On the trail of the quark-gluon plasma at RHIC

outline

Why collide heavy ions?QCD and the phase transition

the Relativistic Heavy Ion Collider + experiments

What have we learned so far?Thermalization & pressure build up – early!(medium-induced) modification of jetsThe control experiment: d+AuMedium effects on fragmentation function?

Conclusions

The Physics of RHIC

Create very high temperature and density matteras existed ~1 sec after the Big Banginter-hadron distances comparable to that in neutron starscollide heavy ions to achieve maximum volume

Study the hot, dense mediumis thermal equilibrium reached?transport properties? equation of state?do the nuclei dissolve into a quark gluon plasma?

Collide Au + Au ions at high energys = 200 GeV/nucleon pair, p+p and d+A to compareAlso polarized p+p collisions to study carriers of p’s spin

Quantum ChromoDynamics

Strong interaction field theory : colored quarks exchange gluons Parallels QED but gluons have color charge

unlike E&M where are uncharged they interact among themselves (i.e. theory is non-abelian):

curious properties

+ +…

short distance:force is weak (probe w/ high Q2, Calculate with perturbation theory)large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) leads to confinementHigh temperature: force becomes screened by produced color-charges

QCD Phase Transition

Basic (i.e. hard) questionshow does process of quark confinement work?how nature breaks symmetries massive particles from ~

massless quarks transition affects evolution of early universe

latent heat & surface tension matter inhomogeneity in evolving universeequation of state compression in stellar explosions

s = 200 GeV: start with pQCD & pp collisions

p-p hep-ex/0304038

Good agreementwith NLO pQCD

Works!A handle on initial NN interactions

also need:2

/( , )

a Nf x Q

2

/( , )ch a

D z Q

Parton distribution functions

Fragmentation functions

Xc(A)

pQCD

BFKL, DGLAP

G-sat.

>2

RHIC

Log Q2

-Log10 x

0

In A+A: QCD in non-perturbative regime

T/Tc

Karsch, Laermann, Peikert ‘99

/T4

Tc ~ 170 ± 10 MeV (1012 °K)

~ 3 GeV/fm3

But, we look for physics beyond simple superposition of NN:

EquilibrationCollective effectsEnergy, color transport in dense mediumDeconfinement?

EOS

Lattice…

Physics is softLattice QCD says:Create these conditions to look for new physics

Experimental approach

Central region has max temperature & density

Head-on = “central” collisions max volume

pT

Thermalization? particle spectra, yieldsPressure developed? particle/energy flowsMedium properties? effects upon probe particlesDeconfinement? c and anti-c remain bound as J/?

RHIC at Brookhaven National Laboratory

RHIC is first dedicated heavy ion collider10 times the energy previously available!

4 complementary experiments

STAR

Is the energy density high enough?PRL87, 052301 (2001)

R2

2c

Colliding system expands:

dy

dE

cRT

Bj 22

11

02

Energy tobeam direction

per unitvelocity || to beam

4.6 GeV/fm3 (130 GeV Au+Au)

5.5 GeV/fm3 (200 GeV Au+Au)well above predicted transition!

history of heavy ion collisions

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

, e+e-, + Kpnd,

Real and virtual photons from q scattering sensitive to the early stages. Probe also with q and g produced early, & passing through the medium on their way out.

Hadrons reflect medium properties when inelastic collisions stop (chemical freeze-out).

high , pressure builds up

Particle production (lots!)

Central Au+Aucollisions

(~ longitudinal velocity)

sum particles under the curve,

find ~ 5000 charged

particles in collision final state

(6200 in 200 GeV/A central

Au+Au)

In initial volume ~ Vnucleus

Rescattering should be important!

Hadron pT spectra – all 4 experiments!

BRAHMS: 10% centralPHOBOS: 10%PHENIX: 5%STAR: 5%

200 GeV/A Au+Au

Protons show velocity boost to beam.Expect if pressure build-up due to rescattering

Data well fit with: Tfo = 110-120 MeV & <t> = 0.5-0.6

Simple quark counting:K-/K+

= exp(2s/T)exp(-2q/T)

= exp(2s/T)(pbar/p)1/3

= (pbar/p)1/3

local strangeness conservation K-/K+=(pbar/p)

= 0.24±0.02 BRAHMS = 0.20±0.01 for SPS

Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV

At y=0

From y=0 to 3

PRL 90 102301 Mar. 2003

Evidence for equilibrated final hadronic stateBRAHMS

More evidence for equilibrated final state

Observed hadron ratios in agreement with thermal ratios!T(chemical freeze-out) ~ 175 MeV

Early state? a barometer called “elliptic flow”

Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy momentum anisotropy

v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

Almond shape overlap region in coordinate space

y2 x2 y2 x2

2cos2 vx

y

p

patan

v2 measured by the experiments

STAR

v2=0.05

130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.04-part cumulants

200 GeV: 0.2< pt < 2.0

Preliminary

200 GeV: Preliminary

- Consistent results- At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

STARPreliminary

v2 predicted by hydrodynamics

STARPRL 86 (2001) 402

Hydro. CalculationsHuovinen, P. Kolb,U. Heinz

pressure buildup explosionhappens fast early equilibration !

Hydro can reproduce magnitudeof elliptic flow for , p. BUTmust add QGP to hadronic EOS!!

Similar conclusion reached byKo, Kapusta, Bleicher, others… rescattering must be very large!

central

a unique probe for physics of hot medium

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet productionProbe: Jets from hard scattered quarks

Observed via fast leading particles orazimuthal correlations between the leadingparticles

But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium

decreases their momentum (fewer high pT particles)“kills” jet partner on other side “jet quenching”

Nuclear Modification of Hadron Spectra?

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

<Nbinary>/inelp+p

nucleon-nucleon cross section

1. Compare Au+Au to nucleon-nucleon cross sections2. Compare Au+Au central/peripheral

Nuclear Modification Factor:

If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT

AA

AA

AA

pp

AuAubinaryAuAuAA Yield

NYieldR

/

2/pp

AuAupartAuAupartAA Yield

NYieldR

/

Au-Au s = 200 GeV: high pT suppression!

nucl-ex/0304022

Au-Au nucl-ex/0304022

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

Central Au + Au

Peripheral Au + Au

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

Suppression: a final state effect?

Hadronic absorption of fragments: Gallmeister, et al. PRC67,044905(2003)Fragments formed inside hadronic medium

Parton recombination (up to moderate pT)Fries, Muller, Nonaka, Bass nucl-th/0301078Lin & Ko, PRL89,202302(2002)

Energy loss of partons in dense matterGyulassy, Wang, Vitev, Baier, Wiedemann…

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Hadron gas

1AuAuR Absent in d+Au collisions!d+Au is the “control” experiment

Not technically quite a Final state effect…

Suppression: an initial state effect?

Gluon Saturation (color glass condensate)

Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of

gluons in the initial state. (gets Nch right!)

• Multiple elastic scatterings (Cronin effect) Wang, Kopeliovich, Levai, Accardi

• Nuclear shadowing

Gribov, Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan,

Balitsky, Kovchegov, Kovner, Iancu …

probe rest frame

r/ggg

dAu AuAuR R RdAu~ 0.5D.Kharzeev et al., hep-ph/0210033

1dAuR

decreases dAuR

Broaden pT :

PHENIX Preliminary 0

PHOBOS Preliminary

STAR Preliminary

Experiments show NO suppression in d+Au!

Do see Cronin effect!

“Cronin” enhancement more pronounced in the charged hadron measurement

Possibly larger effect in protons at mid pT

Implication of RdAu? RHIC at too high x for gluon saturation…

(h++h-)/2

0

0 RAA vs. predictions

PHENIX Preliminary

shadowing

anti-shadowing

Theoretical predictions:

d+Au: I. Vitev, nucl-th/0302002 and private communication.

Au+Au: I. Vitev and M. Gyulassy, hep-ph/0208108, to appear in Nucl. Phys. A; M. Gyulassy, P. Levai and I. Vitev, Nucl. Phys. B 594, p. 371 (2001).

Initial state: mult. scatt.,shadowing + final state dE/dx (Au+Au)

Also: Kopeliovich, et al (PRL88, 232303,2002)

predict RpA~1.1 max at pT=2.5 GeV projectile as color dipole

Centrality Dependence

Dramatically different and opposite centrality evolution of AuAu experiment from dAu control.

Jet Suppression is clearly a final state effect.

PHENIX Preliminary results, consistent with PHOBOS data in submitted paper

Au + Au Experiment d + Au Control

Back-to-back jets observed in d+Au

• jet pair production also looks independent of Ncoll

• observe no (big) suppression in back-to back jets!

• probably some jet broadening due to initial multiple scattering…

ST

STAR

PHENIX preliminary

Particle mix at RHIC is different!

p/ ~1 at high pT

in central collisionsHigher than in p+por jets in e+e-collisions

Hydro. expansion at low pT

+ jet quenching at high pT?

Medium modified fragmentationfunction?

Vitev&Gyulassy nucl-th/0104066

Do the baryons scale with Ncoll?

hard/soft process interplay? Quark recombination?Medium modification of fragmentation function?

Baryon yields not suppresed Ncoll at pT = 2 – 4 GeV/c

Au+Au

To help sort it out, study initial state effects

=

d+Au PHENIX preliminary

low pT

high pT What is initial state multiplescattering mechanism?

How is production of mesons and baryons affected? fragmentation function…

0-20%most central Ncoll=779

20-40%most central Ncoll=296

40-90%most central Ncoll=45

Proton

R.L. Thews, M. Schroedter, J. Rafelski Phys. Rev. C63 054905 (2001): Plasma coalesence modelfor T=400MeV and ycharm=1.0,2.0, 3.0 and 4.0.

L. Grandchamp, R. Rapp Nucl.

Phys. A&09, 415 (2002) and Phys. Lett. B 523, 50 (2001):Nuclear Absorption+ absoption in a high temperature quark gluon plasma

A. Andronic et. Al. Nucl-th/0303036

Deconfinement? Does colored medium screen c+cbar?

Can’t tell yet, but don’t see EXTRA (thermal) J/

Look at J/

conclusions

Rapid equilibration! Strong pressure gradients, hydrodynamics worksConstituent scattering cross section is very large

EOS is not hadronic The hot matter is “sticky” – it absorbs energy

See energy loss, disappearance of back-to-back jetsd+Au data says: final state, not initial state effect

So, the stuff is dense, hot, ~ equilibrated AND NEW!

OK, why not announce QGP discovery?J/ suppression or not? Next runTinitial? direct photon analysis underway by PHENIXProperties not as expect for plasma – looks like gluon liquid

A few mysteries…

Hydro describes single + multi-particles

• How to increase R without increasing Rout/Rside???

EOS?initial T & r profiles? emissivity?

But FAILS to reproducetwo-particle correlations!

Elliptic flow of high momentum particles

pT (GeV/c)

v2baryons cross mesons(not expected from hydro)

Still flowing at pT = 8 GeV/c? Geometry? v2 a bit too big…Mix of flow + jets???

Total charm quark cross section at RHIC

Cross section fits into expected energy dependenceNo evidence for strong energy loss of charmed quarks…

Centrality dependence of charm quarks

Compare themeasurement to (PYTHIA) an event generator tuned for pp collisions…

no largesuppressionas for lightquarks!

Spectra of electrons from c e + anything

Why no big energy loss for heavy quarks?

no x4 suppressionfrom peripheral to central,as predicted fordE/dx=-0.5GeV/fm!

But (we squirm) - Is 40-70% peripheral enough? error bars still big!

is d+Au same as p+Au?

p

n ZDC

Neutron tagged eventsenhance peripheral collisions

<Ncoll> = 5.0 / 3.6Could be Ncoll dependenced+Au looks very similar to p+Au

Particle Composition at high pT

(h++h-)/20 ~ 50% greater in central than peripheral at mid pT

similar again for pT>5 GeV/c

Central

Peripheral

Run 2001/2002 Au-Au 200 GeV:s

Locate RHIC on phase diagram

Baryonic Potential B [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-out

deconfinementchiral restauration

Lattice QCD

atomic nuclei

fit yields vs. mass (grand canonical ensemble)

Tch = 175 MeV B = 51 MeV

These are the conditions when hadrons stop interacting

T

Observed particles “freeze out” at/near the deconfinement boundary!

Au+Au at sNN=200GeVv2 of mesons & baryons to higher pT

Consistent between PHENIX and STAR

pT < 2 GeV/c v2(light) > v2(heavy) Explained by hydro. expansion

pT > 2.5 GeV/c v2(light) < v2(heavy) Can it be explained by some

combination of geometry + jet quenching?

Quark coalescence?S. Voloshin, nucl-ex/0210014R. Fries et al., nucl-th/0301087D. Molnar et al. nucl-th/0302014

In-medium fragmentation fn?

Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001)

QCD Phase Transition

Basic (i.e. hard) questionshow does process of quark confinement work?how nature breaks symmetries massive particles from ~

massless quarks transition affects evolution of early universe

latent heat & surface tension matter inhomogeneity in evolving universeequation of state compression in stellar explosions

J/ suppression was observed at CERN at s=18 GeV/A

Fewer J/ in Pb+Pb than expected!Interpret as color screening of c-cbar

by the mediumInitial state processes affect J/ tooso interpretation is still debated...

collaboration

J/yield

Adler et al., nucl-ex/0206006

A puzzle at high pT

Still flowing at pT = 8 GeV/c? Unlikely!!

Nu Xu

Vitev: they can get v2 right

C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006

K. Filimonov [STAR Collab.],arXiv: nucl-ex/0210027

b=7 fmb~7 fm

• There is a quantitative difference Calculations/fits with flat or continuously growing

2 .v const 2 / .ln Tv p

Check against high-pT data (200 AGeV)

Same for 0-50%

• The decrease with pT is now supported by data• For minimum bias this rate is slightly slower

See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)

Npart/2

Nbinary

PHENIX 130

BRAHMSPRL88(02)

? 2003 ?

STAR 130

hch

15% too many particles, baryons over-quenched, but predicted the suppressionBUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?

kT dependence of R

Centrality is in top 30%

•Broad <kT> range : 0.2 - 1.2 GeV/c •All R parameters decrease as a function of kT consistent with collective expansion picture. • Stronger kT dependent in Rlong have been observed.

kT : average momentum of pair

Comparison of kaon to pion

In the most 30% central

Comparison with hydrodynamic model

Recent hydrodynamic calculation by U.Heinz and P. F. Kolb(hep-ph/0204061)

kT dependence of Rlong indicates the early freeze-out?

Hydro w/o FS

Hydro at ecrit

• Assuming freeze out directly at the hadronization point. (edec = ecrit)

• Standard initialization and freeze out which reproduce single particle spectra.

Centrality is in top 30%

kT dependence of Rout/Rside

A. EnikizonoQM2002

C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV:

.)(25.009.016.1 syst @0.25 GeV/c

HBT PUZZLE

P.Kolb

Small Rout implies small

Large Rside implies large RSmall Rbeam impliessmall breakup ~10 fm/c

Jet Evidence in Azimuthal Correlations at RHIC

near-side correlation of charged tracks (STAR)trigger particle pT = 4-6 GeV/c distribution for pT > 2 GeV/c

signature of jets

also seen in (0) triggered events (PHENIX)trigger particle pT > 2.5 GeV/c distribution for pT = 2-4 GeV/c

M. Chiu, PHENIX Parallel Saturday

QM2002 summary slide (Peitzmann)

Identifying Jets - Angular Correlations

Remove soft background by subtraction of mixed event distribution

Fit remainder:Jet correlation in ; shape taken from PYTHIAAdditional v2 component to correct flow effects

PHENIX Preliminaryraw differential yields

2-4 GeV

Verify PYTHIA using p+p collisions

(neutral E>2.5 GeV + 1-2 GeV/c charged partner)

||<.35 ||>.35

ake cuts in to enhance near or far-side correlationsBlue = PYTHIA

In Au+Au collisions

1-2 GeV partner

(neutral E>2.5 GeV + charged partner)

||<.35 ||>.35

1/N

trig d

N/d

1/N

trig d

N/d

Correlation after mixed event background subtraction

Clear jet signal in Au + AuDifferent away side effect than in p+p

Jet strengthSee non-zero jet strength as partner pT increases!

jets or flow correlations? fit pythia + 2v2vjcos(2)

partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c

1/N

trig d

N/d

v2

vj

1-2 GeV/c

How many particles are produced?

dNch/d = 640

Rises somewhat faster than Npart

Charged hadron correlations - small

•Fit charged correlations with v2 + Gaussian (fixed pT)•Jet signal visible via

Width of near-side Gaussian decreases with pT

No significant centrality dependence on near-side

Cor

rela

tion

wid

th

jT

pT Correlation width jT/pT

How do high pT yields scale?

vs. binary collisions:continuous decrease as

function of centralityfactor ~ 3.5 from

peripheral to central vs. participants:

first increase, then decrease as function of centrality

for Npart > 100 have 3 change (scaling or no?)

surface emission? re-interactions?accident?

18% scaling uncertainty from corrections

Opaque, expanding source would mean:

2222222 2)()( xtso YXRR

)(outX

)(sideY

29.13

5)(

)(

spheres

shellhalfs

R

R

65.012

5)(

)(

sphereo

shellhalfo

R

R

Opaque Expanding

Rischke RIKEN workshop (2002): Such strong xt correlations probably require a lack of boost-invariance...

Energy Dependence

Assumptions:in Lab in C.M.

Energy density (Bjorken):

2% most central at sNN=200 GeV:

5.5 GeV/fm3

From AGS, SPS to RHIC:

Transverse energy and charged particle multiplicity densities per participant consistent with logarithmic behaviour

d

dX

dy

dX

d

dX

dy

dX2.1

dy

dE

Rt

2

1

cfm

AfmR

/1

18.1 3/1

PHENIX preliminary

PHENIX preliminary

So, is there jet quenching?

Suppression observed to 8 GeV/c! (in 3 independent measurements)

Theory agrees with data when quark, gluon energy loss is included

NB: 2 examples here, others also must add some kind of medium modification of the fast quarks/gluons

In initial or final state?

Look at “transverse mass” mT2 = pT

2 + m02

— is distribution e-E/T?i.e. Boltzmann distribution from thermalized gas?

hadron spectra: , K, p and antiprotons

130GeV/A

yes !

Protons are flatter velocity boost to beamResult of pressure built up

Spectral shapes

<pT> for radially expanding hadrongas with Tth and <>

<pT> in pp with “Tch” = 170 MeV and <>=0pp no rescattering, flow or equilibrium

STAR

preliminaryF. Wang

min bias 200 GeV Au+ Au

v2 at high pT

Note pbar/p behavior

Centrality dependence only for pT > 3 GeV/c

Peripheral collisions have quite a few protons at mid-y

Anti-particle/particle ratios vs. rapidity

At y=0 (central coll.)

pbar/p = 0.75 ±0.04

K-/K+ = 0.95 ±0.05 = 1.01 ±0.04

Nearly baryon-free at central y, but not complete transparency

Larger contribution of protons nearer rapidity of the Au beams

PRL 90 102301 (Mar. 2003)

BRAHMS