A new kind of matter at the Relativistic Heavy Ion Collider?

The Physics of Heavy Ion Collisions

Create very high temperature and density mattersimilar to that existing ~1 sec after the Big Bangdistance between hadrons comparable to that in neutron starsstudy only in the lab – relics from Big Bang inaccessible

Goal is to characterize the hot, dense mediumexpect QCD phase transition to quark gluon plasmadoes medium behave as a plasma?find density, temperature, radiation rate, collision frequency,

conductivity, opacity, Debye screening length?probes: passive (radiation) and those created in the collision

Collide Au + Au ions for maximum volumes = 200 GeV/nucleon pair, p+p and d+A to compare

Quantum Chromo Dynamic phase transition

Colored quarks interact by exchange of gluons

Color charge of gluons gluons interact among themselves theory is non-abelian curious properties at large distance: confinement of quarks in hadrons

+ +…

At high temperature and density: force is screened by produced color-chargesexpect transition to free gas of quarks and gluons

non-perturbative QCD - calculate on lattice

T/Tc

Karsch, Laermann, Peikert ‘99

/T4

Tc ~ 170 ± 10 MeV (1012 °K)

~ 3 GeV/fm3

required conditions to study quark gluon plasma

~15% from ideal gas of weakly interacting quarks & gluons

42

30Tg

Where are we?

1039 1045

1010

1012

QGP at RHIC?

1051 1057

345

332

33

104~)(

200~,37

)2

(~2.1

)3()2(

4)(

mTn

MeVTg

TgTg

Tg

Tn

RHIC at Brookhaven National Laboratory

RHIC collides p, Au ions: 200 GeV/nucleon in c.m.10 times the energy previously available!

4 complementary experiments

STAR

pressure builds up

probing early stage of heavy ion collision

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Kpnd,

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

, e+e-, +Real and virtual photons emitted as thermal radiation.

we focus on mid-rapidity (y=0), as it is the CM of colliding system 90° in the lab at collider

Hard scattered or heavyq,g probes of plasma formed

Study simple complex systems: p+p, “p”+A, A+A collisions

p-p PRL 91 (2003) 241803

Good agreementwith NLO pQCD

is QCD the right theory at RHIC? Perturbative for high p transfer processes? pp collisions: it works!

Have a handle on initial NN interactions by scattering of q, g inside N

We also need:2

/( , )

a Nf x Q

2

/( , )ch a

D z Q

Parton distribution functions

Fragmentation functions

0

pQCD in Au+Au? direct photons

[w/ the real suppression]

( pQCD x Ncoll) / background Vogelsang/CTEQ6

[if there were no suppression]

( pQCD x Ncoll) / ( background x Ncoll)

Au+Au 200 GeV/A: 10% most central collisions

[]measured / []background = measured/background

Preliminary

At high pT, it also works!

TOT

pT (GeV/c)

Is the energy density high enough?

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

PRL87, 052301 (2001)

R2

2c

Colliding system expands:

dy

dE

cRT

Bj 22

11

02

Energy tobeam direction

per unitvelocity || to beam

value is lower limit: longitudinal expansion rate, formation time overestimated

Collective effects? Pressure: 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 2cos2 v

x

y

p

patan

y2 x2 y2 x2

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

Hydro. CalculationsHuovinen, P. Kolb,U. Heinz

v2 reproduced by hydrodynamics

STARPRL 86 (2001) 402

• see large pressure buildup • anisotropy happens fast • early equilibration !

central

Hydrodynamics assumes early equilibrationInitial energy density is inputEquation of state from lattice QCDSolve equations of motion

Collective effect probes early phase

Hydrodynamics can reproduce magnitudeof elliptic flow for , p. BUT correct mass dependence requiressofter than hadronic EOS!!

Kolb, et al

NB: these calculations have viscosity = 0medium behaves as an ideal liquid

Pressure felt by all hadrons

v2 scales ~ with # of quarks! evidence for quarks as relevant d.o.f. when pressure built up

Need large for fast equilibration & large v2

Parton cascade using free q,g scattering cross sections underpredicts pressure

Lattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sections

Properties of this stuff?

Pressure built up during collision -> observed collectivityviscosity small, interaction large

Quasiparticles may experience binding, butare not color neutral as at T<Tc.

Do we expect screening?quark & gluon density is high…estimate = <PE>/<KE><PE>=g2/d = g2(~4-6) /(41/3T) <KE> ~ 3T

so plasma parameter

RHIC is in regime of strongly coupled plasmaAs in warm, dense plasma at lower (but still high) TBut strong interaction rather than electromagnetic

“external” probes of the medium

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet productionHard scattering of q,g early.Observe fast leading particles,back-back correlations Before creating hadron jets, scattered quarks induced to radiate energy (~ GeV/fm) by the colored medium-> jet quenching

AA

AA

AA

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

nucleon-nucleon cross section<Nbinary>/inel

p+p

pp

AuAubinaryAuAuAA Yield

NYieldR

/

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

PRL91, 072301(2003)

look for the jet on the other sideSTAR PRL 90, 082302 (2003)

Central Au + Au

Peripheral Au + Au

near side

away side

peripheral central

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

Medium is opaque!

lost energy is thermalized

Away side jet is significantly broadened

preliminary

•recover the energy in hadrons ~ 500 MeV.

• comparable to <pT> of the bulk medium

Au+Au 0-5%

pT > 200 MeV

STAR Preliminary

Property probed: density

Au-Au

d-AudAu

Induced gluon brehmsstrahlung pQCD calculationdepends on number of scatterings

Agreement with data:initial gluon density

dNg/dy ~ 1100

~ 15 GeV/fm3

hydro initial state same

Lowest energy radiation sensitive to infrared cutoff.

*deuteron-gold control experiment with no suppression

A puzzle

p/ ~1 at high pT

in central collisions

> p+p, d+Au> peripheral Au+Au

>jets in e+e-collisions

PRL 91 (2003) 172301

Hydro. expansion at low pT + jet quenching at high pT.

coalescence of boosted quarks into hadrons

enhances mid pT hadrons baryons especially

pQCD spectrum shifted by 2.2 GeV

Teff = 350 MeV

R. Fries, et al

Are extras from the thermal medium?

But jet-like properties too…

Baryons scale

Ncoll !

Mesons, including are suppressed Effect governed by quark content

~RA

A

pions

protons

are baryons from jets or not?

• measure probability of jet-like partner for baryons & mesons in pT range of excess

•decrease for baryons in most central Au+Au

• expected from recombination of purely thermal quarks

Allow fast quark to grab partner from medium

Many baryons ARE from jets, but medium modifies those jets

Summary of medium properties

Extract from models, constrain by jet suppression & v2

Values from I. Vitev; others consistent

Energy loss <dE/dz> (GeV/fm) 7-10 0.5 in cold matter

Energy density (GeV/fm3) 14-20 >5.5 from ET data

dN(gluon)/dy ~1000 200-300 at SPS

T (MeV) 380-400 Experimentally unknown as yet

Equilibration time0 (fm/c) 0.6 Parton cascade agrees

Opacity (L/mean free path) 3.5

Is there quark gluon plasma at RHIC? qualified “yes”Intense debate in community about standard of proof

Evidence is mounting that RHIC creates a strongly coupled, opaque plasma

With aid of hydrodynamic, l-QCD and p-QCD models: ~ 15 GeV/fm3

dNgluon/dy ~ 1000

int large for T < 2-3 Tc Medium modifies hadronization of moderate energy jets Will learn screening properties via cc bound states Are poised to characterize this new kind of plasma

radiation rate, conductivity, collision frequency Color Glass Condensate? Maybe at x ~ 10-3 (y>2)

conclusions

Saturation of gluons in initial state(colored glass condensate)

Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse.

Saturation at higher x at RHIC vs. HERA due to nuclear size

suppressed jet cross section; no back-back pairsr/ggg

Mueller, McLerran, Kharzeev, …

d + Au collisionscent/periph. (~RAA)

Color screening properties?

Lattice predictions for heavy quarks

40-90%most central Ncoll=45

0-20%most central Ncoll=779

20-40%most central Ncoll=296

Data inconclusive, but x50 underway

NB: need temperature too!

PHENIX PRELIMINARY

Open charm: baseline is p+p collisions

fit p+p data to get the baseline for d+Au and Au+Au.

Measure charm via semi-leptonic decay to e+ & e-

, photon conversions are measured and subtracted

Curves are the p+p fit, scaled by the number of binary collisions

No large suppression as for light quarks!

PHENIX PRELIMINARY

Implications of the results for QGP

Ample evidence for equilibration initial dN(gluon)/dy ~ 1000, energy density ~ 15

GeV/fm3, energy loss ~ 7-10 GeV/fm

Very rapid, large pressure build up requiresparton interaction cross sections 50x perturbative

How to get 50 times pQCD ?

• Lattice indicates that hadrons don’t all melt at Tc!c bound at 1.5 Tc Asakawa &

Hatsuda, PRL92, 012001 (2004)

charmonium bound states up to ~ 1.7 Tc Karsch; Asakawa&Hatsuda

, survive as resonances Schaefer & Shuryak, PLB 356 , 147(1995)

q,g have thermal masses at high T. s runs up at T>Tc? (Shuryak and Zahed)would cause strong rescattering

qq meson

spectral function

E. Shuryak

Implications of the results for QGP

Ample evidence for equilibration v2 & jet quenching measurements constrain initial gluon

density, energy density, and energy loss parton interaction cross sections 50x perturbative

parton correlations at T>Tccomplicates cc bound states as deconfinement probes!

Hadronization by coalescence of thermal,flowing quarksv2 & baryon abundances point to quark recombination

as hadronization mechanismJet data imply must also include recombination between

quarks fromjets and the thermalized medium medium modifies jet fragmentation!

Also see evidence for equilibrated final state

Calculate hadron yields in Grand Canonical ensembleObserved hadron ratios in agreement with thermal ratios!T(chemical freeze-out) ~ 175 MeV

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

From fit of yields vs. mass (grand canonical ensemble):

Tch = 176 MeV B = 41 MeV

These are the conditions when hadrons stop interacting

T

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

v2 recombination of flowing quarksnucl-ex/0305013

above p forpT < 2 GeV/c, in agreement with hydrodynamicsThen crosses over.Values ~ saturateat high pT

geometry?

v2/quark ~ constant create hadronsby coalescence of quarks from boosted distribution

The yellow band represents the set of alpha values consistent with the data at the 90% Confidence Level.

No large suppression as for light quarks!

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

How about Color Glass Condensate?

Pt (GeV/c) Pt (GeV/c)

Rda

Rda

Peripheral d+Au (like p+p)

Central: Enhancednot suppressed PHENIX preliminary

y=0

Xc(A)

pQCD

BFKL, DGLAP

G-sat.

>2

RHIC

Log Q2

No CGC signalat mid-rapiditySo, perhaps

But at forward rapidity reach smaller x

y = 3.2 in deuteron direction x 10-3 in Au nucleus

Strong shadowing, maybe even saturation?

d Au

Phenix Preliminary

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!

Why no energy loss for charm quarks?

“dead cone” predicted by Kharzeev and Dokshitzer, Phys. Lett. B519, 199 (1991)

Gluon bremsstrahlung:kT

2 = 2 tform/transverse momentum of radiated gluon

pT in single scatt. mean free path

~ kT / gluon energy But radiation is suppressed below angles 0= Mq/Eq

soft gluon distribution is

dP = sCF/ d/ kT2 dkT

2/(kT2+ 2 0

2) 2not small forheavy quarks!causes a dead cone

Look at “transverse mass” mT2 = pT

2 + m02

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

, K, p, pbar spectra indicate pressure

yes !

Protons are flatter velocity boost to beamResult of pressure built up

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

Au+Au at sNN=200GeVv2 of mesons & baryons

v2

1) High quality M.B. data!!!

2) Consistent between PHENIX and STAR

pT < 2 GeV/c v2(light) > v2(heavy)

pT > 2.5 GeV/c v2(light) < v2(heavy)

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

Study simple complex systems: p+p, “p”+A, A+A collisions

did something new happen at RHIC?

Study collision dynamics (via final state)

Probe the early (hot) phase

Equilibrium?hadron spectra, yields

Collective behavior?i.e. pressure and expansionelliptic, radial flow

vacuum

QGP

Must create probes in the collision itself: predictable quantity, interact differently in QGP vs. hadron matter

fast quarks/gluons, J/fast quarks/gluons, J/, D mesons, D mesonsthermal radiation

we look for physics beyond simple superposition of NNat low momentum/large distance scales:

See jet suppression! a final state effect?

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

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

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

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 :

What about heavy quarks?

J/Test confinement: do bound c + c survive? or does QGP screening kill them?

Open CharmExtra heavy quarks from hot, dense gluon gas?Do the c quarks lose energy like the light quarks?

Need (a lot) more statistics (currently being collected)

But can take a first look…

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 heavily debated...

collaboration

J/yield

Energy loss is initial or final state?

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

Jet Suppression is final state effect of dense medium

Au + Au Experiment d + Au Control

PHENIX preliminary