The Quark Gluon Plasma at RHIC

outline

What’s a plasma?and why do we expect one from quarks and gluons?

The tools to make and study quark gluon plasma What do we see at RHIC?

collective flowopacity of the matterexcitations of the matter?J/ suppression to search for deconfinement

Conclusionswhat we have foundwhat HAVE we found?

what is a plasma?

4th state of matter (after solid, liquid and gas) a plasma is:

ionized gas which is macroscopically neutralexhibits collective effects

interactions among charges of multiple particlesspreads charge out into characteristic (Debye) length, D

multiple particles inside this lengththey screen each other

plasma size > D

“normal” plasmas are electromagnetic (e + ions)quark-gluon plasma interacts via strong interaction

color forces rather than EMexchanged particles: g instead of

Quarks, gluons, hadrons

6 quarks: 2 light (u,d), 1 sort of light (s)2 heavy (c,b), 1very heavy (t)flavor & color quantum numbers

Quarks are bound into hadronsBaryons (e.g. n, p) have 3Mesons (e.g. ): 2 (q + anti-q)

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

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

EM interactions: exchanged photons electrically unchargedgluons 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 gas of free 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

Create high(est!) energy density mattersimilar to that existing ~1 sec after the Big Bangcan study only in the lab – relics from Big Bang inaccessible

T ~ 200-400 MeV (~ 2-4 x 1012 K)~ 5-15 GeV/fm3 (~ 1030 J/cm2)R ~ 10 fm, tlife ~ 10 fm/c (~3 x 10-23 sec)

Characterize the hot, dense mediumdoes medium behave as a plasma? coupling weak or strong?What’s the density, temperature, radiation rate, collision

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

to get there: collide BIG ions at v ~ c

ideal gas or strongly coupled plasma?

Huge gluon density! estimate = <PE>/<KE>

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

<KE> ~ 3Tg2 ~ 4-6 (value runs with T) ~ g2 (41/3T) / 3T

so plasma parameter

quark gluon plasma should be a strongly coupled plasmaAs in warm, dense plasma at lower (but still high) T

how does it compare to interesting EM plasmas?

> 1: strongly coupled, few particles inside Debye radius

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

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

0

look at radiated & “probe” particles

as a function of transverse momentumpT = p sin

with respect to beam direction90° is where the action is (max T, )

midway between the two beams! pT < 1.5 GeV/c

“thermal” particles from the bulk of the plasma

pT > 3 GeV/c

fast particles – mostly part of jets of hadrons coming from hard scattered q or g

produced very early, probe the plasma

search for collectivity (plasma feature)

dN/d ~ 1 + 2 v2(pT) cos (2) + …

“elliptic flow”

Almond shape overlap region in coordinate space

x

yz

momentum space

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 can reproduce magnitudeof elliptic flow for , p. BUT mass dependence →softer than hadronic EOS!!

Kolb, et al

NB: these calculations have viscosity = 0medium behaves as perfect liquid!

Elliptic flow scales as number of quarks

0 1 2 pT/n (GeV/c)

v2 scales ~ with # of quarks! quarks are the particles when the pressure is built up

even charm quarks flow!

measure D→e± + hadrons

Mcharm = 1.3 GeV

D’s flow to ~2 GeV/c

then expect e from B decays (Mb~4 GeV → shouldn’t flow)

collective flows tell us: RHIC creates matter – not just collection of particles the matter catches even the heavy c quarks!

How to actively probe the deep interior?

p

pT

measure “hard scatterings”of q,g at large pT

“transverse momenta”

Direct Photon Spectra in Au+Au

q + g → q + Should not interact

with the color charges

data and theory agree → calibrated probe

pQCD works in the complex environment of two nuclei (Au+Au ) colliding at high energies

peripheralN

coll = 12.3 4.0

centralN

coll = 975 94

strongly interacting probe: a different story!

suppression persists to 20 GeV/c!

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

nuclear modification factorratio of data on previous slide

so we see: photons shine, pions don’t

Direct photons are not inhibited by hot/dense medium Pions (all hadrons) are inhibited by hot/dense medium

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

Central Au + Au

Peripheral Au + Au

Medium is opaque!

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

Pedestal&flow subtracted

Yes!

QGP properties, so far

Extract from models, constrain by dataEnergy loss <dE/dz> (GeV/fm) 7-10 0.5 in cold matter

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

above hadronic e density!

dN(gluon)/dy ~1000 From energy loss, hydro huge!

T (MeV) 380-400

Experimentally unknown as yet

Equilibration time0 (fm/c) 0.6 From hydro initial condition; cascade agrees very fast!

NB: plasma folks have same problem & use same technique

Opacity (L/mean free path) 3.5 Based on energy loss theory

Equation of state? Early degrees of freedom and their ? Deconfinement? Thermalization mechanism? Conductivity?

How to get fast equilibration & large v2 ?

parton cascade using free q,g scattering cross sections doesn’t work! need x50 in medium

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

What is going on?

The objects colliding inside the plasma are not baryons and mesons

The objects colliding also do not seem to be quarks and gluons totally free of the influence of their neighbors

Quarks and gluons are interacting, but are not locally (color) neutral like the baryons & mesons. Neutrality scale likely larger, as expected for a plasma.

strongly coupled gas of atoms

M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. ThomasScience 298 2179 (2002)

weakly coupled

strongly coupled ↓ ellipticflow!

what about the heavier quarks?

e± in Au+Au vs <Ncoll>*p+p

peripheral collisions

central collisions

c quark suppression is nearly as large as for pions!

where does the lost energy go?

transferred to the plasma? does the medium respond?

look at “away side” jet’s particles near thermal pT

M.Miller, QM04

(1/N

trig)d

N/d

()

STAR Preliminary

cGeVp

cGevpassocT

trigT

/42.0

/64

a sonic boom?

g radiates energykick particles in the plasmaaccelerate them along the jet

+/-1.23=1.91,4.37 → cs ~ 0.33 (√0.33 in QGP, 0.2 in hadron gas)

PHENIX

dN

/d(

)

jury is still out…

How about the screening length?

J/Test confinement:

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

RHIC

NA50 and NA60 show suppression in Pb+Pb & In+In

suppression follows system size

Normal nuclear absorption from p+A data: = 4.18±0.35 mb

At CERN (√s = 17 GeV):

At RHIC:

CuCu

200 GeV/c

AuAu

200 GeV/c

dAu

200 GeV/c

J/ muon arm

1.2 < |y| < 2.2

mea

sure

d/e

xpec

ted

At RHIC:

CuCu

200 GeV/c

AuAu

200 GeV/c

dAu

200 GeV/c

AuAuee

200 GeV/c

CuCuee

200 GeV/c

J/ muon arm

1.2 < |y| < 2.2

J/ eeCentral arm

-0.35 < y < 0.35

At RHIC:

CuCu

200 GeV/c

AuAu

200 GeV/c

dAu

200 GeV/c

AuAuee

200 GeV/c

CuCu

62 GeV/c

J/ muon arm

1.2 < |y| < 2.2

J/ eeCentral arm

-0.35 < y < 0.35

Factor ~3suppression

in central events

CuCuee

200 GeV/c

At RHIC:

J/ muon arm

1.2 < |y| < 2.2

J/ eeCentral arm

-0.35 < y < 0.35

Factor ~3suppression

in central events

Data show the same trend within errors for all beams and even at √s=62 GeV

RAA

vs Npart

: PHENIX and NA50

NA50 data normalized at NA50 p+p point.

Suppression similar in the two experiments, although the collision energy is 10 times higher (200GeV in PHENIX & 17GeV in NA50)

What suppression should we expect?

Models that were successful in describing SPS datafail to describe data at RHIC

- too much suppression -

can get better agreement with data

if add formation of “extra” J/ by coalescence of c and anti-c from the plasma

(not necessarily unique or correct explanation!)

take a deep breath…

What did we expect for QGP?

What SHOULD we expect?

weakly interactinggas of quarks & gluons

quarks & gluons retain correlations,medium exhibits liquid propertiesJ/ may survive better

NB: the (quasi-)bound states are not your mother’s hadrons!

Evidence that RHIC creates a strongly coupled, opaque plasma energy density & equation of state not hadronic!must search for plasma phenomena, not asymptotic freedom

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

dNgluon/dy ~ 1000

int large for T < 2-3 Tc can get at properties of this new kind of plasma

opacity, collision frequency, EOS, screening, speed of sound, conductivity

Open questionsDo heavy quarks really thermalize in the QGP?Initial temperature (direct radiation from plasma?)Discovery announcement soon?

conclusions

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

Assume all distributions described by a temperature T and a (baryon) chemical potential : dn ~ e -(E-)/T d3p

One ratio (e.g., p / p ) determines / T : pbar/p = e -(E+)/T / e -(E-)/T = e -2/T

Second ratio (e.g., K / ) provides T predict others

Does final state reflect a thermal distribution?

Tf ~ 175 MeV

black holes at RHIC?

Not the usual ones that come to mind!energy and particles get out (we see them)rate of particle production scales from non-QGP

producing collisions – so no evidence of eating ANY external mass/energy

This experiment has been done MANY times by naturehigh energy cosmic rays impinging on atmosphere

Recent paper by Nastase uses mathematics of black holes developed by Hawking, but forces and behavior (and sizes) are quite different

Possibility of plasma instability → anisotropy

small deBroglie wavelength q,g point sources for g fieldsgluon fields obey Maxwell’s equationsadd initial anisotropy and you’d expect Weibel instability

moving charged particles induce B fieldsB field traps soft particles moving in A directiontrapped particle’s current reinforces trapping B fieldcan get exponential growth

(e.g. causes filamentation of beams) could also happen to gluon fields early in Au+Au collision

timescale short compared to QGP lifetimebut gluon-gluon interactions may cause instability to

saturate → drives system to isotropy & thermalization

do heavy quarks thermalize?

elliptic flow?

energy loss!

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

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

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!

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

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

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

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.

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

“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)

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

Evolution of the Universe

Nucleosynthesis builds nuclei up to HeNuclear Force…Nuclear Physics

Universe too hot for electrons to bindE-M…Atomic (Plasma) Physics 104

K

E/M Plasma

Too hot for quarks to bind!!! Too hot for quarks to bind!!! 1012 KStandard Model (N/P) Physics

Quark-Gluon

Plasma??

Too hot for nuclei to bind 1010 KNuclear/Particle (N/P) Physics Hadron

Gas

SolidLiquidGas

Today’s Cold UniverseGravity…Newtonian/General

Relativity

pressure builds up

probing a 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

The Scope of the Tools (!)

STARspecialty: large acceptancemeasurement of hadrons

PHENIXspecialty: rare probes, leptons,

and photons

Energy density of matter

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