assumes a hot QCD matter formation in pp collisions together with coalescence as a mechanism of hadronisation.

The ALICE measured baryon production in pp collisions at the LHC challenges the concept of “jet universality”, according to which the parton fragmentation is universal between collision systems and can be constrained from e+e− results. This also then implies the breakdown of multiparton interaction (MPI)-based event generators implementing jet universality

(b) The results of the strangeness suppression factor are compatible within uncertainties in the LEP measurements. These results indicate that producing prompt strange D mesons relative to prompt non-strange D mesons in e+e−, ep, and pp collisions shows no significant dependence on the collision system and energy.

(c) The charm production crosssection measurement is found to be compatible with the upper edge of the fixed order plus the next-to-leading logarithms (FONLL) approach and the next-to-next-to-leading order (NNLO) predictions within uncertainties. The precise measurements at the LHC can provide valuable constraints to reduce the theoretical uncertainties on the calculations of cc production at midrapidity in pp collisions.

Enjoy reading more about these beautiful measurements in the above link to the paper draft in arXiv.

Thanks. With best wishes, bedanga

Dear Students,

ALICE has a new interesting paper from UPC https://arxiv.org/abs/2305.06169 If you get time on the weekend have a look. Those interested in the physics of gluon saturation/CGC etc will find it useful.

ALICE has made the first measurement of the cross-section for incoherent photonuclear production of J/ψ vector meson as a function of the Mandelstam |t| variable (related to the square of the momentum transferred during the interaction). The measurement was carried out using ultra-peripheral collisions of Pb nuclei at a centre-of-mass energy per nucleon pair of 5.02 TeV and Bjorken-x range (0.3–1.4)×10^−3. These are events in which the outgoing nucleus is not in the same quantum state as the incoming. The experimental manifestation is that the target dissociates into a system of particles, maintaining the rapidity gap to the produced vector meson. In order to produce the vector meson and nothing else, there cannot be a net colour charge transfer to the target. This requires at least two gluons to be exchanged with the target, and the cross-section consequently scales approximatively as the gluon density squared. The incoherent cross-section measures how much the scattering amplitude fluctuates between the different possible initial state configurations.

Further, the Mandelstam variable |t|, is related through a Fourier transform to the distribution of nuclear matter in the impact-parameter plane. This measurement provides information about parton distributions inside a nucleus. At large |t|, the slope of the incoherent spectrum is controlled by the size of the smallest object that fluctuates. On the other hand, at small |t|, one is sensitive to fluctuations at a long distance scale. At asymptotically large energies, where the black disc limit (gluon saturation) is excepted to be reached, there are no event-by-event fluctuations. In this limit, the incoherent cross-section is expected to be suppressed.

The ALICE results confirm the importance of sub-nucleon fluctuations to describe the measured incoherent J/ψ process at high energies, representing the first experimental step to use the quantum fluctuations of the gluon field to search for saturation effects in heavy nuclei.

Thanks. with best wishes, Bedanga

Dear Students,

A nice paper from ALICE has appeared in arXiv:

https://arxiv.org/pdf/2304.10928.pdf

If you get a chance, take a look. It deals with Ultra-peripheral collisions.

These collisions are very exciting because ions that the LHC accelerates are also carriers of electromagnetic fields and hence are a huge source of photons. These reactions occur when the ions pass by each other with impact parameters larger than the sum of their radii. The exchange of virtual photons between the nuclei mediates the interaction. The number of photons scales as the square of the nuclear charge while typical photon energies scale with the Lorentz contraction of the nuclei and so increase with beam energy.

There are several UPC processes, but the current paper is interested in the photoproduction of a vector meson (specifically J/Psi) in photo-nuclear interactions, where the vector meson is reconstructed from its decay products (here, dimuons). Further, we are interested in Coherent production – the photon interacts coherently with the whole nucleus.

How does ALICE select such events –

(1) In ALICE, such a UPC event sample was recorded using a hardware trigger that requires at least two unlike-sign tracks in the muon spectrometer (as we are interested in J/Psi), and no activity in the V0A, ALICE Diffractive (AD) Detectors - ADA, and ADC. Basically, events where any kind of hadron activity is highly suppressed.

(2) The coherence condition, both in the emission of the photon and in the interaction with the nuclear target, constraints the transverse momentum of the produced di-lepton or vector meson to be of the order of 1/2R_Pb— where R_Pb is the radius of the lead nucleus — which corresponds to a pT ∼ 60 MeV/c. ALICE puts pT selection criteria on dimuon of less than 25 MeV/c.

Now what is the physics this paper tries to address: the vector meson (spin 1 mesons like J/Psi, K*, Phi) production models have assumed that the vector meson has the same helicity as that of the initial photon that interacted with the target, i.e. technically called as s-channel helicity conservation (SCHC). However, there is no fundamental physics principle that justifies SCHC. Experimental measurements of polarisation and spin parameter measurements of photoproduced vector mesons at high energies could provide important insights as to whether this assumption is correct.

The experimental signature is the vector meson will have transverse polarisation if the vector meson keeps the polarisation of the incoming photon. That is, if you measure the polarization parameter (three lambdas in equation 1 of the paper) and if all three are consistent with zero, an isotropic angular distribution is obtained; when (λθ , λφ , λθφ ) = (1, 0, 0) the vector meson is transversely polarised, and (λθ , λφ , λθ φ ) = (−1, 0, 0) corresponds to a purely longitudinal polarization.

ALICE got the following values:

λθ = 0.75 ± 0.25 ± 0.24

λφ = 0.03 ± 0.03 ± 0.02

λθφ = 0.10 ± 0.05 ± 0.06

That means following (λθ, λφ, λθφ ) = (1, 0, 0) picture, which means coherently produced J/ψ mesons are transversely polarised as required for s-channel helicity conservation hypothesis.

Have fun reading the paper.

Thanks. With best wishes, bedanga

P.S: Several of you told me that you liked the summary of papers that I have been sending, you have started to put in a habit to read papers - thank you for the encouragement.

Dear Students,

I come to you with another interesting paper, related to testing QCD thermodynamics in heavy-ion collisions. This time it is from STAR collaboration. It was published a few weeks back but thought you all would be interested to look at it.

High energy heavy-ion collisions in the laboratory produce a form of matter that can test Quantum Chromodynamics (QCD), the theory of strong interactions, at high temperatures. One of the exciting possibilities is the existence of thermodynamically distinct states of QCD, particularly a phase of de-confined quarks and gluons. An important step in establishing this new state of QCD is to demonstrate that the system has attained thermal equilibrium.

Recently it was predicted by Lattice QCD that thermodynamics of QCD matter will lead to cumulant ratios (first order cumulant related to mean, second order to variance, third order to skewness, fourth order to kurtosis and so on …) of event-by-event net-baryon (net-proton a proxy) number distributions to show the following ordering:

C3/C1 > C4/C2 > C5/C1 > C6/C2 – https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.074502

The measurements were carried out by STAR using event-by-event net-proton distributions and found that the above ordering is more-or-less (within uncertainties, large at lower collision energies) followed over collision energies of 200 to 7.7 GeV.

However at collision energy of 3 GeV the cumulant ratios show a reverse ordering: C3/C1 < C4/C2 < C5/C1 < C6/C2.

Very interesting to see this reverse of ordering (reverse of sign as well). It is also qualitatively consistent with a non-QGP model expectation such as UrQMD at 3 GeV.

See the paper

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.082301 for more

details.

Higher statistics data at collider energies will tell us more.

Thanks. With best regards, bedanga

Dear Students,

I come to you with a new recent paper from our experiment ALICE. If you are interested in flow studies you may want to read this paper.

One of the current goals of the ALICE experiment is the precise measurement of properties of Quark Gluon Plasma which is longer lived at LHC due to higher collision energies. Flow observables play an important role.

In flow studies we have read that the initial coordinate space anisotropy can be quantified in terms of eccentricity coefficients and corresponding symmetry plane angles. These then due to interactions and depending on the hydrodynamic response get converted to anisotropy in momentum space. This momentum space anisotropy is characterized by Fourier harmonics and the corresponding symmetry planes. In early flow study days it was assumed (for small eccentricities) the fourier coefficient is linearly proportional initial eccentricity and the initial coordinate space symmetry plane angle is proportional to final momentum space symmetry angle.

With deeper studies it was realized that anisotropies in momentum and coordinate space are related instead with the matrix equation and the two symmetry planes need not be the same in general. Also came the flow fluctuations, odd harmonics and several other developments. These necessitated or led to the development of new observables called Symmetric cumulants describing correlations between squares of Fourier coefficients and Anti-symmetric cumulants involving different moments of the flow amplitudes without any influence from the symmetry planes. Soon it was realized they are very sensitive to initial conditions in heavy-ion collisions and temperature dependence of transport coefficients of QGP.

Theory groups: Duke, Nat. Phys. 2019, used ALICE measurements of two-particle correlations, charged-particle multiplicities, etc. and Jyväskylä 2022, PLB 2022, additionally utilized ALICE measurements of higher-order flow observables in Pb–Pb collisions to constrain the temperature dependence of shear viscosity to entropy density of QGP (see attached plot).

ALICE's new paper (https://arxiv.org/abs/2303.13414 ) has developed a set of new flow observables that are sensitive to the hydrodynamic transport coefficients η/s and ζ /s and can potentially further reduce the uncertainties of the extracted transport coefficients. The chi-square test of data-to-model comparisons (see the attached plot with color code indicating the chi-square values) with two sets of best-fit parameterizations given by maximum a posteriori from the above two independent Bayesian analysis groups are obtained. Generally, the Jyväskylä parameterization describes the new data better. However, it also seems a more careful choice of observables (independent sets) is needed to reduce the uncertainties in the extracted transport coefficient further.

Hope some of you start such analysis with better observables with ALICE data.

Thanks, With Best Regards, Bedanga

Dear Students,

Another very nice ALICE paper on J/Psi production (my focus while reading it was on the midrapidity measurements). If you get a chance look at the figure 11 (right panel) of the paper below. I remember getting deeply involved in the discussions of observable getting formulated/finalized around 2013/2014. I was not an author of the paper, but the idea for this observable was very interesting to miss the physics discussion. That paper had predicted the observable to have a distinct pattern for LHC energy and now it is observed. The measurements for SPS and RHIC were already available then. Unfortunately the paper that time did not make it to a high impact journal in spite of authors trying hard.

rAA = _AA / _pp

The charmonium suppression has long been considered as a signal of the quark-gluon plasma created in relativistic heavy ion collisions. It turned out there are two kinds of hot nuclear matter effects on the charmonium production, the dissociation and the regeneration. The two affect the charmonium yield in an opposite way, and the degree of the both increases with increasing colliding energy. The observable pT nuclear modification factor rAA for J/ψ turns out to be sensitive to these two effects. As seen from the Figure 11 in the new ALICE J/ψ measurements at midrapidity (https://arxiv.org/abs/2303.13361 ),

- a stronger dissociation and weaker regeneration at SPS leads to rAA > 1 (observed);

- The strong competition between the dissociation and regeneration leads to a flat centrality dependence of rAA ~ 1 at RHIC (observed).

- A dominant regeneration at LHC makes rAA < 1 (now observed).

Thanks, With Best Regards, Bedanga

Dear Students,

There is a new paper in ALICE, which I think almost all of you will be interested in reading.I read it today morning. In most of your analysis you have an uncertainty due to material budget and we usually take a number given to us from some source. This new paper deals with this topic and how now it is reduced as discussed below. If you have time and want to know more read the ALICE paper below.

https://arxiv.org/abs/2303.15317

In experiments like ALICE, having accurate experimental conditions in simulations are essential to obtain various corrections factors to be applied to data. One of the dominant sources of systematic uncertainty in several measurements arises from how accurately we simulate the material budget of the experiment. The material budget is usually expressed in terms of radiation length, the mean distance over which a high-energy electron loses all but 1⁄e of its energy by bremsstrahlung. The average uncertainty on this is of the order +/- 4.5%. For example, it is the most significant contributor to the uncertainty in extracting low momentum excess direct photons, which carry information of the temperature of the QGP. Using two new data-driven methods, ALICE has reduced this to +/- 2.5%.

These methods provide calibration weights when data is compared to simulation to appropriately scale the Monte Carlo to better reflect the detector conditions. These methods are general and could be applied to any experiment similarly. The two methods for calibration weights are obtained using the (a) radial distributions of reconstructed photon vertices in experimental data. It makes use of the Time Projection Chamber gas material budget (chemical composition, temperature and pressure of the gas, hence the TPC gas density is monitored well in the experiment and known to the per mil level) and (b) robustness of the ratio of the number of reconstructed photons to the number of reconstructed charged particles, assuming 90% of charged particles are charged pions and same is true for photons from neutral pions. Such calibration has also resulted in a better understanding of the momentum resolution of low momentum tracks where the multiple scattering contributions are the largest.

Thanks, With Best Regards, Bedanga

Dear Students,

When you get time, have a look at this short and nice recent paper which also reflects how well our detectors in ALICE work and perform. Such precise measurement allows us to connect to interesting physics.

See ALICE paper - https://inspirehep.net/literature/2637684

The Λ baryon is the lightest hyperon with quark content uds and its lifetime has been measured by experiments in 1960s-1970s as 263.2 +/- 2 picoseconds.

ALICE with its detectors ITS, TPC and TOF has made unprecedentedly precise measurements of the Λ lifetime and of the relative difference between the lifetimes of Λ and anti-Λ. The confidence range of the Λ lifetime is reduced by approximately a factor of three with respect to the PDG average, allowing for an important test of the CPT symmetry in the strangeness sector. This measurement is also a fundamental reference for the studies of the properties of hypernuclear states.

Thanks, With Best Regards, Bedanga

Dear Students,

Reading papers and those beyond your own work helps us know more about the field in which we work. I am sure you all are aware of it. Earlier used to read a lot of papers every day, now it is limited and that too only to experimental work mostly. I thought I would share with you some of the most important findings that I gathered reading an experimental paper in our field and hope to inspire you to also take a look at it. You can also share with me if you found something additional and equally interesting in the same paper when you get a chance to read it.

I found a new paper from ALICE on the jet axis angle in nucleus-nucleus collisions: https://arxiv.org/abs/2303.13347 very interesting and it could actually make some definitive conclusions. Let me share with you what I understood reading the paper.

There are three kinds of axis one can measure in jets - standard axis using, say anti-KT algorithms, Soft Drop axis obtained after removal of soft radiation (groomed jet) and the winner-take-all (WTA) that uses a momentum recombination algorithm with the direction is that of the most energetic constituent of the jet. Although all three angles between the jet axes considered here involve some form of transverse momentum resummation, studying them addresses several important jet-quenching physics mechanisms: (a) the relative suppression of gluon vs quark jets, (b) intra-jet transverse-momentum broadening and (c) the ability of the medium to resolve a colour dipole as two independent colour charges. This later part decides the medium resolution length. Uncertainty principle arguments suggest that wider splittings are formed earlier in a vacuum than narrower splittings. Heavy-ion collisions would result in wider splittings traversing a longer path in the medium on average.

The new measurements reported - disfavors intra-jet momentum broadening, favours distributions being dominated by quark-initiated jets relative to gluon-initiated jets and favours the medium resolving a splitting immediately after it happens (medium resolution length close to zero, incoherent energy loss of jets)

Thanks, With Best Regards, Bedanga