The Highest Energy Density Matter: Quark Gluon Plasma

The physics of the quark gluon plasma

Energy densities characteristic of the early universeWhy expect a quark gluon plasma?

Experimental study at RHICExperiments and probes of the hot, dense matter

What happens?Energy loss of fast quarks, gluonsCollective motion: hydrodynamic flow

First glimpse of the matter’s properties Underlying degrees of freedom & hadronization Conclusions & open questions

Locate on energy density map

high energy density: > 1011 J/m3

P > 1 MbarI > 3 X 1015W/cm2 Fields > 500 Tesla

QGP energy density > 1 GeV/fm3

i.e. > 1030 J/cm3

Quarks, gluons and hadrons

6 quarks: 2 light (u,d), 1 sort of light (s), 2 heavy (c,b), 1very heavy (t)Besides flavor, also have color quantum number

In normal life: quarks are bound into hadronsBaryons (e.g. n, p) have 3Mesons (e.g. ) have 2 (1 quark + 1 anti-quark)

Colored quarks interact by exchange of gluons (also have color) Quantum Chromo Dynamics (QCD)

Field theory of the strong interaction Parallels Quantum Electrodynamics (QED)

but in EM interactions, exchanged photons electrically uncharged

gluons carry color charge

QCD phase transition

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 – lattice gauge theory

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

So, collide BIG ions at v ~ c

Reach T 170 MeV; > 3 GeV/fm3

Characterize the hot, dense mediumEvidence for QCD phase transition to quark gluon

plasma?does medium behave as a plasma? coupling weak or

strong?What’s the density, temperature, radiation rate,

collision frequency, conductivity, opacity, Debye screening length?

Probespassive (radiation) those created in the collision (“external”)

RHIC at Brookhaven National Laboratory

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

4 complementary experiments

STAR

The Scope of the Tools (!)

STARspecialty: large acceptancemeasurement of hadrons

PHENIXspecialty: rare probes, leptons,

and photons

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 q,g(short wavelength) probes of plasmaformed

So what happens?

Lots of information collected over past 4 years

I’ll focus on a few important results

Start with the “external” probesMade in the first step in the collisionRate and spectrum calculable with (perturbative) QCD look for differences…Start by benchmarking the probe!

Start simple p+p collisions

p-p PRL 91 (2003) 241803

Good agreementwith NLO pQCD

p QCD works for high p transfer processes!

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

g+q +q

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

“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!

Suppression: a final state effect?

1AuAuR

Such a medium is absent in d+Au collisions!But Au provides all initial state effects of q, g

wavefunction inside a Au nucleus

d+Au is the “control” experiment

Hot, dense medium causes

PHENIX Preliminary 0

PHOBOS Preliminary

STAR Preliminary

Experiments show NO suppression in d+Au!

Centrality Dependence

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

Jet Suppression is clearly a final state effect.

Au + Au Experiment d + Au Control

PHENIX preliminary

Are back-to-back jets there in d+Au?

Pedestal&flow subtracted

hadronsleadingparticle suppressed

q

q

?

Yes!

So this is the rightpicture for Au+Au

Collective motion? 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

The data show

Particle emission really is azimuthally anisotropic

Magnitude of the anisotropy grows with beam energy, then flattens

c.m. beam energy

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

So, what do E loss & collectivity tell us?

Medium is “sticky” or opaque to colored probes Thermalization must be very fast (< 1fm/c) Constrain hydrodynamic, energy loss models with data:

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 enough for fast equilibration & large v2 ?

Parton cascade using free q,g scattering cross sections underpredicts pressure must increase x50

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

Plasma coupling parameter?

Very high gluon densityestimate = <PE>/<KE>

using QCD coupling strength<PE>=g2/d d ~1/(41/3T)

<KE> ~ 3Tg2 ~ 4-6 (value runs with T) ~ g2 (41/3T) / 3T so plasma parameter NB: such plasmas known to behave as a liquid!

May have bound q,g states, but not color neutral So the quark gluon plasma is a strongly coupled plasma

As in warm, dense plasma at lower (but still high) TBut strong interaction rather than electromagnetic

Underlying degrees of freedom?

Should be quarks and gluons in the early (QGP) stage

Experimental evidence for this?Look at production of final state particles with

different number of quarksBaryons have 3, mesons have 2

Recall comparison of proton and pion collective flow constrained equation of state

More energetic baryons in Au+Au

p/ ~1 at high pT

in central collisions

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

>jets in e+e-collisions

PRL 91 (2003) 172301

Hydrodynamic expansion common velocity boost

heavier particles boosted to higher pT

enhances mid pT hadrons baryons especially

Do the hadrons get boosted?or

Coalescence of boosted quarks into hadrons?

pQCD spectrum shifted by 2.2 GeV

Teff = 350 MeV

R. Fries, et al

Are extras from the thermal medium?

Elliptic flow scales as number of quarks

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

equilibrated final state, including s quarks

Hadron yields in agreement with Grand Canonical ensembleT(chemical freeze-out) ~ 175 MeV(multi)strange hadrons enhanced over p+p, but QGP

hypothesis not unique explanation

How about J/ suppression?

J/Test confinement:

do bound c + c survive? or does QGP screening kill them?

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

But can take a first look…

Color screening?

Lattice predictions for heavy quarks 40-90%most central Ncoll=45

0-20%most central Ncoll=779

20-40%most central Ncoll=296

This data inconclusive

Evidence is mounting that RHIC creates a strongly coupled, opaque plasma still is debate in community about standard of proof

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

dNgluon/dy ~ 1000

int large for T < 2-3 Tc Equation of state not hadronic! Have evidence that thermalized q,g produce the particles

observed in the final state Will learn screening properties via cc bound states Are poised to characterize this new kind of plasma

T, radiation rate, conductivity, collision frequency

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)

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

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

Charm production scales as Ncoll!

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

No large suppression as for light quarks!

PHENIX PRELIMINARY

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

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!

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!

But baryons show jet-like properties too…

Baryons scale

Ncoll !

follows the mesons Kinematics determined by quark content

~RA

A

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 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

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:

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

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

Predicted signatures of QGP

Debye screening of color charges (Matsui & Satz)Suppresses rate of c-c bound state (J/

Strangeness enhancement (Rafelski & Mueller)Tplasma ~ masss-quark

Enhances strange hadron yields (e.g. K/ increased)

Jet quenching (Gyulassy & Wang)Dense medium induces coherent gluon radiationDecreased yield of jets of hadrons from hard scattered

quarks or gluons

Where does the lost energy go?

Away side jet is significantly broadened

preliminary

• recover the energy in hadrons ~ 500 MeV• comparable to <pT> of the bulk medium• lost energy is thermalized

Au+Au 0-5%

pT > 200 MeV

STAR Preliminary

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

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 Broaden pT :

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.

Evolution of the Universe

Nucleosynthesis builds nuclei up to HeNuclear Force…Nuclear Physics

Universe too hot for electrons to bindE-M…Atomic (Plasma) Physics

E/M Plasma

Too hot for quarks to bind!!!Too hot for quarks to bind!!!Standard Model (N/P) Physics

Quark-Gluon

Plasma??

Too hot for nuclei to bindNuclear/Particle (N/P) Physics Hadron

Gas

SolidLiquidGas

Today’s Cold UniverseGravity…Newtonian/General

Relativity