Zooming in on the QGP?Heavy-Ion Collisions: an Overview

Thomas UllrichHard Probes 2005

June 10, 2006

2

“The physical picture emerging from the four (RHIC) experiments is consistent and surprising. The quarks and gluons indeed break out of confinement and behave collectively, if only fleetingly. But this hot mélange acts like a liquid, not the ideal gas theorists had anticipated.”

M. Riordan, W. Zajc, Sci. Am., May 2006, 34-41.

3

Azimuthal Anisotropy: Elliptic Flow

D. Morrison, SQM’06

dN/d ~ 1+2 v2(pT)cos(2) + …. = atan(py/px) v2 = cos2

v2: 2nd harmonic Fourier coefficient in dN/d with respect to the reaction plane

EoS & geometry-driven momentum anisotropy

Density & geometry-driven absorption anisotropy

R

Elliptic flow observable sensitive to early evolution of system

Large v2 is an indication of early thermalization

4

v2: RHIC Measurements

Rich set of data on v2 at RHIC: h±, ±,0, K±,0, p,p, ,, , Ω, ϕExperiments: dN/dy|y=0 → dN/dpT → v2

v2: Magnitude, pT, centrality and mass dependence, dependence (h± only)

v2 stronger than at SPS, AGS v2() > v2(K) > v2 (p) ~ v2()

200 GeV Au+AuSTAR preliminary

min. bias (0-80%)

pT (GeV/c)

5

The Method Matters: v2EP, v22 versus v24

In more central Au+Au collisions the difference between v22 and v24 increases from 10% at low-pt to about 40-50% at intermediate-pt

Difference smaller for > 2 Huge for Cu+Cu (reason unknown)

W. Gang QM’05

Phys. Rev. C 72 (2005) 014904

v2EP: Assume all correlations between particles due to flowv22: Non flow correlation contribute order 1/Nv24: Non flow correlation contribute order 1/N3

6

Ideal hydro overpredicts flow init. conditions, viscosity?

Afterburner (Teaney, Hirono, Bass ...) Adds dissipative hadron phase “Late” viscosity in hadron phase Break up thermalization: viscosity

effectNeed to fix initial conditions

CGC conditions → need “early” viscosity

BGK/Glauber OK?

Hydrodynamics: Modeling High-Density Scenarios

Full 2(3)-d Hydrodynamics

• EoS :1st order phase transition QGP + excluded volume model

Cooper-Fryeformula

RQMD, UrQMD, JAM, …

t fm/c

final stateinteractionsMonte Carlo

Hadronization

TC TSW

The “era” of the afterburner

7

Elliptic & Radial Flow: Non-Viscous Hydro Works

200 GeV Au+AuSTAR preliminary

pT (GeV/c)

v 2 (

pT)

0.20

0.15

0.10

0.05

0

0

P. Huovinen

P. Huovinen

all: full hydro, all compare to v2EP Au+Au central , √s = 200 GeV

Hydro pQCD

RHIC: Tfo~ 100 MeV, T ~ 0.55 c

D. d'Enterria, D. Peressounko , EPJ .C46:451-464

8

Hydro Limits: Like a Perfect Liquid?

Hydro- dynamics without any viscosity describes heavy ion reactions at RHIC (QGP+H EoS) Thermalization time = 0.6 fm/cEnergy Density: =20 GeV/fm3

=spatial eccentricity = y2-x2/y2+x2S=overlap area

Hydro: Kolb, Sollfrank, Heinz PRC62:054909

Issues No consistent set of hydro calculation that describes all observables Lattice inspired EoS in ideal hydro does as poorly as a hadron gas EoS Before we can make a connection to the EoS using v2(pT,m) much more

work needed in theory (test different EoS, viscosity, hadronic phase)

9

Another Word of Caution … v4/(v24)2

Incomplete thermalization? Need theory input how this would look

in microscopic model

Ratio v4/v22 is sensitive to degree of thermalization

Borghini, Ollitrault nucl-th/0506045:

v4/v22 = 0.5 for ideal

hydro (more accurate for increasing values of pT)

Observed integrated ratio larger than unity

For more peripheral collisions increasing fast as function of transverse momentum

STAR Preliminary

10

Constraints on Viscosity - What Do We Know?

s = 4/3sT (sound attenuation length)

pQCD: s/ = 0.18/(T) ~ 0.18(for s = 0.5 ?)

AdS/CFT: s/ = 1/(3T) ~ 0.11

What we urgently need is viscoushydro but experts tell us this is really hard 3 years away (always?)

One attempt: D. Teaney1st correction to thermal distribution function of an expanding gas estimate viscous corrections to spectra and elliptic flow using boost invariant blast wave model

Lack of Proof ≠ Proof of Lack

11

Parton Transport Model – MPC & Ideal Hydro

Maybe a “Perfect” Fluid - But Not Ideal

What’s going on?a) hard EoS + strongly dissipative evolutionb) softer EoS + negligible dissipative evolution (system stays in perfect local

kinetic equilibrium – how, why?) What’s about 2↔3 processes?

D. Molnar, P. Huovinen PRL94 (’05)

Same initial conditions for MPC & hydro

Very opaque system but still dissipative even for gg 50 mb, ~ 0.1 fm

Still 30% smaller than ideal hydro (diff. established early ~ 1 fm/c)

Note: ~ 3 mb in pQCD (2 ↔ 2) Note: ~ 1/

~ 47 mb as good as it gets troublesome ?

12

The gg ↔ ggg PuzzleZ. Xu and C. Greiner, PRC 71, 064901 (2005)

3+1 dimensional cascade calculation solving on-shell Boltzmann equations for partons including inelastic gg ↔ ggg pQCD processes

),(),(),( pxIpxIpxfp ggggggggg

new development(Z)MPC, VNI/BMS

0.2 fm/c

0.5 fm/c1 fm/c2 fm/c3 fm/c4 fm/c

Thermal (exp.) spectra after ~1 fm/c

13

What’s Going On ?

Old idea about the transport cross-section is wrong (Xu, HQ 2006) This issue addresses the questions on

Fast thermalization (0 ~ 0.6 fm/c) in parton cascade models to describe v2?

Is it the solution or is something wrong ?Need to check more signatures (RAA ...)

Issue needs to be solved!

54~ionequilibrattoofoncontributi

ionequilibrattoofoncontributi

gggg

ggggg

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It Started 20 Years Ago …

Key Idea: Melting in the plasma Color screening of static potential between heavy quarks in deconfined matter Suppression of states is determined by TC and their binding energy Color screening Deconfinement QCD thermometer Properties of QGP

Coincides with Helmut’s Birthday !

15

Quarkonia – Lattice QCD …

Recent developments: Heavy Quark potential?

Singlet free energy: F1 (entropy term?) Singlet energy: V1

When do states really melt? Neither F1 nor V1 are potentials, they are models!

spectral functions (results consistent with V1)

J/ melts at 1.5-2.5 TC

Tdiss(y’) Tdiss(cc)< Tdiss((3S)) < Tdiss(J/y) Tdiss((2S)) < Tdiss((1S))

F. K

arsch, RH

IC-II S

cience

Workshop

Is the sequential suppression pattern the smoking gun?

16

Just When You Think You Start to Understand …Charmonia correlators: potentials vs. lattice (Mócsy, Petereczky, hep-ph/0512156)

Lattice reliable calculation of c and c correlators Taking F,V as plain (Cornell) potentials calculate correlators Temperature-dependence of c

and c lattice correlators is not explained with screened Cornell potential.

Screening likely not responsible for quarkonia suppression time scale of screening not small

compared to time scale of heavy quark motion

Suppression from collisions with thermal gluons only? dNJ//dt = Ng dis

Lattice: Datta, et al.,PRD 69 (2004) 094507

Potential model with screening : Á. Mócsy, P.P, hep-ph/0512156

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SPS: NA50 and NA60

Normalized to

Drell-Yan

Using Glauber &

nuclear absorption (from pA studies)

Suppression beyond nuclear absorption

observed in

Pb+Pb and In+In

at √s ~ 17 GeV

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Origin of Suppression at SPS ? J/absorption by produced hadrons (comovers)

Capella and Ferreiro, Eur.Phys.J. C42 (2005) 419

J/suppression in the QGP and hadronic phases including thermal regeneration and in medium properties of open charm

and charmonium states Grandchamp, Rapp, Brown, NPA715 545; PRL. 92 212301; JPG 30 S1355

Or is it much simpler? F. Karsch, D. Kharzeev, H. Satz, hep-ph/0512239

Assume:1. NJ/(observed) = 0.6 NJ/ + 0.4 Nc

(compatible w Hera-B data)1. J/ doesn’t melt2. c dissociation = ’ dissociation

Right or wrong, it shows how importantthe missing c measurement is!

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Charmonium at RHIC: Screening and Regeneration

General Observations:Models with regeneration, i.e., single charm quarks combining in the later stages to form J/’s – match the observed RHIC suppression better!

Most actual models have suppression + various regeneration mechanisms Rapp - PRL 92, 212301 (2004)

screening & in-medium production Thews - PRC73 (2006) 014904c

pQCD NLO charm + recombination Andronic - PLB57, 136 (2003)

statistical hadronization model with screening of primary J/’s + statistical recombination of thermalized cc’s

Kostyuk – PRC 68, 041902 (2003) statistical coalescence + co-movers or

QGP screening Bratkovskaya – PRC 69, 054903

(2004) hadron-string dynamics transport

Zhu - PLB607, 107 (2005) J/ transport in QGP co-movers,

gluon breakup, hydro for QGP evolution no cold nuclear matter, no regeneration

sum

regeneration

screening

M.

Lei

tch

, S

QM

’06

20

regeneratio

n

dire

ct

Rec

ombi

ned

only

Regeneration Narrowing of pT and y?

pT broadening lies in between Thews direct & in-medium formation suggesting some regeneration.

Recombination predicts a narrower rapidity distribution with an increasing Npart.

Strange: Going from p+p to the most central Au+Au : no significant change seen in the shape of the rapidity distribution.

No Recombination

Thews et al.

21

The Promise of Jet Tomography

Simplest way to establish the properties of a system Calibrated probe Calibrated interaction Suppression pattern tells about density profile

Heavy ion collisions Hard processes serve as calibrated probe Suppression provides density measure

+

=

p+p medium

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Calibrated Probe: Hard InteractionsHard interactions pQCD

Factorization holds PDF (initial) x NLO x FF (final)

Example of probes well reproduced in the forward direction, STAR PRL 92

(2004) 171801 proton production in p+p, STAR

nucl-ex/0601033 Direct production, PHENIX Phys. Rev. D

71 (2005) 071102

There are however “issues” (Strange) baryons, e.g. Charm (at RHIC via D → e + X)

Significant uncertainties in the calculations, so for precision the baseline needs to be measured

N.B.: RHIC contributes substantially in improving FF (e.g. AKK)

STAR preliminary

p+p→0+X

S.S. Adler et al, PRL 91 241803

p+

p/

FO

NL

L

23

Application to Heavy Ion Collisions: Initial Results

Strong suppression in Au+Au collisions, no suppression in d+Au: Effect is due to interactions between the probe and the medium Established use as a probe of the density of the medium

PHENIX: Phys. Rev. Lett. 91 (2003) 072301

STAR: Phys. Rev. Lett. 91 (2003) 072304

PHOBOS: Phys. Rev. Lett. 91 (2003) 072302

BRAHMS: Phys. Rev. Lett. 91 (2003) 072303

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference

24

InterpretationGluon radiation: Multiple final-state gluon radiation off the produced hard parton induced by the traversed dense colored medium

Formalisms: BDMPS (thick plasma), GLV (thin plasma)

Mean parton energy loss medium properties:

Eloss ~ gluon (gluon density) Eloss ~ L2 (medium length)

~ L with expansion Characterization of medium

transport coefficient

gluon density dNg/dy Deduced initial gluon density at =

0.2 fm/c: dNglue/dy ≈ 800-1200 ≈ 15 GeV/fm3 (in static medium)

Pion gas

Cold nuclear matter

RHIC data

sQGP

QGP

Baier’s plot

43

forcecolor of Range

2

scattering ofDensity

2

ˆ

ˆ4

ckq

LqC

E

T

SR

q

25

Central RAA Data

Increasing density

Towards the Quantitative: Limitations of RAA

Surface bias effectively leads to saturation of RAA with density

Challenge: Increase sensitivity to the density of the medium

K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511

A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38(2005) 461

Distributions of parton production points in the transverse plane

26

One Way: Increased Precision and Reach

Successful RHIC runs in 2004 (Au+Au) and 2005 (Cu+Cu) Probe systematic with beam energy, system size Increased reach from larger datasets:

e.g. 0 to 20 GeV/c with high precision still flat

H. Buesching, HQ06

27

Black and White

Medium extremely black to hadrons, limiting sensitivity to density Medium transparent to photons (white): no sensitivity Is there something grey?

S.S. Adler et al, Phys. Rev. Lett. 94, 232301 (2005)

28

Grey Probes

Not all models are completely black at the parton level

Significant differences between predicted RAA, depending on the probe

Experimental possibility: increase sensitivity to the properties of the medium by varying the probe

Wicks et al, nucl-th/0512076

29

Distinguishing Power Between Gluon and Quark?

In the NLO calculation that best describes p+p data, significant difference in the gluon contribution between proton and pion spectra

Yet, no significant difference in suppression: no difference between gluon and quark energy loss?

Ruan WW06, I. Vitev, nucl-ex/0603010.

pT [GeV/c]

AKK, private communication

30

STAR Preliminary

NA57: G. Bruno, A. Dainese: nucl-ex/0511020

RCP at Lower EnergiesIs the observed baryon/meson anomaly just due to , K suppression?

No, at √s=17.3 GeV, baryons enhanced w/o pion suppression

Recombination present in all systems? If the baryon enhancement is from a larger pT kick (or flow)

for baryons, why doesn’t B/M increase when the spectrum is steeper?

Baryon/meson splitting at SPS and RHIC (200 & 62 GeV ) is the same

31

light

(M.Djordjevic PRL 94 (2004))

Getting Heavy: Heavy Flavor Energy Loss

In 2001, Dokshitzer and Kharzeev proposed “dead cone” effect charm quark small energy loss

Recent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027, 2005; M. Djordjevic et al., PRL 94, 112301, 2005.

Heavy quarks will be important to understand the Energy Loss mechanisms and the competition between them

32

RAA,v2 of Non-Photonic Electrons Charm energy loss via electrons from semileptonic D, B decays

Experimental challenge: subtraction of photonic electron background Evidence of large heavy quark energy loss! Substantial elliptic flow for pT < 3 GeV/c

STAR, QM05

Hendrik, Greco, Rappnucl-th/0508055

w.o. B meson (c flow)w. B meson (c,b flow)

33

1982 - Initial idea Bjorken, Fermilab-Pub-82/59-THY early estimates < 0.3 GeV/fm

2005 - Collisional loss in expanding QGP for heavy quarks Mustafa: PRC72, 014905, Mazumder

et al., PRD71, 094061 rebirth of collisional energy loss

coll. loss + rad. loss non-photonic electron (charm) RAA

Wicks et al. nucl-th/0512076 finite size effects: Yes - Peigne et al.,

hep-ph/0509185No – Djordjevic, nucl-th/0603066

The Return of the QCD Collisional Energy Loss

Radiative Energy LossN. Armesto et al, nucl-ex/0511257

‘there must be something else ...’

b

c

34

A. Peshier, hep-ph/0605294

Take running coupling into account

independent of jet energy for T > 1.5 TC considerably

larger than previous estimates

In BJs collisional loss formula

(adaption of rel. Bethe-Bloch)

What is s in a QGP?

A fixed parameter?

Isn’t it running ?

The Question That Has Always Puzzled Me …

Ds

Bjgq

m

ETT

dx

dEln~ 22,

)(~ 2D

APcol mT

dx

dE

35

Beyond Single Particle Spectra

Overlap zone has ellipticity path length dependence can be probed with dihadron correlations

Dihadron correlations introduce geometric biases different from RAA Surface bias in trigger hadrons: longer path lengths

Photon-hadron and beauty-hadron correlations have yet different biases No surface bias in trigger photons: full path length distribution?

T. Renk, nucl-ex/0602045

36

Dijets - Dihadrons High Trigger and High Associated pT

Clear jet-like peaks seen on near and away side in central Au+Au – width not broadened

Away-side yield strongly suppressed to level of RAA

No modification of shape in the longitudinal (zT) or transverse ( width) directions

Strong set of additional constraints on E-loss models

STAR, nucl-ex/0604018

STAR Preliminary

dN/d

z TA

u+A

u/d+

Au

STAR Preliminary

d+Au

1/N

trig

dN

/d() Au+Au 20-40% Au+Au 0-5%

8 < pT(trig) < 15 GeV/c pT(assoc)>6 GeV

D(zT)

1

0

37

Where Does the Energy Go? Lower the associated pT to search for

radiated energy Additional energy at low pT BUT no longer

collimated into jets Active area: additional handles on the

properties of the medium? Mach Shocks, Cherenkov Cones …

e.g. Renk and Ruppert, Phys. Rev. C 73 (2006) 011901

Leadinghadrons

Medium

away

near

PHENIX, QM05 and nucl-ex/0507004STAR, Phys. Rev. Lett. 95 (2005) 152301

AA

/pp

away pT (GeV/c)

pT trigger ~ 4 GeV/cPHENIX preliminary

38

Zooming in on the *QGP* your favorite plasma goes here s,w,bs,c,…

Soft and Medium Sector – Data Driven RHIC, SPS: plenty of data – all you can eat

except low energy scan at RHIC (hunt for critical point) need more theory (manpower)

bounds on viscosity ?EoS ?short thermalization time ?

Hard Sector – Theory Driven ? need more measurements (running time & upgrades)

heavy flavor sector (Quarkonia, B, D)-jetcorrelation studies (e.g. 3 particle)

theoryenergy loss: collisional vs. radiative, what else?

LHC – the clock is ticking … will it simplify or complicate our picture ?

“The good news: The sQGP may not be as difficult to understand as many have feared.”

Berndt Müller

I hope this is true …

Let this conference be a success and let us a step further in this direction

Get well soon Miklos !