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Perspectives for heavy ion physics
Jean-Yves Ollitrault
Theoretical physics, Saclay
Open Symposium 2006
CERN Council Strategy Group
January 30 - February 1, 2006
LAL - Orsay, France
Outline
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What are heavy ions about?
A brief history of heavy-ion collisions
What happens in a heavy ion collision?
What are the essential observables?
What are the theoretical approaches?
What have we learnt from previous experiments?
What do we expect from heavy ions at LHC?
What do we expect from fixed-target experiments?
Concluding remarks
What are heavy ions about?
What are the goals of elementary particle physics?
1.
Find new terms in the Lagrangian of the universe
2.
Determine accurately the free parameters of the known, standard
model, lagrangian.
If you think this is the end of the story, then heavy ion physics does
not belong to particle physics.
Yet heavy ion physics belongs to fundamental physics, in the sense
that is allows us to study experimentally new phenomena which are
interesting from the point of view of theory.
Interaction between theory and experiment may be stronger in
heavy ion physics than in any other branch of particle physics:
A lot of theoretical progress triggered by experiment and vice-versa
What are heavy ions about?
Heavy ion collisions create matter
of extremely high density.
The initial goal was to probe
experimentally the QCD phase
diagram.
The scope is now wider: study
various properties of the highdensity phase
A brief history (from J. Schukraft)

AGS @ Brookhaven (1986 - 1998)
 Beam:
Elab < 15 GeV/N, s ~ 4 GeV/N
 Users:
400
Experiments: 4 big, several small

SPS @ CERN (1986 - 2003)
 Light Ions(O, S) : 1986 – 1992 Heavy Ions (In, Pb): 1994 - 2003
 Beam:
Elab =40, 80, 160, 200 GeV/N, s < 20 GeV/N
 Users:
600
Experiments: 6-7 big, several small, 3 ‘generations’

RHIC I @Brookhaven (2001 – 2012 ?)
 Beam:
s < 200 GeV/N
 Users:
1000
 Experiments: 2 big, 2 small
What happens in a heavy-ion collision?
Two Lorentz-contracted nuclei collide
Hard QCD processes: high-pt jets, heavy quarks
Direct photons
High-density, strongly-interacting hadronic matter (quarkgluon plasma?) is created and expands, and eventually
reaches the detectors as hadrons.
2 types of observables
Decay products of the fireball:
1st-year measurements (high luminosity not required)
• Yields of identified hadrons
• Momentum spectra (pt, y and azimuthal angle φ)
• Quantum correlations (Bose-Einstein interferometry)
Probes of the early stages of the collision:
These usually require a few more years
• Electromagnetic probes (dileptons,
thermal photons)
• Heavy quarks and quarkonia
• High-pt particles (jet quenching)

z
y
x
What are the theoretical approaches?
1. Lattice QCD
Lattice calculations go beyond the
mere equation of state of QCD
matter.
Example: how does a heavyquark bound state, such as the
J/ψ, behave in a hightemperature medium?
(from Asakawa, Hatsuda,
hep-lat/0308034)
This was stimulated by
experiments (J/ψ suppression)
What are the theoretical approaches?
2. Analytical calculations at high T
Important progress made in perturbative calculations at high
temperature, using improved resummations schemes. 3 examples:
1.
2.
3.
Energy loss of a hard parton through a quark-gluon plasma (jet
quenching) (Baier Dokshitzer Mueller Peigné Schiff hep-ph/9608322)
Photon production by a quark-gluon plasma was computed to leading
order only fairly recently (Arnold, Moore, Yaffe, hep-ph/0111107)
Perturbative calculations of the eq. of state are in agreement with lattice
calculations down to a few Tc (Blaizot Iancu Rebhan, hep-ph/0005003)
Exact calculations are also a source of inspiration: viscosity of N=4
supersymmetric QCD was computed using the AdS/CFT
correspondence (Policastro Son Starinets hep-th/0104066)
Important progress recently made in understanding quantum field
theory ouf of equilibrium (e.g. Aarts et al hep-ph/0201308)
What are the theoretical approaches?
3. high-energy QCD
A lot of progress has been made in the last 2 years in understanding
the high-energy limit of QCD: analogy with reaction-diffusion dynamics
Munier, Peschanski, hep-ph/0309177
higher energy
Dilute gas
CGC: high density gluons
Ab-initio calculations for heavy-ion collisions are possible in the
framework of the color glass condensate.
But the produced particles may interact: final-state interactions
What have we learnt from previous experiments?
1. Particle yields from the fireball
Hadron abundances are well
described by « thermal » fits,
i.e., by Boltzmann factors
(2 parameters only)
The temperature seen at SPS
and RHIC = the deconfinement
temperature from lattice QCD!
What have we learnt from previous experiments?
2. Elliptic flow
dN
1

(1  2v1 cos   2v2 cos 2  ...)
d 2

z
y
Interactions among the produced particles:
pressure gradients generate positive
elliptic flow v2
x
y
x
py
x
px
Elliptic flow is NOT a
small effect: indication of
collective (fluid-like) motion
Clear mass-ordering:
lower v2 for heavier
particles at given pt
The only explanation of
the mass ordering is that
the fluid velocity is
relativistic v~0.7 c
Hydro by Huovinen et al.
hydro tuned to fit central
spectra data.
What have we learnt from previous experiments?
2. Elliptic flow
PRC 72 (05) 014904
200 GeV Au+Au
min-bias
What have we learnt from previous experiments?
3. Jet quenching
One of the most striking results
from RHIC:
Suppression of high-pt particles
in central nucleus-nucleus
collisions compared to the
expectation from proton-proton
collisions
This is probably due to « jet
quenching », i.e., the energy lost
by fast particles traveling through
the dense medium
The heavy-ion program at LHC
From RHIC to LHC: colliding energy x30, particle density (expected) x2
A dedicated Heavy-Ion experiment:
ALICE
And also heavy ion studies in ATLAS and CMS
Why are we interested in LHC energies ?
1. Lifetime of the quark-gluon plasma
At RHIC, the time spent in the high-density (quark-gluon plasma) phase is
relatively small (typically 5 fm/c): what we see is a dirty mixture…
The expected particle density at LHC is a factor of 2 higher, which leads to
qualitative changes: relativistic hydrodynamics alone should do a good job
in describing all soft hadronic (fireball) observables.
Furthermore, ALICE does better than any heavy-ion experiment in the soft
region (particle identification at low pt)
Why are we interested in LHC energies ?
2. High-pt physics
• At RHIC, « jet quenching » is seen only through single-particle
(leading hadron) spectra, and two- (recently three-) particle
correlations. But a leading hadron has little to do with a jet.
• Here also, the situation is qualitatively different at LHC: one hopes to
reconstruct jets
• High-pt are, of course, statistics-limited signals: γ-jet correlations
(‘golden channel’ to study jet quenching): order 1000 events/year
with pt > 30 GeV
Fixed-target experiments are still alive!
SPS will search for the onset of deconfinement
Temperatures in Pb-Pb
collisions at SPS are just
below the critical point of
QCD, predicted by lattice.
Collisions of smaller nuclei yield
somewhat higher temperatures,
coming closer to the critical point.
Large fluctuations are expected
near a critical point. This study,
with an improved detector, is one
of the goals of the future-NA49
experiment.
The future NA49 experiment will
also investigate further the
anomaly (sharp peak) in
strangeness production.
The NA60 experiment at SPS
• An upgrade of the famous SPS dimuon experiment using a
technological breakthrough: radiation-hard silicon pixel detectors:
what we used to see
what NA60 sees
Opposite-sign muon
pairs
w 
Background(s)
h
Signal dimuons
This can be used to study issues related to chiral symmetry restoration at high T,
such as modification of ρ mass and width
Other fixed-target experiments
The Facility for Antiproton and Ion Research (FAIR) at. the GSI
laboratory in Darmstadt has 5 scientific pillars, one of which is the
study of nuclear matter with heavy-ion beams at laboratory energies
8-40 GeV per nucleon (lower end of SPS energy range).
The goal is to study the onset of deconfinement, as in the SPS
program, with dedicated experiments.
Concluding remarks
• We have not always been very good at foreseing the future…
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The NA38/NA50 experiment at CERN was initially planned to study «
thermal dileptons » but its most exciting results came from measurements
of J/ψ production (anomalous suppression)
When RHIC started, the most exciting results were expected from jet
quenching and « event-by-event » fluctuations, but elliptic flow was thought
to be marginal.
• Claims are somewhat ahead of discoveries: SPS claimed the
discovery of a « new form of matter », but this was better seen at
RHIC; similarly, RHIC recently claimed to have produced a « perfect
liquid »… but it seems to me that this is really what we are going to
see at LHC, as LHC is the first machine that will produce a longlived quark-gluon plasma (and this was known since many years)