Exploring superdense matter at RHIC

Goals of experiments at RHIC

Collide Au + Au ions at high energy130 GeV/nucleon c.m. energy in 2000s = 200 GeV/nucleon in 2001

Achieve highest possible temperature and densityas existed ~1 sec after the Big Banginter-hadron distances comparable to

that in neutron starsheavy ions to achieve maximum volume

Study the hot, dense matter do the nuclei dissolve into a quark gluon

plasma?do partons/hadrons thermalize?characteristics of the phase transition?transport properties of the quark gluon

plasma? equation of state?

Use RHIC to study QCD

Hadron properties governed by QCDforce between quarks: exchange of

colored gluons

How does confinement work? What are the properties of deconfined

matter?

QCD is non-abelian:gluons can interact with gluonscalculations challenging

at short distance:force is weak(probe w/ high Q2, perturbative)

at large distance:force is strong(probe w/ low Q2,non-perturbative)

Deconfinement temperature, energy density?

QCD on the lattice predicts:

Karsch, Laermann, Peikert ‘99

/T4

T/Tc

Tc ~ 170 ± 10 MeV (1012 °K)

~ 3 GeV/fm3

101044 gluons, q, q’s gluons, q, q’s

Initial collision probability given by nuclear structure functionsfollowed by parton cascade

Evolution of a heavy ion collision

Experiments ask:did something new happen?

Collision dynamics (via hadronic final state)

Probe the early (hot) phase

Equilibrium?hadron spectra, yields

Collective behaviori.e. pressure and expansion?elliptic, radial flow

matter box

vacuum

QGP

Particles created earlyin predictable quantityinteract differently withQGP and normal matterfast quarks, J/fast quarks, J/, strange, strangequark content, quark content, thermal radiation

RHIC at Brookhaven National Laboratory

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

4 complementary experiments

STAR

Address via experiment:

Temperatureearly in the collision during plasma phase

Densityalso early in the collision, at maximum

Are the quarks confined or in a plasma?Use probes of the medium to investigate

Properties of the quark gluon plasma:equation of state (energy vs. pressure)how is energy transported in the plasma?

Density: a first look

summing particles under the curve, find ~ 5000 charged particles in collision final state

initial volume ~ Vnucleus

(~ longitudinal velocity)

Central Au+Aucollisions

Is energy density high enough?

4.6 GeV/fm3

YES - well above predicted transition!50% higher than seen before

PRL87, 052301 (2001)

dy

dE

cRT

Bj 22

11

02

R2

2c

Colliding system expands: Energy tobeam direction

per unitvelocity || to beam

elliptic flow as “barometer”

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

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

y2 x2 y2 x2

2cos2 v

x

y

p

patan

Almond shape overlap region in coordinate space

Large v2: the matter can be modeled by hydrodynamics

STARPRL 86 (2001) 402

Hydro. CalculationsHuovinen, P. Kolb and U. Heinz

v2 = 6%: larger than at CERN or AGS!

pressure buildup explosionpressure generated early! early equilibration !first hydrodynamic behavior seen

mT2 = pT

2 + m02

mT - m0 = transverse kinetic energy

Protons are flatter velocity boost

charged hadron spectra

Many high pt baryons!

hydrodynamicalcalculation agrees with data

Teaney, Lauret, Shuryaknucl-th/0110037

nucl-ex/0203015

As many baryons as pions at pT> 2 GeV/c

Conditions in hadronic phase at RHIC

Collisions atRHIC approachzero net baryondensity

¯

_

s

B---B

Braun-Munzinger, Magestro, Redlich, Stachel, hep-ph/0105229

Tch = 175 MeVB = 51 MeV

Analyze with Grand Canonical Ensemble: fit particle ratios for mB, T

pde

pde

p TE

TE

B 3)(

3

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

At the time of chemical equilibrium amonghadrons

Mystery #1

How come hydrodynamics does so well on elliptic flow and momentum spectra of mesons & nucleons emitted

… but FAILS to explain correlations between meson PAIRS?

pT (GeV)

Possible explanations:non-uniform particle density distribution!(i.e. Hydrodynamics is not explosive enough

middle not depopulated)Shape of correlation function different at RHIC

Hard scattered partons as probeof early collision stage

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet production

Probe: Jets from hard scattered quarks

Observed via fast leading particles orazimuthal correlations between the leadingparticles

But, before they create jets, the scatteredquarks radiate energy (~ GeV/fm) in thecolored medium

decreases their momentum fewer high momentum particles beam “jet quenching”

hadron pT spectra

PHENIX data STAR data

Baseline: inclusive pt distribution in p+p collision

Fit power law: pp =

d2N/dpt2 = A (p0+pt)

-n

Should be dominatedby leading hadronsfrom jets

Both h & 0 below p+p

Peripheral (60-80% of geom):

<N binary collisions> = 20 6

central (0-10%): <N bin coll> = 905 96

PRL 88, 022301 (2002)

Jet quenching in central Au + Au collisions?

Charged deficit seen by both STAR & PHENIX

charged

central coll central

pp

/Yield N

Yield

transverse momentum (GeV/c)

Phys. Rev. Lett. 88, 022301 (2002)

STARpreliminary

0 loweras h ½ baryons

A closer look at high pT

PHENIX preliminaryYield scales with Nbin.coll?

NO

Yield scales with Npart?

high pT : should be from hardprocesses, but see scaling with # of binaryNN collisions decrease with increasingcollision centrality (quenching effect!?)

Can we confirm jets?

STAR preliminary

Correlation of 4 GeV/c trigger hadronWith particle of pT > 2 GeV/c(v2 effect removed)

= 0.27 0.9 rad (as for jets in pp)

How much energy loss at RHIC?

<dE/dx> = 0.25 GeV/fm

scaled pp

shadowing +initial mult. scattering

but we know system is not static!

With expansion:<dE/dx> 7.3 GeVfor 10 GeV/c jets

X.N. Wang & E. Wang, hep-ph/0202105

energyloss

EM probes at RHIC

PHENIX looks for J/ e+e- and

There is the electron.

A needle in a haystack

must find electron without mistaking a pion for an electron at the level of one in 10,000

Ring ImagingCherenkovcounter to tagthe electrons“RICH”

See cherenkovlight in CO2

vpart. > cmedium

We do find the electrons

Energy/Momentum

PHENIX sees some “extra” electrons

they come from charm quarks c D meson

e + K +

J/ analysis is underway now

0 ee

ee, ee

0ee, 3

0ee, ee

conversion

ee

ee

Electron enriched sample (using RICH)

All tracks p=0.8-0.9 GeV

Mystery #2

If jets from light quarks are quenched, shouldn’t charmed quarks be suppressed too?

nucl-ex/0202002

Theorists: yes (some), no (others)Enhancement balanced by e loss?

Unprecedented energy density! > crit

Early thermalizationvery explosive collisions matter at early

time has a stiff equation of statehydrodynamics works (mostly)

Chemical equilibration with Tch ~ Tc

Probe early phase with hard partonssee a deficit energy loss!

Some mysteriesHydro misses 2 particle correlationsNo energy loss by c,cbar quarksJ/ to come (from higher L data)

QGP? Most likely… pA reference needed

Conclusions

Gluon saturation at RHIC?

Wavefunction of low x partons overlap and the self-coupling gluons fuse,

thus saturating the density of gluons in the initial state

treat as classical field!

In nucleus rest frame

r/ggg

The saturation scale:

pT2 ~ sNc 1/A2/3 dNg/dy (G(x,pT

2))

(A, b dependent) mT scaling of hadrons & suppressed gluon jet production

expect saturation effects at higher x than at HERA effect present in initial state at RHIC?

1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987).

Venugopalan, McLerran, Kharzeev, etc.

What’s next?

To rule out conventional explanations extend reach of Au+Au data measure p+p reference p+Au to check effect of cold nuclei on

observables study volume & energy dependence

• are jets quenched & J/ suppressed???

Measure momentum & flight time;calculate particle mass

(dE/dx) = .08 dE/dx

pions

e

kaons

protons

STARSTAR

or measuremomentum+ energy loss in gas detector

also

Identify hadrons

PHENIX measures 0 in PbSc and PbGl calorimeters

0’spT >2 GeV, asym<0.8

in PbSc

excellentagreement!

PRL 88, 022301 (2002)

J/ suppression observed at CERN

Fewer J/ in Pb+Pb than expected!

But other processes affect J/ tooso interpretation is still debated...

J/yield

Something new at RHIC?

Compare to a baseline, or controluse nucleon-nucleon collisions at the same energy

To zero’th order Au + Au collisionsa superpositionof N-N reactions(modulo effect ofnuclear binding andcollective excitations)

Hard scattering processes scale asnumber of N-N binary collisions <Nbinary>

so expect: YieldA-A = YieldN-N . <Nbinary>

nucleons

PHENIX at RHIC

2 Central spectrometers2 Forward spectrometers3 Global detectors

Philosophy: optimize for signals / sample soft physics

Thermal Properties

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

measuring the thermal history

, e+e-,

+Kpn

d,Real and virtual photons from quark scattering is most sensitive to the early stages. (Run II measurement)

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

Hydrodynamic flow is sensitive to the entire thermal history, in particular the early high pressure stages.

Known effects

tppA ppA

X.N.Wang, nucl-th/0104031

pA and AA data at lower energy showexcess above unity:“Cronin effect”

(multiple scattering)

From compilation of X.N. Wang

RA

A(p

T)

parton energy loss, if any, is overwhelmed by initial state soft multiple scattering!

In Pb + Pb at CERN

Crossing at ~ 1.5 GeV/c

Is SPS-RHIC comparison fair?

Same pt implies different x!

RHIC

if pT(had) / pT(jet) ~ 1 then xT ~ x(parton) at y=0

xT =

Nuclear shadowing at RHIC?Zheng Huang, Hung Jung Lu, Ina Sarcevic: Nucl.Phys.A637:79-106,1998 (hep-ph/9705250 )

Shadowingof structure functions small in RHIC x range!!

Gluon shadowingshould be even less

pt comparison OKdeficit shadowing!

quark structure function

Effect of flow + quenching?

hydro boosts baryons to higher pT

Jet quenching should reduce yield (by ~3-5)baryons less depleted as less likely to be

leading particles in fragmenting jet

Vitev & Gyulassy Phys. Rev. C65 (2002) 041902 pbar/

pi-

Correlations at high pT

jet correlations weak or missing!Reaction plane results a mystery...

Hydrodynamicsno longer dominates

Correlation method on HIJING picks out back-to-back particles from jets

For datacorrelation & reaction plane methods agree

J. Rak