Challenges in Research
The Making of ALICE PMD Technical Design Report
and Addendum to the TDR
I was a graduate student working in the WA98 experiment @
CERN, when the Indian Heavy-ion experiment collaboration asked
me to contribute to ALICE experiment for which the group was writing the Technical Design Report (TDR). Only
on the successful defence of this TDR will the Photon Multiplicity
Detector (PMD), the fully Indian effort, become part of the
experiment. I was clearly told this work will not be part of
my PhD thesis. All programs were in C++ and my programing
expertise that time was
ONLY in fortran (WA98 analysis software was based on fortran).
I have to dedicate completely two months (leaving aside my
thesis work) to this effort and almost
90% of it alone at CERN (with not so encouraging financial
support @ CERN, that time given to the PhD students). The job was to: do the test beam analysis to show the
protype detector of PMD works and Physics simulations to show
the detector will work in the high multiplicity environment at
LHC. With no one to help at CERN I started with a fortran to C++ book given by Terry Awes (WA98
spokesperson) to work on the job. The first program to look at
was pmdmon.C. Added to this, in the first
meeting with the ALICE team at CERN, our group leader mentioned that we may not be able to
complete the TDR on time as one of our main person working on
it is gone temporarily for 2-3 years from the group. On top
of this negative atmosphere was the fact that PMD group at
CERN had two slow desktops and one of which was with
Black/White screen. What followed was a historic and most
memorable experience of my research career. I completed the
task assigned, helped complete 2 chapters out of the 5 in the
TDR. The confidence gained that I can independently take up
such challenges was immense and one of the key turning points
of my career.
ALICE PMD TDR
The TDR was submitted on time before the deadline of
30th September 1999. It was successfully defended in the
LHCC. A party was given by the then ALICE spokesperson -
Jurgen Shuckraft. Since I was asked by the group to be back to India
just the day before
the LHCC presentation, I missed these. However I was happy
that our PMD TDR was through.
Few years later, the PMD group was told that the position
of the PMD which was suppose to be at 5.8 m from the
interaction point covering a pseudorapidity range of 1.8 to
2.4 has to be moved somewhere else or removed from the experiment. This
was because the most important inner detector services and structural
components virtually formed a 20 radiation length wall before
the PMD. What followed was a battle for existence of PMD in
ALICE. We need to now find a new optimal position of PMD at
which we had to re-calculate the physics performance ability
of our detector and defend that we can do the physics we
promised in the new configuration to LHCC. The work not only
involved redoing all the physics performances but also a great
deal of work on implementing the material in front of PMD in
GEANT. I was called upon to help. It was a great opportunity
to enhance my knowledge in implementing geometries in GEANT
and we did it. The PMD detector geometry and the associated
programs to read out the data from GEANT was one of the
several of my contribution. The documentation was an Addendum to the PMD
TDR and was sucessfully defened and justified at CERN.
ALICE PMD TDR - Addendum
It was heartening to see PMD sucessfully take data at LHC, siting 3.6
m away from the interaction point and several
students doing there PhD with the analysis of the data taken by the detecctor.
The First Physical Review Letters from PMD
India had decided to publicly express itself as a nuclear power country
in 1998 and there were sanctions imposed. As a result we could not
become part of the RHIC experiments in USA. As the situation eased and we
were successfully collecting data with the prototype PMD detector
for ALICE, I requested our group leader about the possibility of
being part of STAR experiment at RHIC. A series of email exchanges,
review by committee and generous funding from DAE we became a part
of the STAR experiment with a promise that we will bring in
interesting physics possibilities in forward rapidity with photons.
A committee was get up with Hans Georg Ritter as the
head to discuss about having PMD as part of STAR experiment. A space
was subsequently found to accommodate PMD which will have overlaping
coverage with a charged particle detector (Forward TPC -
Prof. P. Seyboth leading it) so that we could do the correlation
studies related to DCC formation. This will be the key physics
addition to STAR physics program. As it turned out (see below)
longitudinal scaling was the highlight measurement!
Although not part of my PhD work, I was assigned to
get the PMD implemented in the simulation framework of STAR
(GSTAR). This is the first step for demonstarting that PMD can do
the physics that it claims to bring to STAR. I arrived at BNL and
got in contact with John Harris the then spokesperson of STAR and
was given a place in a makeshift building. I was suppose to interact
with Pavel Nevski (and Maxim Potekhin) the STAR computing and
software incharges. To my surprise, eventhough I had gained
expertise to work in GEANT and
also in C++, the geometry codes were in META LANGUAGE!! .. the files
are typically named as pmdgeom.geo ...by November 2000 I had
succeed in having an excellent working relation with Paval (who to
my surprise came to BNL even in weekends to discuss with me, a rare
thing for those who know Pavel!). The PMD was implemented in GSTAR -
for more details take a look at this STAR Webpage created (one such
page) to document this implementation on 29th November 2000 : PMD.HTML. This is also the time I came in contact with Olga Barannikova
.. with whom we had lot of interactions later in STAR (She went on
to become the STAR Council Chair)..... We then went on to write the
technical design report of STAR PMD We also then went on to publish
the detector related work in NIM paper
After sucecssfully building PMD (with some level of
learning experience of the associated difficulties, sparkings, large
leakage currents .. EMI effect .. role of graphite coating on the
detectors and so on...), installing
the PMD in STAR .. we took data of Au+Au collisions at 200 GeV and
62.4 GeV. The detector I must admit ran with lots of difficult,
frequent trips .. pedatstal related issues .. LV issues .. FEE issues
.. once data started coming .. teams were made to look and get into
the physics ... a team was stationed in BNL and another in VECC guest
house (somehow I was not at either of the place .. but wanted to look
at that data ..) and focusing on 200 GeV .. so I with a young student
Pawan Netrakanti decided we will look at only 2 days worth data at
62.4 GeV. So that there is not much competetion to show results first
.... We immediately realised several issues with the data .. cleaning up the
data for physics quality was just a first big step .. removing channel
zero effect, low ADC frequent hits .. etc etc ..GAIN variations ... we had to rewrite
virtually all aspects of the PMD reconstruction chain .. which was
different from which the rest of the PMD team were using .. most
cruial part was the ASSOCIATION MAKER and CULSTER MAKER ...finally we
got the multiplicity distributions of photons with our detector
.. while the rest of the team was struggling with the high
multiplicity environment in 200 GeV collisions ... As
you can guess .. no one was ready to accept our results .. several
presentations happended sometimes it was heated exchanges .. I must
say Dr. Viyogi always supported us and Zhangbu the then spectra PWG
convenor really gave full support... after all checks and numerous
presentation (mostly because we had done all the software in stand
alone mode .. and had to be integrated into common farmework ..) we
succeeded in convincing everyone that our analysis was correct. I
could realise then how it feels to put your neck out and make a
measurement with a detector never done before at an energy& system where data
does not exist for reference comparison. Later on subsequent measurements by others validated
our prgrams and results.
With analysis agreed in the PMD group and STAR we
started writing up the work for a paper. During that time, RHIC had
re-discovered (already seen i pp collisions) energy dependence of
longitudinal sclaing/Limiting fragmentation of
particle multiplicity in heavy-ion collisions, but there were two
contradictory results from PHOBOS and BRAHMS on the CENTRALITY
dependence of longitudinal scaling, one showing it exists and other
showing it does not, both using inclusive charged particles and both
published in PRL. Somehow
our data was the only one at RHIC which were for identified particles
the photons (from pi0) at
forward rapidity .. we jumped on this and showed that longitudinal
sclaing exists for heavy-ion collisions!. We wrote the paper ~ 4 pages as
per Physical Review Letters requirement .. and circulated it for
various stages of approval in the PMD group and STAR. None in the PMD
group belived it will be a PRL and Dr. Viyogi told us .. brief reports
are also of 4 pages .. try PRL if we do not succeed we will go for
Brief reports (which only publishes incremental research work)! STAR
spectra PWG (Zhangbu) supported us a lot ..even encouraging us to go
for search for dark photons!!! .... .but not all in STAR were
also convinced ...wrote : "it is not a slam duck" .. but we were convinced , so
was Zhangbu (with whom we later on had several collaborative work and
he also became the STAR spokesperson).
THE PAPER GOT PUBLISHED ... my dream of having a PRL
with PMD was full filled (I tried in WA98 but could not succeed)
... Here is the referee comments in first round from
Physical Review Letters which lays out the importance of the work:
"Information on the rapidity distributions of produced particles in pp and
A+A collisions is an important topic of investigation, and this letter
provides interesting new information on this topic and is therefore worthy
of publication in PRL.
It has been demonstrated for pp collisions that the multiplicity
distributions in the fragmentation region near to the beam rapidity are
independent of incident energy, an observation which is referred to as
limiting fragmentation. Limiting fragmenation has recently been shown to
be true for A+A collisions by both the PHOBOS and BRAHMS experiments at
RHIC. It has furthermore been suggested by BRAHMS that for A+A collisions,
the multiplicity per participant pair is independent of centrality in the
limiting fragmenation region, while PHOBOS has concluded that this claim
is not true. It has been suggested that the breakup of the spectator
matter destroys the centrality scaling. The present letter presents the
multiplicity distributions of photons in the forward rapidity region.
Since photons predominantly result from pi0 decays, the photon measurement
provides information on the pi0 distribution and is not sensitive to
spectator or fragmenation baryons. The letter presents important first
results which indicate that the photon (i.e. pion) yield/participant pair
is independent of incident energy and centrality. The letter is worthy of publication in Physical Review Letters, ......"
The full comments can be seen in editor email to me: Round #1 and Round
#2. The paper we are refering to can be found here: ArXiv and
PRL. We went on to also publish a long detail paper in Physical
Review C., the ArXiv version can be found here.
The Physics Reports on DCC and my first Quark
Matter Plenary Talk
During the time when I was doing PhD it was
generally felt in our community that a PhD student should not travel abroad to
conferences/workshops etc even to present his work. I recall
ditinctly that in 1999 Quark Matter Conference (highest conference in
our field) in Italy the
organizers had given me support in terms of accommodation and
registration fees being waived for presenting my work at the
conference. The senior scientists in our community from India hearing
this actually wrote to the chair of the conference to give these support to other
senior scientists of the group instead of supporting me. The organizers refused!
Given these prevailing situation, I was one day surprised to receive
the invitation letter from the chair of the Quark Matter
Confernce being held in July 2002 in Nantes, France, Prof. Hans Gutbrod, to give a Plenary Talk on my PhD thesis
work on search for Disoriented Chiral Condensates (DCC) in high
energy heavy-ion collisions. Never before a freshly (I was awarded
PhD degree in February 2002) completed PhD student
from India had received such an honor. With support from Prof. Bikash
Sinha, DAE generously send me a letter which said that "The President of India
is pleased to allow .... and DAE will finanically support the
travel....". So I was able to travel for the first time to the
highest conference in my field.
My Plenary talk was scheduled to be the last talk of
the day and after that, immediately later in the evening, was the conference
banquet. So obviously when I rose to give the talk, I saw several
people getting up to leave the hall to prepare for the banquet (go to
the hotel rooms and freshen up etc ). That included theorists who
worked directly on the experimental work I was about to present -
Prof. Krishna Rajagopal, Prof. Sean Gavin, Prof. Volker Koch etc ..I
started my talk with my eye wandering towards the door in saddness
that people who I want to hear, comment and give suggestion are leaving
the room ....by the time I went to 2nd/3rd slide of my talk, I saw all of them
stopped and were listening to the talk ! What an honour ... I
realised the key to giving a good talk is to be passionately proud
of your work (however small it may be) and giving your utmost best in
carrying out the work, as if onces existence depended on it...
There was great appreciation of the talk and the
experimental work we had carried out on the search for DCC (formed if
chiral transition occurs in QCD matter) by several
people just after the talk and afterwards ... then as I was stepping
down from the stage, I saw a young man walk up to the podium
.. congratulated me and said the limit I have put on DCC production
at SPS energies at CERN from my PhD experiment (WA98) is exactly what he
had estimated in the theory work on the same topic for his PhD thesis!
I immediately said are you Dr. Julien Serreau, the student of
Prof. Andre Kryzwicki (whose and J. D. Bjorken's papers we have read a lot). He said
yes and I said ofcourse I am aware of your work and I have mentioned
it in the talk. What followed was great scientific discussions .. which
ultimately culminated in we together getting to write a review on both
theory and experimental aspects related to DCC at the invitation of
the editors of Physics Reports !
Physics Reports on
DCC [ArXiv Version]
It was one of the great scientific experience (I was
just 30 years old) I had in writing the
review which got published in one of the most prestigious journals for
reviews in Physics - Physics Reports. This gave me a lot of confidence
for writing papers. Subsequently, after the review was
published, I was made to realise that I did ruffle few people feathers, and the consequences
continued for a long time .. another story to be told on some other
day ....but this lesson taught me a lot and hence I have always
supported my students to go to conferences during PhD and presenting
their work, several of my PhD students have presented their work in
Quark Matter Conference during PhD time. I have also encouraged them
to write and publish single author papers during PhD period, which I
will continue to do !
How to extend the Particle Identification to
higher momentum in STAR
Heavy-Ion Phenomenology at VECC