Status of Quark-Gluon Plasma and saturation effects at RHIC

1
Status of Quark-Gluon Plasma and saturation
effects at RHIC

• Fouad RAMI
• Institut de Recherches Subatomiques, Strasbourg

• Introduction
• Status of QGP at RHIC
• ?Particle multiplicities
• ?Elliptic flow
• ?High pt suppression jet quenching
• High density gluon saturation (CGC)
• ?d-Au data
• ?Forward rapidities
• Summary perspectives

F.Rami, IReS Strasbourg



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Main Goals in RHIC experiments
Phase Diagram of Nuclear Matter
F.Karsch, hep-lat/0106019
Lattice QCD
Energy Density/T4
?
TC ? 160MeV (?B 0)
Temperature
? Large experimental program
F.Rami, IReS Strasbourg



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Relativistic Heavy Ion Collider _at_ BNL
BRAHMS
PHOBOS

• First Physics Run June 2000

PHENIX

• 2000-2005 5 runs

STAR

• Several systems/energies
• AuAu _at_ 200 GeV
• _at_ 130 GeV
• _at_ 62.4 GeV
• CuCu _at_ 200 GeV
• _at_ 63 GeV
• dAu _at_ 200 GeV
• 62.4 GeV
• pp _at_ 200 GeV
• (reference data)

RHIC accelerates all species from p to Au
F.Rami, IReS Strasbourg



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Particle multiplicities at RHIC
Central AuAu event measured by STAR/TPC, _at_ 130
GeV
? Very large number of charged particles per event
dNch/d??0 650 (at 200GeV) ? much higher than
at SPS
Number of charged particles per unit of rapidity
at ?0
F.Rami, IReS Strasbourg



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dNch/d? at Mid-Rapidity Energy
Dependence

• Large increase from SPS to RHIC
• (almost a factor of 2)
• ? Higher energy densities

• An estimate of e 5 GeV/fm3
• at 200GeV (Bjorken model)
• PHENIX, PRL87(2001)052301
• ? Well above the critical
• density ( 1 GeV/fm3)

• AuAu data much larger than pp
• ? Not a simple superposition
• Medium effects ? important role
• in AA collisions

F.Rami, IReS Strasbourg



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Elliptic Flow

• Elliptic Flow
• Pressure converts spatial
• anisotropy into p-space
• anisotropy

Reaction plane
STAR, PRL90(2003)032301
?
MR
Collective Flow Collective
expansion of Nuclear Matter following the
compression phase

• Fourier decomposition of the
• azimuthal distributions

dN/d? F0 (1 ? 2vicos(i?))
? Response to early pressure
v2 2nd harmonic Fourier coefficient ?
Measure of Elliptic Flow
F.Rami, IReS Strasbourg



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Elliptic Flow at RHIC

• Much larger elliptic flow at RHIC
• ? high degree of thermalization
• (multiple interactions of
• produced particles)

• Supported by the good agreement
• with hydrodynamical model

F.Rami, IReS Strasbourg



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High pt Suppression Jet Quenching

• Particles with high pts (above 2GeV/c)
• are primarly produced in hard scattering
• processes early in the collision
• ? Probe of the dense and hot stage

• pp experiments ? Hard scattered
• partons fragment into jets of hadrons

Experimentally ? Suppression in the high pt
region of hadron spectra (relative to pp)
F.Rami, IReS Strasbourg



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High pt Suppression at RHIC

• At RHIC
• ? Significant suppression

New phenomenon at RHIC

• Not observed at lower energies
• SPS(PbPb) ? Enhancement
• ? due to initial state multiple
• scattering (Cronin effect)
• well known in pA collisions

• Suppression ? consistent with
• partonic energy loss (Quenching)

But, it might be also due to saturation of
gluon densities (initial state effect) ? Jets
do not lose energy but they are produced in
a smaller number
Compare AA and dA (Run3, Control experiment)

• Gluon sat. ? Suppression in dAu
• Quenching ? No suppression in dAu

F.Rami, IReS Strasbourg



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Initial or Final State effect ?
? Same conclusion
Final State effects are dominant in
central AuAu at RHIC as expected
from the formation of a hot and dense
medium of partonic matter
F.Rami, IReS Strasbourg



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Summary of the main experimental observations
for central AuAu collisions
? All of these results are consistent with the
existence of a dense partonic state of
matter characterized by strong collective
interactions
Main conclusion of the 4 RHIC White Papers (to
be published in Nucl.Phys.A)
F.Rami, IReS Strasbourg



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High Density Gluon Saturation at RHIC

• Several global features of AuAu and dAu
• collisions at RHIC can be reproduced by the
• Color Glass Condensate model (high density
• gluon saturation in the initial state)

McLerran, hep-ph/0402137
As x becomes smaller and smaller, the gluon
density increases faster ? driving force toward
saturation
BRAHMS
F.Rami, IReS Strasbourg



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Forward measurements in dAu collisions
Sensitivity to smaller-x values

• BRAHMS spectrometers measure in the
  d-fragmentation region

From y0 to y4 ? x values lower by 10-2 ?
One could hope to see the occurrence of a
suppression effect
D.Kharzeev et al, hep-ph/0307037

F.Rami, IReS Strasbourg



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What do we expect?
RpA Nuclear Modification Factor

• At RHIC energies
• ? Cronin effects
• predominant at
• mid-rapidity

• At more forward ys
• ? Transition from
• Cronin enhancement
• to a suppression
• effect

• This is what one would expect if there is an
  effect
• of gluon density saturation in the initial
  state

F.Rami, IReS Strasbourg



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What do we see in the data?
For pt2 GeV/c
(?4deg)

x 5?10-4

• Transition from Cronin enhancement to
  suppression
• Qualitatively consistent with the expected
  behavior for CGC

F.Rami, IReS Strasbourg



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Rapidity Dependence
F.Rami, IReS Strasbourg



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Results from other RHIC experiments
Reference from peripheral collisions

• Good agreement between BRAHMS and PHENIX
• PHOBOS ? consistent results (limited y-range)

F.Rami, IReS Strasbourg



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Comparison to CGC calculations

• Quantitative CGC calculations for dAu _at_
  ?SNN200 GeV

Nuclear Modification Factor
Nuclear Modification Factor
F.Rami, IReS Strasbourg



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Summary Perspectives

• Results obtained so far at RHIC for central
  AuAu collisions are
• consistent with the formation of a dense
  partonic state of matter
• characterized by strong collective
  interactions

• Strong hints of saturation effects at RHIC (from
  dAu data)
• ? CGC might provide the initial conditions for
  A-A collisions at RHIC

The task now ?

• Characterize the properties of this dense
  partonic state of matter

• Confirmation of the Color Glass Condensate

? will require further experimental tests (more
sensitive probes)

• RHIC (upgrades ? improved physics capabilities)
• LHC ? much higher energies (smaller x)

F.Rami, IReS Strasbourg



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Backup slides
F.Rami, IReS Strasbourg



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Future experimental progran at RHIC

• Next 5 years ? Significant detector upgrades

• Improved vertexing for charm measurements
• Better particle id (TOF)
• Low-mass dilepton measurements
• Expanded forward coverage (? low-x physics)

• Longer term ? Significant upgrade of the machine
  (RHIC II)
• based on electron cooling
  (? higher luminosities)

• Additional upgrades
• Proposal for a new detector to exploit the
• increased luminosity

? Extend jet-related measurements to much higher
pts (into the perturbative regime)
Physics program of RHIC II still under discussion
F.Rami, IReS Strasbourg



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Main Goals in RHIC experiments
Phase Diagram of Nuclear Matter
F.Karsch, hep-lat/0106019
Lattice QCD
Energy Density/T4
?
TC ? 160MeV (?B 0)
Temperature
?B ?F/?NB
Explore and characterize the QGP
F Free energy NB Baryonic Number
(baryon anti-baryon)
Study QCD matter at high densities
? Large experimental program
F.Rami, IReS Strasbourg



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Space-time evolution of a heavy-ion
collision at collider energies

• There are several stages in the collision

Emission of hadrons
(t ? 20fm/c)
Parton interactions take place during first stages
Initial State (vc)
Dense Medium
CGC ?
F.Rami, IReS Strasbourg



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Two Large Detectors at RHIC
F.Rami, IReS Strasbourg



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Two Small Detectors at RHIC
BRAHMS 2 spectrometers (movable) Magnets,
Tracking Chambers, TOF, RICH

• Detailed measurements of momentum
• spectra and yields of charged hadrons
• over a wide range of rapidities
• (including the forward kinematical region)

F.Rami, IReS Strasbourg



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Thermal model parameters from particle ratios

• Statistical Model Analysis
• ?Analysis of particle ratios measured at RHIC in
  a grand canonical ensemble with baryon number,
  strangeness and charge conservation

P.Braun-Munzinger et al, PLB518(2001)41
Thermal model parameters at chemical freeze-out

• ?B 465 MeV
• T1747 MeV
• Similar analysis at ?SNN200GeV
• ?B 298 MeV
• T1777 MeV

?SNN130GeV
Agreement -gt indicates a high degree of chemical
equilibration Flow -gt hydro (thermalisation ..)
F.Rami, IReS Strasbourg



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dNch/d? at Mid-Rapidity Energy
Dependence

• Saturation models
• also reproduce the
• measured multiplicities

F.Rami, IReS Strasbourg



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Very high charged hadron multiplicities
dNch/d??0 553 ? 36
dNch/d??0 625 ? 55
Number of charged particles per unit of
rapidity in the MR (at ?0)

• Much higher multiplicities than at CERN-SPS
  (PbPb)

F.Rami, IReS Strasbourg



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Very high energy densities

• Transverse energy distributions measured by
  PHENIX (calorimetry)

• Central events ?
• dET/d??0 500 GeV ( ? SPS)

• Using Bjorken estimate for the energy
• density (J.D.Bjorken, PRD27(83)140)

? ?BJ 4.6 GeV/fm3

• At 200GeV) ? ?BJ 5 GeV/fm3
• factor of 1.6 larger than at SPS

?BJ 3.2 GeV/fm3 for PbPb at SPS (NA49,
PRL75(1995)3814)
Well above the value expected for the Critical
Energy Density (?crit 1 GeV/fm3)
F.Rami, IReS Strasbourg



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Energy Density from Transverse Energy
Measurements
Bjorken formula for thermalized energy density
J.D.Bjorken PRD27(83)140
time to thermalize the system (t0 1 fm/c)
6.5 fm
Longitudinal expansion of thermalized system
F.Rami, IReS Strasbourg



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EVENT CHARACTERIZATION COLLISION
CENTRALITY
AuAu _at_ ?SNN130GeV

• Measured with Multiplicity
• Detectors (TMA and SiMA)

Central b0
Peripheral b large
Central ?
? Peripheral

• Define Event Centrality Classes
• ? Slices corresponding to different fractions
  of the cross section

• For each Centrality Cut
• ? Evaluate the corresponding number of
  participants Npart and
• number of inelastic NN collisions
  NCOLL (from Glauber Model)

F.Rami, IReS Strasbourg



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Elliptic Flow pt dependence
Kolb Heinz, nucl-th/035084
? Good agreement for central and
mid-central events But overpredicts v2 for
peripheral events (b ? 10fm) ?
incomplete thermalization
F.Rami, IReS Strasbourg



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Elliptic Flow from Parton Cascade Transport
Calculations
Molnar Gyulassy, NPA 697 (2002) 495

• Calculations with ? transport opacities ?

• Agreement with data if ? is very large
• ? large number of interactions
• among the fireball constituents
• (partons)

• ? very large ? hydro limit
• (pt lt 1.5GeV/c)

• Parton cascade predicts
• saturation at high pts
• (observed in the data)
• High pt particles escape the
• fireball before having suffered
• a sufficient number of rescattering
• to thermalize their momenta.

F.Rami, IReS Strasbourg



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Flow is Sensitive to Early Stages

• ? Elliptic flow builds up in the first
• instants of the collision (before
• hadronization) and then stays
• constant

• Rescattering converts the initial space
• anisotropy of the overlap region to the
• momentum anisotropy of elliptic flow
• ? v2 is sensitive to the number of
• interactions and can be considered
• as a measure of thedegree of
• thermalization at early time.

v2 is proportional to the
parton-parton scattering
cross section used in the
calculations
F.Rami, IReS Strasbourg



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Elliptic Flow Sensititivity to the EoS
Hydro calculations Huovinen, Kolb
Heinz NPA698 (2002) 475

• Elliptic flow builds up and saturates
• early in the collision
• ? sensitivity to high density EOS

? Hydrodynamical mass splitiing (observed
in the data) underpredicted by EOS/H
EOS/Q ? quark gluon plasma EOS (hard) EOS/H ?
pure hadron resonance gas (soft)
F.Rami, IReS Strasbourg



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Nuclear Modification Factor RAA
PHENIX, PRL91(2003)072301
Scaled pp reference
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Nuclear Modification Factor RAA
For peripheral collisions ? NCOLL scaling works
well
Nucl-ex/0410003 (PHENIX White paper) ? NCOLL
scaling of hard processes has been also checked
using direct photons, which are produced via hard
scattering processes but do not loose energy in
the medium since they have no color charge
F.Rami, IReS Strasbourg



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Evaluation of Npart and NCOLL
Npart Nucleons that interact inelastically
in the overlap region between the
two interacting nuclei NCOLL Number of binary
nucleon-nucleon collisions (one nucleon
can interact successively with several
nucleons if they are in its path)

• Use Glauber Model
• Nucl.Phys.B21(1970)135

• Main assumption
• Independent collisions of part. nucleons
• Nucleons suffer several collisions along
• their incident trajectory (straight-line)
• without deflection and without energy
• loss

• Nucleons inside nuclei distributed
• according to a Woods-Saxon density
• profile
• Interaction probability between 2
• nucleons is given by the pp cross
• section
• Calculate the overlap integral at
• a given impact parameter

pp Np1 and NCOLL1 pA Np1 and NCOLLgt1
F.Rami, IReS Strasbourg



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High pt suppression Theoretical
calculations
Vitev Gyulassy, hep-ph/0209161

• pQCD based calculations incorporating
• parton energy loss via medium induced
• gluon radiation are able to reproduce
• the data
• Energy dependence can be explained
• by the competition between quenching,
• nuclear shadowing and Cronin effect

• Realistic hadronic calculations
• (Cassing, Gallmesiter and Greiner,
  hep-ph/0311358)
• ? unable to reproduce the observed effect

F.Rami, IReS Strasbourg



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Another evidence in favor of Jet Quenching
Azimuthal Correlations
STAR, PRL91(2003)072304

• Azimuthal correlations between
• a high pt particle (trigger) with
• 4 ? pt ? 6 GeV/c and all other
• particles with pt above 2 GeV/c
• ? Indirect way to identify the
• formation of jets

• pp ? Clear two-jet signal
• (back to back correlation)
• dAu ? The signal survives
• Central AuAu ? The signal disappears

• Strong experimental evidence
• for Jet Quenching in AuAu

Jets are deflected in the medium ? destroys the
coplanarity of the 2 jets
F.Rami, IReS Strasbourg



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High Density Gluon Saturation

• e-p scattering at HERA

?Gluon density in a proton increases strongly
from large x to small x (xfraction of E
transfered to the gluon)
Saturation at high density QS Saturation scale
F.Rami, IReS Strasbourg



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McLerran, hep-ph/0402137
F.Rami, IReS Strasbourg



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Forward measurements in dAu collisions
Sensitivity to smaller-x values

• BRAHMS spectrometers measure in the
  d-fragmentation region

• To reach small x in the gluon distribution of
  the Au nucleus

xAu mt/?S ? e-y
? Go very forward
? Larger saturation scale QS Qs2(x) Q02 ?
(x0/x)?

• Qs2 ? A1/3 (Thickeness effect)

? Saturation scale in Au larger than in p
(saturation can be probed at lower x)

• No final state effects in dAu

From y0 to y4 ? x values lower by 10-2 ?
One could hope to see the occurrence of a
suppression effect
D.Kharzeev et al, hep-ph/0307037
F.Rami, IReS Strasbourg



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Kinematics of p-Au
A is p and B is Au Energy and momentum
conservation xL xA - xB (2MT/vs)sinh
y kA kB k
xAxB MT2/s A solution to this system is
xA (MT/vs) ey xB (MT/vs) e-y
y is the rapidity of the detected particle
(xL,k) xL is its logitudinal momentum
fraction xA (xB) is the longitudinal momentum
fraction of the projectile (target) parton
F.Rami, IReS Strasbourg



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Centrality Dependence
Reference from peripheral collisions
Highest density of gluons in central
collisions ? Largest suppression
F.Rami, IReS Strasbourg



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CGC calculations Predictions for LHC
Predictions for LHC
p-A collisions

• Stronger suppression
• at LHC (smaller x)

F.Rami, IReS Strasbourg



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Accessible x range at RHIC and LHC

• LHC ? higher energies, higher rapidities
  (smaller x)
• p-A (and AA) deeply inside the saturation
  regime
• Possibility to probe saturation also in pp

F.Rami, IReS Strasbourg



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Nuclear modification factor for h- and h

F.Rami, IReS Strasbourg



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Nuclear modification factor for mesons and
baryons
anti-proton data not corrected for anti-lambda
feed down
- Difference between baryons and mesons - Related
to parton recombination? (Hwa et al,
PRC71(2005)024902
F.Rami, IReS Strasbourg



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