Relativistic Heavy Ion Physics: An Experimental Review

Saskia Mioduszewski

22 July 2003

2

Outline• Physics Goals: deconfinement and chiral symmetry

restoration

• Overview of the Program• Global Observables

– charged-particle multiplicity– flow

• Other Experimental Highlights– J/ suppression– low mass dilepton enhancement– high pT suppression

• Summary

Lattice QCD at Finite Temperature• Coincident transitions: deconfinement and chiral symmetry restoration

F. Karsch, hep-ph/010314

Critical energy density:4)26( CC T

TC ~ 175 MeVC ~ 0.7 GeV/fm3

Ideal gas (Stefan-Boltzmann limit)

B=0)

Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero:

At high temperature and/or baryon density

Constituent mass current mass Chiral Symmetry (approximately) restored.

MeVqq 3)250(

0qq

4

Schematic Phase Diagram of Strongly Interacting MatterSchematic Phase Diagram of Strongly Interacting Matter

Baryonic Potential B [MeV]

T

em

pera

ture

T [

MeV

]

0

200

250

150

100

50

0 200 400 600 800 1000 1200

AGS

SIS

SPS

RHIC

quark-gluon plasma

hadron gas neutron stars

early universe

thermal freeze-outdeconfinementchiral restoration

Lattice QCD

atomic nuclei

P. Braun-Munzinger, nucl-ex/0007021

Test QCD under extreme conditions and in large scale systems

Search for deconfined QGP phase

SISAGS SPS RHICLHC

From high baryon density regime to high temperature regime

5

How to Observe QGP in Heavy Ion Collisions

Some tools to distinguish QGP from dense hadron gas:

– Direct observation of deconfinement: suppression of J/ – High energy density: interaction of jets with medium– High temperature: direct photons/dileptons– Chiral symmetry restoration: meson properties (m,) expected to be modified in medium– Equilibration at early stage large pressure collective expansion: flow

6

History of High-Energy A+B Beams

• BNL-AGS: mid 80’s, early 90’s

O+A, Si+A 15 AGeV/c sNN ~ 6 GeV

Au+A 11 AGeV/c sNN ~ 5 GeV

• CERN-SPS: mid 80’s, 90’s

O+A, S+A 200 AGeV/c sNN ~ 20 GeV

Pb+A 160 AGeV/c sNN ~ 17 GeV

• BNL-RHIC: early 00’s

Au+Au sNN ~ 130 GeV

Au+Au, p+p, d+Au sNN ~ 200 GeV

7

The RHIC Experiments

STAR

8

Global Observables Reflect the conditions of the system after freeze-out,

after resonance decays

• Charged-Particle Multiplicity- helps constrain models- reflects produced entropy

• Flow- collective expansion, rescattering- pressure

9

AA collisions are not all the same

Nuclei are extended objects– Impact parameter– Number of

participants– Centrality ( % from total inelastic

cross-section)

100% 0 %

Participants

Spectators

Spectators

Charged-Particle Rapidity Distribution

BRAHMS (0-5%): Nch (||<4.7) = 3860 ± 300NA49 (0-5%): Nh

- (|y| < 3) = 695 ± 30

- Factor of 3 more particles produced at RHIC than at SPS - Wider distribution

Enhancement of particle production for central collisions at mid-rapidity.

Particle production scales with Npart at high rapidities ( >3).

h-

NA49

dn/dy

RHIC

SPS

BRAHMS

11

From SPS to RHIC :

* dNch/dy increases by

~70% at sNN

= 130 GeV

* dNch/dy increases by

~90% at sNN

= 200 GeV

ln(sNN

) dependence from

AGS to RHIC

sNN

Dependence of dNch/dy

AGS

SPSRHIC

12

Radial Flow

– Expansion of system due to pressure

– Heavier particles shifted to higher pT

– Observable: <T> from slopes of mT spectra as a function of mass

– Spectra can be described by hydrodynamic models for pT< 2-3 GeV/c and mid-peripheral to central events

13

Single Particle Spectra (low pT)

• Decreasing slope for increasing particle mass and centrality

T. Ullrich QM2002

14

Single Particle Spectra for most central events (0-5%)

• proton yield ~ pion yield @ 2 GeV• consistent with hydrodynamic model calculations (e.g. comparison to 130 GeV data - Teaney, Lauret, Shuryak nucl-th/0110037)

PHENIX Preliminary PHENIX Preliminary

Au+Au at sqrt(sNN) =200GeVAu+Au at sqrt(sNN) =200GeV

J. Burward-Hoy, QM2002

Mean Transverse Momentum vs. Npart

<pT> increases with Npart and particle mass, indicative of radial expansion

Relative increase with Npart greater for (anti)p than for , K

J. Burward-Hoy, QM2002

closed symbols: 200 GeV

open symbols: 130 GeV

Hydrodynamic Model Fit to the Spectra

PHENIX:Freeze-out Temperature

Tfo = 110 23 MeV

Transverse flow velocity

T = 0.7 0.2 < T> ~ 0.5

Most central collisionsfor 200 GeV data

Ref: E. Schnedermann, J. Sollfrank, and U. Heinz, Phys. Rev. C 48, 2462 (1993)

Au+Au at sqrt(sNN) =200GeV

STAR:

Tfo ~ 100 MeV

T ~ 0.6

J. Burward-Hoy, QM2002

17

Mid-Rapidity mT spectra at SPS

M. van Leeuwen QM2002 (NA49)

NA57, H. Helstrup, this conference:

Tfo = 131 ± 10 MeV

<T> = 0.47 ± 0.02

18

Elliptic Flow in Non-central Collisions

Early state manifestation of collective behavior: • Asymmetry generated early in collision, quenched by expansion observed asymmetry emphasizes early time

x

y

p

patan2cos2 vSecond Fourier coefficient v2:

Coordinate space: initial asymmetry

Momentum space: final asymmetry

multiple collisions (pressure)

py

px

Strong elliptic flow signal strong (collective) pressure Large and fast rescattering (early thermalization) v2 dependent on mass (predicted by hydro P. Huovinen et al, PLB 503 (2001) 58).

Elliptic Flow

20

Elliptic Flow

• SPS: v2 ~ 0.03

• RHIC: v2 ~ 0.055

Wetzler QM2002

E877: Phys.Lett.B474:27-32, 2000CERES: QM2001INPC 2001 nucl-ex/0109017 STAR: PRC66 (2002) 034904NA49 Preliminary

130 GeV data

21

Flow: Comparison of SPS and RHIC

• Radial Flow: pressure can build up over entire dynamics– <T> ~ 0.4 - 0.5 at SPS– <T> ~ 0.5 - 0.6 at RHIC

• Elliptic Flow: pressure must build up before asymmetry of system has diminished– v2 ~ 0.03 at SPS– v2 ~ 0.06 at RHIC

• Moderate increase in <T> more pressure at RHIC• Significantly larger v2 is evidence for early build-up of

pressure• According to hydrodynamic models early

thermalization at RHIC (~0.6fm/c - Heinz, Kolb

Nucl.Phys.A702:269-280,2002 )

22

Energy Density

Energy density a la Bjorken:

dy

dE

τπR

1ε T

2

fm/c 12.0τ

fm/c 1τ

A 1.18R

RHIC

SPS

1/3

38~6.0 GeV/fmfm/c

dET/dy ~ 720 GeV (S. Bazilevsky

QM2002, PHENIX PRELIMINARY)

35~1 GeV/fmfm/c

Estimate for RHIC:

23

Other Highlights of Program

• Global observables properties of collision dynamics, EOS

• Other probes for signatures of QGP– J/ suppression deconfinement– low mass dileptons chiral symmetry restoration

– high pT suppression density of produced medium and energy loss

24

J/ suppression: probe of deconfinement

• An “old” signature of QGP formation: (Matsui and Satz PL B178, (1986) 416).

• At high enough color density, the screening radius < binding radius J/ will dissolve

• Observation: Anomalous suppression in Pb-Pb collisions* beyond normal nuclear absorption abs

~ 4-6 mb

25

J/ suppression: Evidence of deconfinement?L. Ramello, QM 2002NA50 Preliminary

Suppression increasing with centrality (discontinuities?)

Exceeds normal nuclear absorption (as measured in p+A)

Many models exist (hadronic and QGP) – data consistent with suggested QGP signature (Matsui, Satz, Kharzeev)

26

• possible signature of the deconfinement phase transition– J/ yield can be

• suppressed more than at SPS - dissolve in QGP (longer lifetime, higher temperature than SPS)

• enhanced - cc coalescence as the medium cools (2 orders of magnitude more production of cc pairs at RHIC)

• important to measure J/ in p+p and d+Au to separate “normal” nuclear effects– shadowing– nuclear absorption in cold matter

• Jmeasurements in leptonic decay channels– J/ e+ e- and J/ in p+p at s = 200 GeV

– J/ e+ e- in Au+Au at sNN = 200 GeV

Charmonium (Jphysics at RHIC

(hep-ex/0307019)

(nucl-ex/0305030)

27

J/ Production at RHIC

normal nuclear absorption:

Pb+Pb at CERN SPS (NA50)

PHENIX, sNN

= 200 GeV • J/-Suppression maybe most compelling QGP evidence at CERN SPS

• Expectation at RHIC energies unclear0 cc pairs

produced per central Au+Au collision

– Possibly enhanced J/- production due to charm-coalescence

-

PLB477(2000) 28 normalized to PHENIX p+p measurement

28

coalescence model (Thews at al.)

y = 1.0

y = 4.0

statistical model (Andronic at al.)

absorption model (Grandchamp et al.)

Model comparisons

• models that predict enhancement relative to binary collision scaling are disfavored

• no discrimination between models that lead to suppression

29

Low-Mass e+e- pairs

No enhancement in pp and pA collisions

Main CERES Result:Strong enhancement of low-mass pairs in A-A collisions

(wrt to expected yield from known sources)

Enhancement factor (.25 <m<.7GeV/c2): 2.6 ± 0.5 (stat) ± 0.6 (syst)

30

Interpretations

scattering off baryons(Rapp, Wambach et al)

-meson broadening Dropping -meson mass(G.E. Brown et al)

annihilation: +- * e+e- (thermal radiation from HG)Cross section dominated by pole at the mass of the em form factor:

2222

42

m )m (m

m m)(F

Plus

or

Add

Onset of Chiral Symmetry Restoration?Dropping -meson mass

(Rapp, Wambach et al)

In-medium -meson broadening(G.E. Brown et al)d.o.f.

hadrons quarks

What happens as chiral symmetry is restored? Dropping mass or broadening (melting)?

32

Fate of Hard Scattered Partons in Au+Au Collisions

• Hard scatterings in nucleon-nucleon collisions produce jets of particles.

• In the presence of a color-deconfined medium, the partons strongly interact (~GeV/fm) losing much of their energy.

• “Jet Quenching”

hadrons

q

q

hadrons leadingparticle

leading particle

schematic view of jet production

33

Nuclear Modification Factor RAA

• in absence of nuclear effects– RAA < 1 at low pT (soft physics regime)– RAA = 1 at high pT (hard scattering regime)

• “suppression” (enhancement, e.g. Cronin effect)– RAA < 1 (> 1) at high pT

Nuclear Modification Factor

RAA (pT ) d2N AA /dpT d

TAA d2 NN /dpT d

<Nbinary>/inelp+p

NN cross section

34

By definition, processes that scale with Nbinary will produce RAA=1.

RAA is what we measure divided by what we expect.

RAA is < 1 at RHIC, but > 1 at SPS

SPS: “Cronin” effect dominatesRHIC: suppression dominates

RAA for 0

Nbinary-scaling

A.L.S.Angelis PLB 185, 213 (1987)WA98, EPJ C 23, 225 (2002)PHENIX, PRL 88 022301 (2002)PHENIX submitted to PRL, nucl-ex/0304022

35

Jet Quenching ?• high pT suppression

reproduced by models with parton energy loss

• other explanations not ruled out, need to measure initial-state effects

without parton energy loss

with parton energy lossWang

Wang

Levai

Levai

Vitev

comparison with model calculations

with and without parton energy loss

Au+Au0+X at sNN = 200 GeV

Wang: X.N. Wang, Phys. Rev. C61, 064910 (2000).

Levai: P.Levai, Nuclear Physics A698 (2002) 631.

Vitev: I. Vitev and M. Gyulassy, hep-ph/0208108 + Gyulassy, Levai, Vitev, Nucl. Phys. B 594, p. 371 (2001).

36

RAA for 0 and charged hadrons

pp

AuAubinaryAuAuAA Yield

NYieldR

/

PHENIX AuAu 200 GeV0 data: nucl-ex/0304022, submitted to PRL.charged hadron (preliminary) : NPA715, 769c (2003).

• RAA is well below 1 for both charged hadrons and neutral pions.

• The neutral pions fall below the charged hadrons since they do not contain contributions from protons and kaons (will be discussed later).

Strong Suppression!

- Consistent observation by all 4 experiments in charged hadron measurement

PHOBOS, R. Nouicer, this conferenceBRAHMS, Z. Yin, this conference

Azimuthal distributions in Au+Au

Near-side: peripheral and central Au+Au similar to p+p

Strong suppression of back-to-back correlations in central Au+Au collisions

Au+Au peripheral Au+Au central

pedestal and flow subtracted

Phys Rev Lett 90, 082302

?

38

d+Au

Au+Au

RAA vs. RdA for charged hadrons and 0

No Suppression in d+Au, instead small enhancement observed (Cronin effect)!!

d-Au results rule out initial-state effects as the explanation for Suppression at Central Rapidity and high pT

Initial State Effects Only

Initial + Final

State Effects

PHENIX (d+Au) hep-ex/0306021submitted to PRL

PHOBOS, R. Nouicer, this conference

BRAHMS, Z. Yin, this conference

39

Azimuthal distributions

pedestal and flow subtracted

Near-side: p+p, d+Au, Au+Au similarBack-to-back: Au+Au strongly suppressed relative to p+p and d+Au

Suppression of the back-to-back correlation in central Au+Au is a final-state effect

40

High pT Measurements at RHIC

d+Au collisions:• No suppression at high pT

• Away-side jet strength consistent with p+p collisions

Peripheral Au+Au collisions:• Hadron yields consistent with Nbinary-scaled yields in p+p

collisions

• Away-side jet strength consistent with p+p collisions

Central Au+Au collisions:• Hadrons are suppressed at high pT (up to 10 GeV/c)

• Away-side jet disappears

Particle Composition in Central Au+Au collisions: What is happening with the protons?

41

Particle Species Dependence of High pT Suppression

No apparent proton suppression for 2-4 GeV/c – different production mechanism ?

PHENIX, nucl-ex/0305036

(Similar effect seen in STAR for vs. Kshort suppression)

peripheralbinaryperipheral

centralbinarycentral

NYield

NYield

//

42

Particle Composition at High pT

• p/ < 0.25 expected from jet fragmentation• observed p/ ~ 0.4 in peripheral, ~ 1 in central

– protons from non-fragmentation sources ?

nucl-ex/0305036

43

Summary

Physics highlights:

• Strong collective expansion at SPS and RHIC

• Evidence for early equilibration at RHIC• SPS: * Anomalous J/ suppression * Enhancement of low-mass dileptons• RHIC: * Suppression of high pT particles and disappearance of away-side jet

Very intriguing results. All consistent with QGP

formation

44

Extra Slides

45

Direct Photons (I)

• Evidence for direct photons in central Pb-Pb collisions?

10-20% excess but 1 effect only

• CERES preliminary result: enhancement = 12.4% ± 0.8% (stat) ± 13.5% (syst)

WA98 WA98

46

Direct Photons (II)

• Comparison to scaled pA: similar spectrum but factor of ~2 enhanced yield in Pb-Pb, again ~1 effect.• pQCD underpredicts direct photon yield

WA98WA98

Hydro calculations :Prompt + QGP Mixed phase HG QGP dominates at high pT

Srivastava and Sinha nucl-th/0006018Srivastava and Sinha nucl-th/0006018

47

Direct Photons Direct Photons:– Photons not originating

from hadron decays like 0

all

direct+

decay

• Direct photon signal seen in Pb+Pb at s

NN=17.3 GeV

• Stronger signal expected at RHIC, because 0 suppressed by factor 5– Suppression appears to

be a final state effect– Direct photons not

affected by final state interactions

pQCD calculation for direct and 0

in p+p at s=200 GeV(Werner Vogelsang):

48

Direct Photon Search

• Au+Au at sNN

= 200 GeV

• No direct photon signal seen within errors

• With further analysis systematic errors will be reduced ...

preliminarypreliminary

preliminary

49

Azimuthal asymmtery (v2) at high pT

STAR

Finite v2 up pT ~ 10 GeV

Hydrodynamics up to pT ~ 2-3 GeV

Jets correlated to reaction plane?

50

Neutral Pion Production in central and peripheral Au+Au collisions

• reference p+p data with same detector

• binary scaling in peripheral Au+Au

• suppression factor

~ 5 in central Au+Au

Binary scaling

Participant scaling

×1/5

0 at sNN = 200 GeVnucl-ex/0304022, submitted to PRL

pp

AuAubinaryAuAuAA Yield

NYieldR

/

51

Particle Spectra Evolution“Peripheral

”

Particle

Physics

“Central”

Nuclear

Physics

52

Centrality Dependence

• Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control.

• High pT hadron suppression in AuAu is due to a final state effect.

“PHENIX Preliminary” results on centrality-dependence consistent with PHOBOS data

Au + Au Experiment d + Au Experiment

53

What might all this mean?

?

Conjecture: core of reaction volume is opaque to jets

surface emission

Consequences: near-side fragmentation independent of system suppression of back-to-back jets suppression of inclusive rates strong elliptic flow at high pT

Compelling picture, but is it right?

54

J/ suppression: Evidence of deconfinement?

PLB 477 (2000) 28

NA50 preliminaryNA50 preliminary

L. Ramello, QM 2002

melting of charmonium states: c (binding energy 250 MeV)

and J/ (650 MeV)

55

Jet correlations: Au+Au vs. p+pSTAR PRL 90, 082302 (2003)

22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v

Back-to-back jets are suppressed in central collisions!

near side

away side

peripheral central

Peripheral Au + Au

Central Au + Au

56

Centrality Determination

For example, in PHENIX:

Use combination of • Zero Degree

Calorimeters• Beam-Beam

Counters(sensitive to 92% of geom)

to define centrality classes• Glauber modeling

to extract N-participants

0-5%

10-15%15-20%

5-10%

PHENIX

57

Centrality Dependence: Comparison to Models

Saturation models reproduce the scaling with centrality and energy dependence!

dN

ch/d/

(0.5

Np

art)

Kharzeev & Levin, nucl-th/0108006Schaffner-Bielich et al, nucl-th/0108048

58

- Centrality Dependence of Pion Suppression -

• smooth increase of suppression with centrality

• neither binary or participant scaling

0 at sNN = 200 GeVnucl-ex/0304022, submitted to PRL

59

The SPS Experiments• 1986 - 1987 : Oxygen @ 60 & 200 GeV/nucleon• 1987 - 1992 : Sulphur @ 200 GeV/nucleon• 1994 - 2000 : Lead @ 40, 80 & 158 GeV/nucleon• 2002 - 2003 : Indium and Lead @ 158 GeV/nucleon

And proton beams for pp and pA reference

studies

NA35 NA36

NA49

NA34/2HELIOS2

NA34/3HELIOS3

NA44

NA45CERES

NA38

NA50

NA60

WA80

WA98

WA85

WA97

NA57

NA52

WA94SO

Pb

multistrangephotonshadrons

dimuons

dielectrons

1986

1994

2000

hadrons

strangeletshadrons

hadronsdimuons

1992

2003

Carlos Lourenco QM01Carlos Lourenco QM01

60

Color Glass CondensateAlternate Explanation

• Nucleons contain many low x partons.

• At some scale, and particular to relativistically contracted nuclei, gluons will saturate phase space and essentially cancel.

• Jets are not quenched, but are apriority made in fewer numbers.

Color Glass Condensate hep-ph/0210033

Gribov, Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu

High x

Low x