High energy heavy ion interactions and the search for the Quark-Gluon Plasma

1
High energy heavy ion interactions and the search
for the Quark-Gluon Plasma

• Alberta Marzari-Chiesa / Univ. TORINO
• Luciano Ramello / Univ. Piemonte Orientale

NURT 2003 - La Habana, Cuba, October 27-31, 2003
2
Plan of the presentation

• Introduction to Quark Gluon Plasma physics and
  heavy ion collisions
• Experimental observables to determine the
  centrality of each collision
• Present and future experimental facilities at
  CERN and at Brookhaven National Lab.
• Some results on the global features of the
  collisions
• Specific QGP signatures enhancement of strange
  particles, charmonium suppression
• Why go to higher energy (experiment ALICE at the
  LHC) ?

3
Nuclear matter and QGP

• Ordinary Nuclear Matter is made of nucleons and
  electrons. Nucleons (and other hadrons) are made
  of quarks a nucleon is made of 3 quarks, a
  meson (?, K, ?,) is made of a quark and an
  antiquark. In ordinary matter the quarks are
  never free they are confined inside the hadrons,
  and their mass is mu?md?300 MeV, ms ?500 MeV.
• The energy density of ordinary nuclear matter,
  for a nucleus of mass number A and radius R
  r0A1/3, ro1.2 fm is 0.14 GeV/fm3

• Quark Gluon Plasma (QGP) is a state of matter in
  which quarks and gluons are free, and their
  mass, being the bare mass, is smaller mu?md?5
  MeV, ms ?150 MeV. QCD lattice calculations
  predict that this state occurs when the density
  is ? ? 10 ?o when density is increased enough
  interpenetration can occur and eventually each
  quark will find very many others in its immediate
  neighbourhood. ... it has no way to remember
  which of these were the partners in the
  low-density nucleonic state (H. Satz - Nature
  324 (1986) 116)

4
Phase transition to QGP

• Such a high density can be obtained
• by compressing baryons
• by heating a mesonic medium, increasing its
  density by particle production in collisions

QCD lattice calculations predict that the phase
transition from ordinary nuclear matter to QGP
can occur for temperatures T150-200 MeV and/or
for energy densities e gt 2.5-3 GeV/fm3
5
QGP formation where (and when) ?

• In early Universe (lt1ms after Big Bang)
• possibly in the core of neutron stars
• a transient state in Heavy Ion collisions

6
Why HEAVY ION interactions ?

• The system must be large (dgtgt1 fm) and of
  long lifetime (? gt 1 fm/c formation time)
• Moreover it must be near equilibrium, and this
  can be realized only if the number of collisions
  per particle is gt 1

• The first condition is satisfied by heavy
  nuclei
• RPb ? 1.2?(208)1/3 ? 7 fm.
• Also the second one is satisfied, since the mean
  free path for a hadron is much lower than the
  dimensions of a heavy nucleus (with ? 0.14 fm-3
  and ? 40 mb 4 fm2 ? lh1/?? 1.6 fm ltlt d ).
  For quarks and gluons lq? 0.5 fm , lg? 0.2 fm
  the situation is therefore even better.
• Other conditions (temperature, energy density)
  cannot be a priori estimated they must be
  determined experimentally.

7
Heavy nuclei are extended objects

• the collision can be quite different, depending
  on the way in which the nuclei interact. The
  parameter that describes the collision is the
  impact parameter b, defined as the minimum
  distance between the centers of the two nuclei

8
Geometrical spectator/participant picture

• The nucleons inside the interaction volume are
  called participants, and the other spectators.
• Spectators proceed almost unperturbed with
  momentum close to the one of the beam
• Participants interact, and many n-n collisions
  occur in the interaction volume, producing
  secondary particles

9
Experimental observables for centrality
NA50 experiment at CERN SPS
Transverse Energy ET
Multiplicity Nch
Forward energy EF
10
Experimental observables (contd)

• Forward Energy is measured with a ZDC (Zero
  Degree Calorimeter). Spectators proceed in beam
  direction at very forward angles (at SPS ? lt0.5
  mrad) and their energy per nucleon is the same of
  the beam. A calorimeter covering angles lt 0.5
  mrad measures the total energy of spectators
  (contribution of secondaries coming from the
  interactions is negligible at these angles). The
  number of spectators (Nspec) can be obtained
  dividing EZDC by the energy per nucleon of the
  beam, Ebeam (158 GeV for Pb at SPS). The number
  of participants Npart will be Npart A
  Nspec A -EZDC/Ebeam

• Transverse energy (defined as ET?Eisin?i)
  depends only on energy deposited in the
  interaction volume. It is therefore proportional
  to the number of participants. ET is invariant
  under the boost from the C.M. system to the lab
  system. Some experiments measure the
  electromagnetic transverse energy ET0, i.e. the
  e.m. showers from the gamma rays which arise
  mainly from the neutral pion decays. ET0 is
  proportional to total transverse energy.

• Charged Multiplicity is measured by (e.g.) a
  silicon detector, and it scales with the number
  of participants as well, similarly to transverse
  energy. The number of charged particles is
  proportional to the total number of particles
  (e.g. pp0p- are produced in equal amounts).

11
Transverse energy distributions
The agreement between the model (Venus 4.12) and
the data is clearly visible. It is therefore
possible, with a rather simple calculation,
convert ET or Nch into the number of participants.
12
Multiplicity distributions (1)
NA57 experiment at SPS charged multiplicity Nch
in the pseudorapidity range 2lt?lt4 measured with
silicon microstrip detectors
Events have been classified in five centrality
classes, corresponding to given fractions of the
total inelastic Pb-Pb cross-section at 158 A GeV
13
Multiplicity distributions (2)
PHENIX, one of the four RHIC experiments,
measures charged multiplicity with two different
detectors
14
Heavy ion facilities at CERN and BNL

• Accelerators for fixed target experiments
• AGS (1986) Si, Au beams
• E(max,lab) 14.5-11.5 AGeV
• SPS (1986) O, S, In, Pb beams
• E(max,lab) 200-160 AGeV

• Colliders
• RHIC (2000) Au beams
• 100100 AGeV
• LHC (2007) Pb beams
• 2.7 2.7 ATeV

_at_SPS experiments extremely specialized in
studying particular phenomena _at_ RHIC and LHC
multipurpose experiments
15
CERN accelerators
16
The SPS heavy ion physics program
dimuons
2003
hadrons
NA60
dielectrons
NA49
multistrange

• 1986 - 1987 Oxygen _at_ 60 200 GeV/nucleon
• 1987 - 1992 Sulphur _at_ 200 GeV/nucleon
• 1994 - 2000 Lead _at_ 40, 80 158 GeV/nucleon
• 2002 - 2003 Indium and Lead _at_ 158 GeV/nucleon

2000
NA45 Ceres
NA50
NA57
photons hadrons
hadrons
strangelets
Pb
NA52
NA44
WA98
WA97
1994
dimuons
hadrons
1992
NA34/3 Helios-3
NA38
WA80
WA94
NA35
NA36
S O
NA34/2 Helios-2
WA85
1986
17
Brookhaven National Lab
18
Complex events

• The events are very complex, having a very high
  multiplicity (? 500 charged tracks a _at_ SPS,
• gt 1000 tracks _at_ RHIC).
• Nevertheless many measurements have been made and
  understood
• ? Formidable experimental challenge expecially
  for tracking

NA49 at CERN SPS uses magnets to separate out
charged particles and Time Projection Chambers to
measure their trajectories
19
RHIC event

• This is one of the first Au-Au collisions
  recorded by the Time Projection Chamber (TPC) of
  STAR, one of the four RHIC experiments

20
Global Measurements (to check whether the phase
transition is possible) (1) temperature

• If a system of particles is in thermal
  equilibrium at temperature T, the transverse mass
  distribution is

? measuring the inverse slope of the mT
distribution, we can obtain T
From the mT spectra (see next slide), it is
evident that they are consistent with an
exponential low. BUT it is also evident that the
slope parameter increases with the particle
mass. This was explained with a collective flow
(expansion of the interaction volume)
which introduces a term that depends on the
particles mass.
21
Temperature and flow from mTspectra
NA49 Blast wave fit indicates temperature of
122-127 MeV and average flow velocity of
0.48 Pions were not included in the blast wave
fit due to significant resonance contribution at
low mT
V. Friese, NA49, Strange Quark Matter 2003
22
mT spectra at RHIC

• First mT spectra from STAR (for negatively
  charged pions) show higher temperature with
  respect to SPS experiments

23
Global Measurements (2) energy density

• It is estimated from transverse energy
• using Bjorkens model
• ?o formation time 1 fm/c
• R1.12 A1/3 fm ? rapidity

It is evident that in central Pb-Pb interactions
the energy density (3.2 GeV/fm3) is well above
the value expected for the phase transition
(?crit ? 1 GeV/fm3)
24
Energy density at RHIC

• ET per Participant per Charged Particle is even
  higher at RHIC

ET/participant is 50 larger than for SPS dET/d?
is 40 higher than SPS, As a consequence, Energy
density is higher approximately, more than 40
larger at RHIC than at SPS since the parameters
of the Bjorken formula were calculated for SPS.
25
Quark Gluon Plasma signatures

• A probe for deconfined matter (QGP) must
• (obviously) distinguish between confined and
  deconfined matter
• be present in the initial stages of the
  interaction (the QGP phase)
• preserve a memory of the initial state during the
  evolution of the system
• Several signatures were proposed, and most of
  them were searched for. The results must be
  carefully studied, taking into account that
• Signals compete with backgrounds emitted from
  normal nuclear sources
• Signals are modified by final-state interactions
  after the QGP phase, as soon as the temperature
  becomes lower, a hadronisation phase occurs, in
  which the quarks become bound
• Here we will present only two signatures
• strangeness enhancement
• J/? suppression

26
How to validate QGP signatures

• The way of analyzing the results is common to all
  the signatures
• The effect is measured in light systems as p-p or
  p-nucleus, where no QGP can be present, and then
  it is extrapolated to heavier systems.
• The extrapolation is made assuming that a
  nucleus-nucleus interaction is the superposition
  of many nucleon-nucleon interactions.
• If the experimental results are different from
  this extrapolation, one concludes that something
  different happened.

27
Strangeness enhancement

• In hadron interactions, strange particle
  production occurs via associated production.
• The reaction that requires the minimum energy is

for which

• Strange anti-baryon production requires more
  energy

In a QGP the energy threshold is lower, being the
energy to produce an couple
28
Multi-strange hadrons

• The strangeness enhancement is not conclusive if
  limited to K/? ratio

• the K production enhancement can in fact be
  explained through rescattering
• ????KK ???????
• BUT
• for multistrange baryons or multistrange
  antibaryons the situation is different

can be produced via
with a very high threshold
or via a long series of interactions
which take a long time (100 fm/c, to be compared
to 5-10 fm/c of a single N-N collision)

• in a QGP with strangeness enhancement factor Es
  the hadrons containing N strange quarks are
  produced with a rate EsN times higher than in an
  environment with no strangeness enhancement. So
  in QGP E?gtE?gtE?

29
Experimental measurement of strangeness
Strangeness production was measured by
experiments WA97 and NA57 at CERN SPS, with Pb
beam at 158 GeV/nucleon
30
Multi-strange hyperon enhancement
WA97 has seen a clear enhancement for ? and
anti-? there is a factor 17 with respect to
extrapolation of p-Be and p-Pb results. NA57
later confirmed the result.
31
K/p ratio vs. center of mass energy
Data at 30 AGeV support phase transition
scenario (Statistical Model of the Early Stage)
Volker Friese (NA49), Strange Quark Matter 2003,
Atlantic City, March 2003
32
K/p ratio vs. energy
BRAHMS results at y0 seem to indicate saturation
of K/p reached at top SPS energy
33
Charmonium suppression

• Quark binding can be dissolved in quark matter.
  The mechanism is similar to the Debye screening
  observed in atomic physics
• The force between the charged partners of a
  bound state is considerably modified, if this
  bound state is placed in an environment of many
  other such objects. The Coulomb potential between
  two electric charges e, separated by a distance
  r, in vacuum is proportional to e2/r. In the
  presence of many other charges it becomes subject
  to Debye screening
• where the screening radius rD is inversely
  proportional to the overall charge density of the
  system.
• If in atomic matter the Debye radius becomes less
  than the atomic radius rA , then the binding
  force between electron and nucleus is effectively
  screened, and the electron becomes free. For
  atomic systems, an increase in density thus
  results in an insulator-conductor transition (H.
  Satz, Nature, 1986).
• Something similar can happen in a deconfined
  medium for the colour charge between a quark and
  an antiquark

34
Charmonium suppression (contd)

• For the colour charge, in a normal nuclear
  medium
• where ?r is the term responsible of the quark
  confinement and ?/r is the Coulomb-like term
• In a QGP where quarks are deconfined and many
  colour charges are present

If rC is smaller than the distance at which a
quark and an antiquark become bound to form a
particle, the bound state cannot be formed. rC
is inversely proportional to the charge density.
Since the quark density is proportional to the
temperature, we expect that rC is decreasing
with temperature.
35
Charmonium suppression (contd)
J/?, ? and ?c are different bound states of the
charm-anticharm system (charmonium) Each of them
has a different bound state radius ri
when temperature T is high enough so that rD(T) lt
ri, then the i-th charmonium state is dissolved
by the QGP.
This means that as soon as 1.1 TC (TC is the
phase transition critical temperature) is
reached, ?c (and ?) cannot be formed, while 1.3
TC is needed to dissolve also J/?.
36
Experimental study of charmonium

• EXPERIMENTS NA38 NA50 at CERN studied the
  charmonium suppression measuring the J/?
  production as a function of the number of
  participants.NA50 is the upgrade of NA38,
  having three centrality detectors instead of one,
  and a higher rate capability.
• J/?s were detected through their decay in ??-
  the experimental apparata consisted therefore
  essentially of a dimuon spectrometer centrality
  detector(s). Characteristic of these experiments
  is the high beam intensity (? 107 Pb/s), due to
  the low J/? production cross section and to the
  low branching ratio in two muons

37
The NA50 experiment
J/?
Drell-Yan
The absorber stops all the hadrons, and only
muons can reach the last chambers. Measuring the
emission angle and the curvature of both muons,
it is possible to reconstruct the ??- invariant
mass
38
The Drell-Yan reference process
Drell-Yan is a rare, hard collision process and
its cross-section scales with the number of
nucleon-nucleon collisions.
A nucleons
B nucleons
This is in effect what is observed had an
absorption been present, it would change the
scaling to (AB)? , with ? lt 1.
Drell Yan reactions are therefore taken as a
reference and many of the J/? results were
presented as a ratio ?J/?/?DY.
39
Nuclear absorption
J/? absorption with respect to Drell-Yan was
already observed by the NA38 experiment. Unfortuna
tely, it was not possible to conclude that the
QGP had been observed since the suppression,
observed in Oxygen and Sulphur interactions, is
already present in p-nucleus interactions. The
plot B??? vs A?B, that for Drell Yan events is
flat, here is consistent with a continous
decreasing pattern from p-p to S-U
interactions B???(J/?) ? (A?B)0.92?0.015 This
behaviour can be accounted for by nuclear
absorption.
40
Anomalous charmonium suppression
The observations in p-A, A-B collisions can be
fitted by the empirical law
where r0 nuclear density, L length of nuclear
matter crossed by the charm quark-antiquark pair
after its formation
L can be calculated using a simple geometrical
model (hard spheres) or with more refined models
of the nuclei.
Going to heavier systems the situation changes
for Pb-Pb the normal nuclear absorption does
not justify the results and an anomalous
additional suppression is clearly present.
41
Anomalous suppression (contd)

• In this figure the ratio J/?/D.Y. is divided by
  the same ratio expected under the hypothesis of
  normal nuclear absorption.
• The number of participants is
• obtained from the measured transverse energy.

42
Anomalous suppression (contd)

• The same analysis is possible with EZDC and Nch
  as centrality variables. Here the ZDC analysis is
  reported. It is clear that the suppression
  pattern is compatible with a double step in EZDC.
  The first could be due to the ? absorption, the
  second to the J/? one. All the models, based on
  normal nuclear effects, are ruled out.

43
Why go to higher energies ?
Significant quantitative improvements in the
experimental conditions are expected when going
from SPS energy to RHIC (already running since
June 2000) and later to LHC (startup foreseen in
2007)
Energy density, volume and lifetime of the plasma
are very much improved by going to RHIC, and even
more by going to LHC
44
More extended baryon free region
A net-baryon free region (no excess of protons
over antiprotons) allows easier comparison with
theory
Net protons distributions indicate high degree of
stopping at AGS energies, less stopping at top
SPS energy and almost full transparency at RHIC
45
Onium suppression revisited
The main advantage of LHC for onium physics
will be the access to Y (beauty-antibeauty)
states this will allow unambiguous confirmation
of the results already obtained from charmonium
studies at lower energies. RHIC is presently
accumulating data on charmonium, which should
allow access to a higher transverse momentum
region than the one previously explored.
46
The ALICE experiment
47
The ALICE Internal Tracking System

• 6 cylindrical layers of silicon detectors
• pixel detectors
• drift detectors
• double sided microstrip detectors

Layer 3 Layer 4
Radius (mm) 14.9 23.8
Ladders 14 22
SDDs per ladder 6 8
48
segmented
2 x 256 anodes
MOS charge injectors for drift velocity
monitoring

• Wafer 5, NTD, 3 k?.cm, 300 ?m
• Active area 7.02 ? 7.53 cm2

guard region implanted HV voltage dividers 256
anodes (294 mm pitch)
49
The Internal Tracking System mechanical support
50
Challenges rates and data volumes
ALICE will push the previous limits of high
energy physics experiments in the direction of
very large data volumes, while other LHC
experiments will be demanding very high trigger
rates and Data Acquisition bandwidth
To insure that ALICE data will be analyzed in a
timely manner, the offline software is being
prepared well in advance of the beginning of data
taking in 2007, and is being tested with M.C.
events. The GRID software technology is being
developed in order to be able to process data
stored in several regional centers in an
efficient way, moving around programs rather than
data.
51
High Energy Heavy Ion bibliography

• C.Y. Wong - Introduction to high energy heavy-ion
  collisions, ed. World Scientific (1994)
• J. Schukraft H. Schmidt - The Physics of
  ultrarelativistic heavy-ion collisions, Journal
  of Physics G 19 (1993)
• Proceedings of the QM (Quark Matter) Conferences
• 1991 - Gatlinburg (USA) - Nucl. Phys. A 544
• 1993 - Borlange (Sweden) - Nucl. Phys. A
  566
• 1995 - Monterey (USA) - Nucl. Phys. A 590
• 1996 - Heidelberg (Germany) - Nucl. Phys. A
  610
• 1997 - Tsukuba (Japan) - Nucl. Phys. A 638
• 1999 - Torino (Italy) - Nucl. Phys. A 661
• 2001 Stony Brook (USA) Nucl. Phys. A 698
• 2002 - Nantes (France) Nucl. Phys. A715