Table Of ContentResearch Reports in Physics
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Nuclear Structure of the Zirconium Region
Editors: J. Eberth, R. A. Meyer, and K. Sistemich
Ecodynamics Contributions to Theoretical Ecology
Editors: W. Wolff, C.-J. Soeder, and F. R. Drepper
Nonlinear Waves 1 Dynamics and Evolution
Editors: A. V. Gaponov-Grekhov, M. I. Rabinovich, and J. Engelbrecht
Nonlinear Waves 2 Dynamics and Evolution
Editors: A. V. Gaponov-Grekhov, M. I. Rabinovich, and J. Engelbrecht
Nonlinear Waves 3 Physics and Astrophysics
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Nuclear Astrophysics
Editors: M. Lozano, M. I. Gallardo, and J. M. Arias
Optimized LCAO Method and the Electronic Structure of Extended Systems
By H. Eschrig
Nonlinear Waves in Active Media
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Problems of Modern Quantum Field Theory
Editors: A. A. Belavin, A. U. Klimyk, and A. B. Zamolodchikov
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By M. A. Savchenko and A. V. Stefanovich
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Editors: Gu Chaohao, Li Yishen, and Tu Guizhang
Nonlinear Waves in Waveguides with Stratification
By S. B. Leble
Quark-Gluon Plasma
Editors: B. Sinha, S. Pal, and S. Raha
Symmetries and Singularity Structures Integrability and Chaos
in Nonlinear Dynamical Systems
Editors: M. Lakshmanan and M. Daniel
Modeling Air-Lake Interaction Physical Background
Editor: S. S. Zilitinkevich
Bikash Sinha
Santanu Pal
Sibaji Raha (Eds.)
Quark-Gluon Plasma
Invited Lectures of Winter School,
Puri, Orissa, India, December 5-16, 1989
With 164 Figures
Springer-Verlag
Berlin Heidelberg New York London
Paris Tokyo Hong Kong Barcelona
Dr. Bikash Sinha
Dr. Santanu Pal
Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, 700 064 Calcutta, India
Professor Sibaji Raha
Fachbereich Physik der Philipps Universitat Marburg, Mainzer Gasse 33,
0-3550 Marburg, Fed. Rep. of Germany
ISBN-13: 978-3-540-51984-3 e-ISBN-13: 978-3-642-75289-6
001: 10.1007/978-3-642-75289-6
This work is subject to copyright. All rights are reserved, whether the whole or part of the material
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© Springer-Verlag Berlin Heidelberg 1990
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2157 13140-543210 - Printed on acid-free paper
Preface
This volume contains the notes of the main lectures presented at the Winter School
on Quark-GIuon Plasma (QGP) held at the Toshali Sands resort near Puri, India,
5-16 December, 1989. The format of the volume can be considered as a compre
hensive synthesis of introductory lectures dealing with the essential fundamentals
of the field and the frontier level ideas, directly relevant for an in-depth understand
ing of the state of the art. The degree of enthusiasm and involvement displayed
by the participants of the School convinced us that a permanent record in the form
of this book would be appropriate.
This text begins with a discussion of experiments and experimental tech
niques: Roberto Salmeron relates the history of the search for signs of QGP in
ultrarelativistic heavy ion collisions, after an exposition of relativistic kinematics.
Michael J. Tannenbaum's contribution is primarily concerned with QGP activities
at Brookhaven National Laboratory.
We continue with several chapters on the theoretical models and formalism.
Jitendra Parikh covers Quantum Chromodynarnics (QCD) and QCD plasma, in
particular, perturbative QCD in many-body systems at finite temperatures, to study
QGP thermodynamics. Frank Close discusses the properties of a single ground
state nucleon in a nuclear medium, and in a volume of high energy density.
This problem is of direct relevance to the phase transition from hadronic to QGP
behaviour. A more detailed knowledge of the glue and its distribution inside
hadrons is a pressing goal.
Narayan Rana and Charles Alcock present the very exciting developments
in our understanding of the Universe a microsecond after its creation - an area
of physics where QCD/QGP related ideas can be applied with some confidence.
Narayan Rana focuses on the geometry and dynamics of the early universe going
over to Big Bang nucleosynthesis; Charles Alcock covers the very recent work of
possible changes in the relative abundances of various nuclei essentially due to
the inhomogeneity introduced by QCD phase transition. The need to invoke dark
matter, its possible origin and its relevance to QCD phase transition is discussed.
It is implicit that previous knowledge of QGP is not essential to understand the
contents of this book. It is hoped that the material gathered here, like the .lectures
on which it is based, will inspire the reader to the same degree of enthusiasm as
experienced by the participants of the Puri School.
Calcutta, Marburg Bikash Sinha
June, 1990 Santanu Pal
Sibaji Raha
v
From the Editors
A little over two years ago, the International Conference on Physics and Astro
physics of Quark-GIuon Plasma (lCPA-QGP'88) organised by us at Tata Institute
of Fundamental Research, Bombay, seemed to have enjoyed a satisfactory measure
of success. To sustain the excitement and motivation generated in that conference
and especially to introduce the young graduate students and fresh post-docs to this
new and rapidly evolving discipline, we decided to organise a more intense school
on the same subject, covering extensively, if not exhaustively, the experimental
and theoretical scenario related to the physics of QGP, starting from the very early
universe to the laboratory experiments of the present generation as well as the next
generation. To many of us, the study of QGP is all the more stimulating, because
it forges, as it were, a bridge between the microcosmos and the macrocosmos.
The Winter School on QGP, December 5-16,1989, was organised at the glo
riously enchanting and picturesque surroundings of Toshali Sands, a spot near
Puri, a famous pilgrimage and sea resort in Orissa, situated in the eastern part of
India. The setting and the surroundings helped a great deal in cementing a sense
of intense involvement and eager anticipation both among the students and the
lecturers.
The format of the school was neither entirely pedagogic nor synoptic but rather
a synthesis of the two. Each lecturer started with an introduction to his subject
accessible to a graduate student generally well prepared but without any prior
expertise in the area of QGP. From there, he brought the students along to the
very forefront of the discipline with remarkable clarity and lucidity. It was indeed
a pleasure to witness at the end of the school the young participants hitherto
unfamiliar with the field converse knowledgeably about the current issues and
bubbling with enthusiasm to start contributing to this growing subject. It is our
firm belief in editing these proceedings that a large share of that enthusiasm will
filter through in the printed pages too, and that this volume will prove to be of
immense help to the beginners and the practising experts in QGP.
On the experimental side, Roberto Salmeron covered single-handedly the entire
area of searches for the signals of QGP in ultrarelativistic heavy ion collisions. His
remarkable set of lectures started very appropriately with a lucid introduction to the
relativistic kinematics of high energy collisions, concepts which are often glossed
over in the usual curricula, leaving room for immense confusion in the minds
of beginners. Salmeron thus rendered yeoman service not only to the students
of the school but to the entire community through his clear enunciation of the
concepts every theorist and experimentalist must know by heart. From there on,
he covered in an inspiring set of lectures the entire state of the art of experimental
VII
QGP searches - from direct photons to hadrons, from two particle interference
(Bose-Einstein correlations) to intermittency, the resonance states of J/psi and the
possible sources of its suppression, the changes in the slope of the average energy
density deposition as a function of the average transverse momentum indicating a
phase transition. Behind all these signals, however, there still remains the enigma
of FRITUOF, somehow managing to get things almost right, hinting that there are
as yet no clear signals of QGP.
Robert Brockman, along with Salmeron, helped to "demystify" the mythology
of the large scale detectors which are part and parcel of ultrarelativistic heavy
ion physics research. He concentrated mainly on the CERN detectors, from TCP
to ZDC, streamer chambers to nuclear emulsions, dimuon spectrometers, BGO
crystals, massive on line data acquisition systems and so on. It was extremely
useful for the Indian participants, especially as India is on the verge of becoming
a full partner in one of the CERN experiments (WA80), where involvement with
the hardware is a must. Is it then possible for us to build a calorimeter or indeed
a TPC - how about drift chambers? It is indeed unusual to delve deep into
the hardware of detectors in a school not solely dedicated to instrumentation,
but Brockman's and Salmeron's attempts were instrumental in stimulating many
young minds at the school to get their hands dirty while keeping the physics goals
firmly in sight. For reasons beyond our control, however, we had to leave the
write-up of Brockman's lectures out of this volume.
The other scheduled lecturer in the experimental section, Mike Tannenbaum,
could not attend at the very last minute. He had planned to cover the features of the
Brookhaven activities in this area, emphasising in particular the role of transverse
energy measurements as a tool for QGP diagnostics. In his absence, Salmeron
gave the students an introduction to this area too. But we are also including
the write-up of Tannenbaum's contribution in this volume for the benefit of the
readers.
On the theoretical side, Jitendra Parikh covered the essential area of QCD and
QCD plasma. His discussion on how to avoid the pitfalls of perturbative QCD
in many-body systems at finite temperature in order to study thermodynamics
and other collective properties of QGP was incisively instructive. Quite clearly,
the young (and often the initiated too) can embark on a rather elaborate and
involved calculation using perturbative QCD, without fully appreciating the fact
that unless one takes into account the collective modes semi-phenomenologically
(at the present state of our understanding of strong interaction physics), one might
end up throwing the baby away with the bath water. The essential foundation of
thermodynamics (or the lack of its validity!) for studying the evolution of the
reaction volume after the collision of two nuclei at ultrarelativistic energies was
also discussed in detail.
Frank Close contributed greatly in bridging the two seemingly (but then again,
why indeed?) uncorrelated areas, the properties of a single nucleon embedded in
a nuclear medium in its ground state on the one hand and on the other hand, what
happens to them when they are in a volume where sufficient energy density has
been pumped in. The very important recent developments in our understanding
of a baryon in a nucleus and exactly how important the glue distribution is for
predicting the structure of a baryon was emphasised. Needless to say, these con-
VIII
siderations can have profound consequences for our understanding of the QGP
signals as well as the phase transition from the hadronic to QGP sector. It is felt
that whether it is the EMC effect involving the hadronic structure in a nucleus
in terms of the QCD degrees of freedom (quarks and gluons) or the study in
volving the QCD phase transition, a more detailed knowledge of the glue and its
distribution inside hadrons or in the fireball is a central issue.
Thus, a more comprehensive study of the atomic nucleus revealing the quark
structure of hadrons (through lepton-nucleus interactions) and its logical extension
to high density/high temperature hadronic matter, eventually dissolving into a
QGP, needs to be performed in a careful and self-consistent manner. The physics
of CEBAF (Continuous Electron Beam Acceleration Facility) and the physics
of RIDC (Relativistic Heavy Ion Collisions) should be mutually complementary,
fitting into the wonderful goal of searching for a unified picture of the ultimate
constituents of the hadronic world.
So what exactly is the message at this time for our search for QGP in the
laboratory and the related theoretical questions? It is probably correct to say that
no definitive signals are yet at hand, the central problem being the pollutants
of the hot hadronic matter, evolving with time beyond the critical temperature
of the phase transition and eventually to freeze-out. All the signals proposed
so far fall short of the acid test, implying that most of the data can perhaps be
explained without necessarily invoking the existence of a QGP, notwithstanding
the unnatural assumptions often needed to make the mundane explanations work.
But it is heartening to note that to date not a single set of data has been found
which would rule out the formation of QGP. Perhaps the theorists have not yet
succeeded in asking sufficiently discerning questions on QGP diagnostics, but
there is always a hint for the future. Is there enough energy deposited? Is the
statistics good enough? This last point is particularly relevant for the direct photon
signals of QGP - a 30% effect will correspond to one direct photon for every
thousand charged particles, not an easy task and one which certainly demands
much longer running time. Strange and anti-strange mesons may help to an extent
but so little is known about the process of hadronisation, final state interactions
and/or the scattering of hadrons from the neighbouring hadrons in a hot dense
medium! Indeed, while we are looking for a novel state of matter, it is imperative
to realise that very little really is known about the properties of hadronic matter
(the background, to wit) at high density and/or temperature. It is high time that
the initiated take a harder look at this issue.
If signals relating to anyone particular species of detected particles may not be
truly clinching; what about their ratios? Yes, it may be difficult to do, involving
the measurement of several different kinds of particles in the same experiment, but
the dividends certainly justify the effort. The ratio of photons to dileptons is an apt
example. Naively, direct photons from QGP arise from the annihilation of quarks
and anti-quarks as well as from the so-called generalised Compton effect (q+g
going to q+photon), a channel absent for diphotons in the lowest order. So the
ratio will clearly provide an all-important "clue" to the elusive glue distribution.
It is felt that looking for several signals in the same experiment and studying their
ratios will clarify many cloudy issues.
IX
Similarly for two particle interferometry! Pion interferometry has been tradi
tionally done for the past 30 years, starting with the epoch-making discovery of
the GGLP effect. As can be gleaned from the lectures of Salmeron, such mea
surements have now become a mainstay of RIDC processes, not to mention also
electron-positron and hadronic collisions at high energy. The enormous impor
tance of these measurements notwithstanding, one has to realise that extracting
physically meaningful information from them is still not an unambiguous endeav
our, what with the final state interactions, parametrisation dependence of the source
distributions and so on. Moreover, pions being predominantly final state products,
pion interferometry measures the size of the reaction volume at or near freeze-out,
at any rate in its hadronic phase. It will thus be most important to devise interfer
ometric experiments which could yield information about the volume of the initial
stage of the evolution.
On the theoretical side, the study of non-equilibrium dynamics and its appli
cation to RHIC processes, especially at the early times of formation, must be
addressed. The efforts of the past few years in unravelling the subtleties of a ki
netic theory with QCD ingredients appear not to have borne the fruit they initially
seemed to promise, the difficulties of formulating a gauge-invariant prescription
being the root cause. We cannot help thinking that there is scope for a break
through there and perhaps some ingenious theorist will take a fresh look at the
problem in the near future. What concerns the signals, a simultaneous analysis
of the central rapidity regime and the fragmentation regime seems important to
understand the role of the baryonic chemical potential, especially in suppressing
the anti-quarks. The evaluation of the production rates of J/psi in hadron-nucleus
and nucleus-nucleus collisions within the framework of a bound state equation
(e.g. Bethe-Salpeter) is an urgent task. We believe that the experimental results
obtained from the runs with sulphur will decide the course theoretical activities
should take, with an eye to the forthcoming lead runs.
The message is quite clear. It is simply not possible to detect QGP by a short
run without elaborate controlled experiments, be that for higher statistics, be that
for scanning for the signals event by event, be that for the study of correlation
experiments. The results obtained at CERN and AGS so far have proved beyond
doubt that there is interesting physics to be learnt from the search for QGP and it
is absolutely imperative to design more detailed experiments to iron out some of
the prejudices and unclarities in our search for the truth.
Going over from the microcosmos to the macrocosmos, the most relevant issue
is the possible consequences of a QCD phase transition on the evolution of the
very early universe, approximately several microseconds after the Big Bang.
Narayan Rana introduced the students to the language, the techniques and the
subtleties of the geometry and the dynamics of the early universe. In an excellent
series of talks, both in terms of thoroughness and pedagogy, he elaborated from
a classical point of view the issue of the observed abundances of the elements
and its deep connection with the inhomogeneity produced in the early universe by
the (presumably) first order QCD phase transition. The lectures covered the basic
concepts of the Standard Big Bang cosmology; the question of the dark matter
whether it is necessary - was discussed. The students were given a taste of the
intricacies involved in the study of primordial nucleosynthesis. Rana paved the
x
way for the students to ask: What are the ingredients that go into the cosmic soup?
How is the latent heat associated with the QeD phase transition taken away to
the ambient hadronic universe? How many layers conducting heat away from one
another exist in the early universe? What is the mechanism of this conduction?
How sensitive are the results to the parameters of the phase transition, the critical
temperature, for example?
Charles Alcock dealt with these questions at length, in a brilliant set of lectures.
The prospect of the first order phase transition leading to a cosmic separation of
the QGP and the hadronic phases, which eventually allow diffusion of baryons
among them, as Alcock expounded, can have very important effects on primordial
nucleosynthesis and on the overall picture of the evolution of the early universe.
The stretching of the time scale associated with nucleosynthesis as a consequence
of the QeD transition is a scenario decidedly absent in the Standard Big Bang
cosmology.
The central issue of the dark matter came into focus again. As was argued
by Witten some time ago, a QeD phase transition may perhaps produce stable
configurations of strange quark matter (called strange nuggets in the literature)
which would be opaque and thus be plausible baryonic candidates for dark matter.
While in recent times the existence of nuggets has been called into question, it
is undeniable that an understanding of the dark matter within the framework of
known physics, without having to invoke exotics like axions or massive neutrinos,
would indeed be a great step forward. Alcock told the students about the results
of detailed calculations done by himself and collaborators on nucleosynthesis in
the presence of a first order QCD transition, where the observed abundances of
all light elements with the singular exception of 7Li come out to be within ac
ceptable limits even when one insists on a baryonically closed universe. The
intriguing question about the reliability and the accuracy of the astrophysical ob
servation of abundances of elements was also discussed, with the conclusion that
the uncertainties are rather large.
Having argued that the QeD phase transition allows the universe to be closed
by baryonic matter, Alcock then addressed the second part of the issue. If the
dark matter is not to be the strange nuggets, could they be Jupiter-like planets?
They would indeed be non-luminous; how can one search for them? Alcock
gave the students an introduction to his group's future project, setting up optical
telescopes in the southern hemisphere to look for the brightening of distant stars
in the Magellanic cloud by the gravitational lensing action of these Jupiter-like
objects. All of us look forward to the results of this experiment.
In addition to the lectures, there were also some specialised seminars delivered
by more initiated participants. We decided not to include those write-ups in this
volume, as the format of those talks would not match the style followed in the
regular lectures where no previous knowledge about the subject was assUmed.
The two weeks at Toshali Sands were a feast, a gourmet meal where each
course was different from the others, yet complementing one another in a way
we had not imagined possible. We are grateful to Springer-Verlag for letting the
whole community savour that taste to some extent. The School was sponsored by
the Department of Science and Technology and the Department of Atomic Energy,
Government of India.
XI
