status of quark-gluon plasma and saturation effects at rhic

Status of Quark-Gluon Plasma and saturation Status of Quark-Gluon Plasma and saturation effects at RHICeffects 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 Sinaia2005

Phase Diagram of Nuclear Matter

Temperature

Lattice QCD

F.Karsch, hep-lat/0106019

TC 160MeV (B = 0)

• Study QCD matter at high densities

• Explore and characterize the QGP

Main Goals in RHIC experiments

Large experimental program

First Physics Run: June 2000

2000-2005: 5 runs

PHOBOS

PHENIX

BRAHMS

Relativistic Heavy Ion Collider @ BNL

p+p 500 1032 Au+Au 200 1026

SNN (GeV) L(cm-2 s-1)

RHIC accelerates all species from p to Au

Two independent rings ~3.8 km in circumference

Several systems/energies Au+Au @ 200 GeV @ 130 GeV @ 62.4 GeV Cu+Cu @ 200 GeV @ 63 GeV d+Au @ 200 GeV 62.4 GeV p+p @ 200 GeV (reference data)

Particle multiplicities at RHIC

Central Au+Au event measured by STAR/TPC, @ 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

Au+Au data much larger than pp Not a simple superposition Medium effects important role in AA collisions

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

Wang & Gyulassy, PRL86(2001)3496

BRAHMS

RHIC (average)

dNch/d at Mid-Rapidity

Energy Dependence

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

Semi-central Collisions

Collective Flow Collective expansion of Nuclear Matter following the compression phase

Response to early pressure

Elliptic Flow Pressure converts spatial anisotropy into p-space anisotropy

Reactionplane

Elliptic Flow

Fourier decomposition of the azimuthal distributions

dN/d = F0 (1 + 2vicos(i))v2 : 2

nd harmonic Fourier coefficient

Measure of Elliptic Flow

STAR, PRL90(2003)032301

STAR, PRL86(2001)402

Au+Au @ 130GeV

Hydro limit

Pb+Pb at SPS

CentralPeripheral

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

Large set of v2 data available at RHIC (STAR, PHENIX) for ≠ particle species All can be reproduced by hydro including the mass dependence

Hydro good agreement for soft particles:• pions up to pt ~ 1.5GeV/c• protons up to pt ~ 2.5GeV/c

More than 95% of the emitted particles

The bulk of the fireball behaves hydrodynamically

To reproduce the large v2 values Hydro evolution must start very early Fast thermalization

hadronsleadingparticle

leading particle

Schematic view of jet production Particles with high pt’s (above ~2GeV/c) are primarly produced in hard scattering processes early in the collision Probe of the dense and hot stage

Experimentally Suppression in the high pt regionof hadron spectra (relative to p+p)

p+p experiments Hard scattered partons fragment into jets of hadrons

In A-A, partons traverse the medium

If QGP partons will lose a large part of their energy (induced gluon radiation) Suppression of jet production Jet Quenching

High pt Suppression & Jet Quenching

RAA =Yield(AA)

NCOLL(AA) Yield(pp)

Scaled pp reference

Nuclear Modification Factor

At RHIC Significant suppression

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 A+A and d+A(Run3, Control experiment)

(h++h-)/20

PHENIX, PRL88(2002)022301

High pt Suppression at RHIC

New phenomenon at RHIC

Not observed at lower energies SPS(Pb+Pb) Enhancement due to initial state multiple scattering (Cronin effect) well known in p+A collisions

Gluon sat. Suppression in dAuQuenching No suppression in dAuF.Rami, IReS Strasbourg

Sinaia2005

Initial or Final State effect ?

Final State effects are dominant in central Au+Au at RHIC as expected from the formation of a hot and dense medium of partonic matter

Same conclusion

Summary of the main experimental observations for central Au+Au collisions

All of these results are consistent with the existence of a dense partonic state of matter characterized by strong collective interactions

Large particle multiplicities

Presence of a dense partonic medium

High energy densities (well above critical)

High degree of collectivity and early thermalization

Main conclusion of the 4 RHIC White Papers (to be published in Nucl.Phys.A)

High Density Gluon Saturation at RHIC

McLerran, hep-ph/0402137

xAs x becomes smaller and smaller,the gluon density increases faster driving force toward saturation

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

No apparent sign of saturation in high pt hadron spectra for d+Au

Those data are for MR particles More forward rapidities

(smaller x values)

BRAHMS

Forward measurements in d+Au collisions Sensitivity to smaller-x values

BRAHMS spectrometers measure in the d-fragmentation region

D.Kharzeev et al, hep-ph/0307037

xAu = mt/S e-y

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

Go very forward

From y=0 to y=4 x values lower by ~10-2

One could hope to see the occurrence of a suppression effect

What do we expect?

CGC at y=0

D. Kharzeev et al, hep-ph/0307037

Very high energy

As y grows

At RHIC energies Cronin effects predominant at mid-rapidity

RpA : Nuclear Modification Factor

At more forward y’s 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

BRAHMS, PRL 93 (2004) 242303

x ~ 10-2

η=0, (h++h-)/2 η=3.2, h-

For pt=2 GeV/c

x ~ 510-4

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

(θ=4deg)

What do we see in the data?

Suppression increases with rapidity as expected for saturation effects

x ~ 10-2

(for pt=2GeV/c)

η=0, (h++h-)/2 η=1, (h++h-)/2

η=2.2, h-

η=3.2, h-

(Min bias)

BRAHMS, PRL 93 (2004) 242303

Rapidity Dependence

Results from other RHIC experiments

Central [0-20%]

BRAHMS PHENIX (hadrons)

d-sideAu-side

nucl-ex/0411054

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

RCP =Yield(0-20%)/NCOLL(0-20%)Yield(60-80%)/NCOLL(60-80%) Reference from

peripheral collisions

Quantitative CGC calculations for d+Au @ SNN=200 GeV

Comparison to CGC calculations

Overall good agreement

Calculations predict also a transition from Cronin enhancement at MR to suppression at larger y’s

So far No alternative explanations within realistic model calculations

D. Kharzeev at al. hep-ph/0405045

=2.2 =3.2

Modifi

Summary & Perspectives

Results obtained so far at RHIC for central Au+Au 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 d+Au data) CGC might provide the initial conditions for A-A collisions at RHIC

Confirmation of the Color Glass Condensate

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

Characterize the properties of this dense partonic state of matter

The task now

will require further experimental tests (more sensitive probes)

Backup slides

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 pt’s (into the perturbative regime)

Physics program of RHIC II still under discussion

• Study QCD matter at high densities

Phase Diagram of Nuclear Matter

Temperature

Lattice QCD

F.Karsch, hep-lat/0106019

TC 160MeV (B = 0)

• Explore and characterize the QGP

Main Goals in RHIC experiments

B = F/NB

F = Free energy NB = Baryonic Number

(baryon – anti-baryon) Large experimental program

Parton interactions takeplace during first stages

Emission of hadrons(t 20fm/c)

There are several stages in the collision

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

Initial State (v~c) Dense MediumCGC ?F.Rami, IReS Strasbourg

Sinaia2005

STAR Solenoidal field

Large Solid Angle TPCSi-Vertex detectorRICH, EM Cal, TOF

Measurements of hadronic observables with a large acceptance and Pid

Event-by-event analyses

PHENIXAxial Field

High Resolution & Rates2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID

Designed to measure simultaneously Leptons, Photons, and Hadrons in selected solid angles

Rare signals such as J/ψ decaying into muons and electrons, direct photons

Two “Large” Detectors at RHIC

BRAHMS2 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)

PHOBOSNearly 4 coverage with Si-detectors

2 Arm Spectrometers (also based on Si)

Total charged particle multiplicity in 4 & Global properties (elliptic flow) Charged hadron spectra (small acceptance)

Ring Counters

Paddle Trigger Counter

Spectrometer

Octagon+Vertex

Two “Small” Detectors at RHIC

B = 46±5 MeV

T=174±7 MeV

Similar analysis at SNN=200GeV

B = 29±8 MeV

T=177±7 MeV

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

SNN=130GeV

P.Braun-Munzinger et al, PLB518(2001)41

Thermal model parametersat chemical freeze-out

Agreement -> indicates a high degree of chemical equilibrationFlow -> hydro (thermalisation ..)

Thermal model parameters from particle ratios

Saturation models also reproduce the measured multiplicities

HIJING – Jet quenching

EKRT (Gluon Saturation)

Wang & Gyulassy, PRL86(2001)3496

BRAHMS

RHIC (average)

HIJING – No Jet quenching

dNch/d at Mid-Rapidity

Energy Dependence

BRAHMS, PLB523(2001)227

0-5% 30-40%

BRAHMS, PRL88(2002)202301

3860 300 4630 370

SNN=130GeV SNN=200GeV

Nch(-4.7<<4.7)

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 (Pb+Pb)

BJ ~ 4.6 GeV/fm3

BJ ~ 3.2 GeV/fm3 for Pb+Pb at SPS (NA49, PRL75(1995)3814)

Well above the value expected for the

Critical Energy Density (crit ~ 1 GeV/fm3)

Very high energy densities

Transverse energy distributions measured by PHENIX (calorimetry)

PHENIX, PRL87(2001)052301

Au+Au @ SNN=130GeV

MBCentral(top 5%)

Central events

dET/d|=0 ~ 500 GeV ( SPS)

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

At 200GeV) BJ ~ 5 GeV/fm3

factor of ~1.6 larger than at SPS

dydz 0

Bjorken formula for thermalized energy density

time to thermalize the system (0 ~ 1 fm/c)

~6.5 fm

J.D.BjorkenPRD27(83)140

Longitudinal expansion of thermalized system

Energy Density from Transverse Energy Measurements

EVENT CHARACTERIZATION COLLISION CENTRALITY

Au+Au @ SNN=130GeV

Measured with Multiplicity Detectors (TMA and SiMA)

Central Peripheral

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

Central b=0

Peripheral b large

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 Sinaia2005

Kolb & Heinz, nucl-th/035084

Elliptic Flow: pt dependence

Good agreement for central and mid-central events But overpredicts v2 for peripheral events (b 10fm) incomplete thermalization

Elliptic Flow from Parton Cascade Transport Calculations

Molnar & Gyulassy, NPA 697 (2002) 495

Calculations with ≠ transport opacities ζ

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

ζ very large hydro limit (pt < 1.5GeV/c)

Parton cascade predicts saturation at high pt’s (observed in the data)

High pt particles escape the fireball before having suffered a sufficient number of rescattering to thermalize their momenta.

Flow is Sensitive to Early Stages

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

v2 is proportional to the parton-parton scattering

cross section used in the calculations

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.

Au+Au at 200GeV

time(fm/c)

Parton Cascade Model (AMPT,Zhang et al, PLB455(1999)45)

Elliptic Flow: Sensititivity to the EoS

Hydro calculations: Huovinen, Kolb & HeinzNPA698 (2002) 475

EOS/Q quark gluon plasma EOS (hard)EOS/H pure hadron resonance gas (soft)

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

RAA =Yield(AA)

NCOLL(AA) Yield(pp)

Scaled pp reference

Nuclear Modification Factor

RAA<1 Suppression relative to scaled pp reference

PHENIX, PRL91(2003)072301

Nuclear Modification Factor RAA

Scaled pp reference

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 directphotons, which are produced via hard scattering processes but do not loose energy in the medium since they have no color chargeF.Rami, IReS Strasbourg

Sinaia2005

Evaluation of Npart and NCOLL

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

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)

Main assumption : Independent collisions of part. nucleonsNucleons 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

p+p : Np=1 and NCOLL=1p+A : Np=1 and NCOLL>1F.Rami, IReS Strasbourg

Sinaia2005

High pt suppression: Theoretical calculations

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

Vitev & Gyulassy, hep-ph/0209161

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

STAR, PRL91(2003)072304

Strong experimental evidence for Jet Quenching in Au+Au

Another evidence in favor of Jet Quenching Azimuthal Correlations

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

Jets are deflected in the medium destroys the coplanarity of the 2 jets

p+p Clear two-jet signal (back to back correlation)

d+Au The signal survives

Central Au+Au The signal disappears

Low energy

High energy

Gluon densityincreases

Small x

Large x

x = Econstituant/Ehadron

High Density Gluon Saturation

Gluon density in a proton increases strongly from large x to small x (x=fraction of E transfered to the gluon)

e-p scattering at HERA

Saturation at high densityQS : Saturation scale

McLerran, hep-ph/0402137

Forward measurements in d+Au collisions Sensitivity to smaller-x values

BRAHMS spectrometers measure in the d-fragmentation region

D.Kharzeev et al, hep-ph/0307037

xAu = mt/S e-y

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

Go very forward

Qs2 A1/3 (Thickeness effect)

Larger saturation scale QS : Qs2(x) = Q0

2 (x0/x)λ

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

From y=0 to y=4 x values lower by ~10-2

One could hope to see the occurrence of a suppression effect

No final state effects in d+Au

A is p and B is Au

Energy and momentum conservation

xL = xA - xB =(2MT/√s)sinh y

kA + kB = k

xAxB = MT2/s

A solution to this system is:

xA = (MT/√s) ey

xB = (MT/√s) e-y

rapidity

Kinematics of p-Au

y is the rapidity of the detected particle (xL,k)xL is its logitudinal momentum fraction

xA (xB) is the longitudinal momentum fractionof the projectile (target) parton

Highest density of gluons in central collisions Largest suppression

=3.2More suppression in central events

Also consistent with CGC matter

Centrality Dependence

RCP =Yield(0-20%)/NCOLL(0-20%)

Yield(60-80%)/NCOLL(60-80%)

Reference fromperipheral collisions

CGC calculations: Predictions for LHC

LHC, =0

RHIC, =3.2

Predictions for LHC

Stronger suppression at LHC (smaller x)

p-A collisions

MQQ : Invariant mass of the QQ pair produced in the hard scatteringyQQ : Rapidity of the pair

A.Dainese, nucl-ex/0311004

X2 = [MQQ/SNN] exp(- yQQ)--X1 = [MQQ/SNN] exp(+ yQQ)--

Accessible x range at RHIC and LHC

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

Nuclear modification factor for h- and h+

Nuclear modification factor for mesons and baryons

anti-proton datanot corrected foranti-lambda feed down

- Difference between baryons and mesons- Related to parton recombination? (Hwa et al, PRC71(2005)024902

status of quark-gluon plasma and saturation effects at rhic

gev p p

gev d au

systemsenergies au au

rhiccentral au au event

phobosphenixstarbrahms

larger elliptic flow

gev cu cu

high pts

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