Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Collisions

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T. Hallman, ICPAQGP5 Feb 2005

Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Collisions Collisions

Tim Hallman

ICPAQGP5

Kolkata, India February 8-12, 2005

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A Definition of the Quark-Gluon PlasmaA Definition of the Quark-Gluon Plasma

QGP a (locally) thermally equilibrated state of matter in which quarks and gluons are deconfined from hadrons, so that color degrees of freedom become manifest over nuclear, rather than merely nucleonic, volumes.Not required:

non-interacting quarks and gluons

1st- or 2nd-order phase transition

evidence of chiral symmetry restoration

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2cos2 v x

y

p

patan

Anisotropic Flowx

yz

px

py

The Strongest Evidence For (Locally)Thermalized State of Matter and EOS with a soft point : Observed Elliptic Flow vs the Predictions of Hydro

Peripheral Collisions

Hydro calculations: Kolb, Heinz and Huovinen

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Soft Sector: Evidence for Thermalization and EOS with Soft Point?Soft Sector: Evidence for Thermalization and EOS with Soft Point?

Systematic m-dependence of v2(pT) suggests common transverse vel. Field

mT spectra and v2 systematics for mid-central collisions at low pT are well (~20-30% level) described by hydro expansion of ideal relativistic fluid

Hydro success suggests early thermalization, very short mean free path and high initial energy density ( > 10 GeV/fm3) Best agreement with v2 and spectra for therm < 1 fm/c and soft (mixed-phase- dominated) EOS ~ consistent with LQCD expectations for QGP hadron

What do the v2 and Hydro results tell us ?

What do we need to understand better ?

Real sensitivity of the Hydro predictions to the EOS and the Freeze-out Treatment

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MODELDATA

Supporting Evidence: Hagedorn Resonance Gas and Particle Multiplicity Ratios at RHIC

Par

ticl

e m

ult

ipli

city

rat

ios

P. Braun-Munzinger, D. Magestro, J. Stachel & K.R

• pT-integrated yield ratios in central Au+Au collisions consistent with Grand Canonical stat. distribution @ Tch = (160 ± 10) MeV, B 25 MeV, across u, d and s sectors.

• Inferred Tch consistent with Tcrit (LQCD) T0 >Tcrit .

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Together with the data at lower energy…

Resonance gas model provides good description of particle yields and their ratios in heavy ion collisions from AGS up to RHIC

For recent review of SPS e.g., F. Becattini, M. Gazdzicki, A. Keranen, J. Manninen & R. Stock; P. Braun-Munzinger, J. Stachel & K.Redlich

For recent review of RHIC, e.g., M. Kaneta & N. Xu, W. Broniowski & W. Florkowski, O. Baranikowa et al.

40 GeV/u Pb+Pb

T = 148 MeV

B = 400 MeV

PBM,Stachel, RedlichNucl-th 0304013

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What do the results tell us?

Chemical freeze-out at RHIC and top SPS energy coincides with phase boundary predicted by LQCD

Data are nearly described by curve of constant critical energy density

Open Questions:

Where is the phase boundaryat lower energy?

Could it be that the full chemical freeze-out curve is coincidentwith the phase boundary?

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Partonic radiative energy loss in dense matter as a means to (indirectly) test deconfinement

Thick plasma (Baier et al.):

glueSglue

Debye

sRBDMS

q

vLqC

E

2

2

ˆ

~ˆ4

L

ELogrdCE jet

glueSRGLV 23 2

,

Linear dependence on gluon density glue: • measure E gluon density at early hot, dense phase

High gluon density requires deconfined matter (“indirect” QGP signature !)

Gluon bremsstrahlung

Thin plasma (Gyulassy et al.):

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ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

<Nbinary>/inelp+p

nucleon-nucleon cross section

Nuclear Modification Factor:

AAhadrons

leadingparticle suppressed

q

q

?

If R = 1 here, nothing newgoing on

High pT (Self-Analyzing) Probes of the Matter at RHICExperimental Tools:

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A comment on the sensitivity of RAA

• Quenching so strong that RAA loses sensitivity to the density of the medium: dominated by unquenched “halo”

• Increased sensitivity only through detailed angular correlations and/or decreasing coupling strength

K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, hep-ph/0406319

Dainese, C. Loizides, G. Paic, hep-ph/0406201

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High pT yields in central Au+Au are suppressed

D. d’Enterria

x5

Factor 5 suppression: huge effect!

Binary Collision scaling

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Hard Sector: Evidence for Parton Energy Loss in Hard Sector: Evidence for Parton Energy Loss in High Density MatterHigh Density Matter

Inclusive hadron and away-side cor-relation suppression in central Au+Au, but not in d+Au, clearly establish jet quenching as final-state phenomenon, indicating very strong interactions of hard-scattered partons or their fragments with dense, dissipative medium produced in central Au+Au.

PHENIX

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Parton energy loss ?• Au+Au suppression (I. Vitev and M.

Gyulassy, hep-ph/0208108) and d+Au enhancement (I. Vitev, nucl-th/0302002 understood in an approach that combines multiple scattering with absorption in a dense partonic medium

• Medium induced radiative energy loss is the only currently known physical mechanism that can consistently explain the high pT suppression.

• From GLV model, initial gluon density dng/dy~1000 is obtained. This corresponds to an initial energy density e~15 GeV/fm3.

• These values are consistent with the energy density obtained from the dET/d measurement as well as ones from the hydro models.

RAA data vs GLV model

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What do the data tell us?What do the data tell us?

Can pQCD models account for orientation- dependence of di-hadron correlation?

pQCD parton energy loss fits to observed central suppression dNgluon/dy ~ 1000 at start of rapid expansion, i.e., ~50 times cold nuclear matter gluon density. ~pT-independence of measured RCP unlikely that hadron absorption dominates jet quenching.

How sensitive is this quantitative conclusion toassumptions concerning: • factorization in-medium• vacuum fragmentation following degradation; • treatments of expansion• initial-state cold energy loss

What do we need to understand better?

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A related question: the initial condition

• Large nucleus (A) at low momentum fraction x gluon distribution saturates ~ 1/s(QS

2) with QS2 ~ A1/3

• A collision puts these gluons ‘on-shell’ ~ A xg(x,Q2) / R2

• Parton-hadron duality maps gluons directly to charged hadrons

• Parton dynamics in a dense system of gluons differs from pQCD

• Saturated gluon density ( CGC ) effective field theory of dense gluon systems provides an appropriate description of the initial condition

D. Kharzeev, E. Levin and L. McLerran, Phys. Lett. B 561 (2003) 93

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dN/d

/ .

5Npa

rt

Npart

Predicted Consequences:

)Λ

Qln(~

)(Qα

1~

A

N2

2S

2SS

CHSaturation in Multiplicity

sNN = 130 GeV Au+AuSTARsNN = 130 GeV Au+Au

Suppression of RdAu at forward Disagreement with expectations based on incoherent multiple scattering in the initial state (i.e. standard factorized pQCD explanation for Cronin enhancement) suppression of high pT

in d+Au at small x

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Comparison of CGC calculations to data

Inclusive h-

Open Charm

Charmonium

K. Tuchin

Inclusive hadrons

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C. Salgado: AA

Comparison of CGC Calculations to the Data

Should the model workat SPS energy ?

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Gluon Saturation: What do the data indicate?Gluon Saturation: What do the data indicate?

How robust is agreement, and how sensitive to assumption of ~one charged hadron per gluon?

• Assuming initial state dominated by g+g below the saturation scale (constrained by HERA e-p), Color Glass Condensate approaches ~account for RHIC bulk rapidity densities dNg/dy ~ consistent with parton E loss.

What do we need to understand better?

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PT is balanced by many gluons

“Mono-jet”

Dilute parton system

(deuteron)

Dense gluon field (Au)

• E > 25 GeV• 4

Beam View Top View

Statistical errors only

25<E<35GeV

35<E<45GeVSTAR

Preliminary

Strengthening the proof with particle correlations

Fixed as

E & pT grows

Large 0+h± correlations

• Suppressed at small <xF> , <pT,>

Consistent with CGC picture

•Consistent in d+Au and p+p at larger <xF> and <pT,>

as expected by HIJING

DIS’04

Fixed as

E & pT grows

Can the boundaries on this diagram be mapped out

experimentally?

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Something that was a surprise..

The first indication that a new mechanism other than universal parton fragmentation is the dominant source of baryons in the intermediate pT range.

Anomalous p/ ratio in 2-4 GeV/c Proton scales with Ncoll,Mesons don’t

PHENIXPHENIX

BRAHMS

= 2.2 = 0

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Also seen and extended to higher pT with topological PID in STAR

Simple fragmentation picture fails for pT less than ~6 GeV/c

p+pbar/h enhancement in Au + Au not fully explained by Cronin effect

Strong baryon/meson modification in Au + Au also in /K0s ratio

PHENIX

STAR preliminary

Observations:

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Extending to the strange sector: RCP of Strange Hadrons

Two groups (2<pt<6GeV/c):

- K0s, K, K*, mesons

- , , baryons

dependence on number of valence quarks

limited to pt<6GeV/c ?

hadron production from quark recombination/ coalescence ?

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Recombination Extended to Elliptic FlowThe complicated observed flow pattern in v2(pT) for hadrons d2n/dpTd ~ 1 + 2

v2(pT) cos (2) is predicted to be “simple’ at the quark level under pT → pT / n , v2 → v2 / n , n = (2, 3) for (meson, baryon)

if the flow pattern is established at the quark level

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A possible explanation: Coalescence + FragmentationA possible explanation: Coalescence + Fragmentation

Duke-model recomb. calcs.

Duke-model recomb. calcs.

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Is recombination the complete answer?

• The “jet-like” correlations of particles with leading baryon rule out the simplest recombination models, which assume a perfectly thermal source of partons.

• At present, no model provides a complete understanding of hadron formation in the intermediate pT regime.

Jet correlation of paticles associated with a leading proton and mesonAssociatedyields similar in all cases

Dominanceof jet-likeproduction

Inconsistent (?)with different suppression formesons/baryons

PHENIX nucl-ex/0408007

||ηη|<0.35|<0.35

||ηη|<0.7|<0.7

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Intermediate pIntermediate pTT: Hints of Relevant Degrees of Freedom ?: Hints of Relevant Degrees of Freedom ?

For 1.5 < pT <6 GeV/c, see clear meson vs. baryon (rather than mass-dependent) differences in central-to-mid-central yields and v2.

v2/nq vs. pT /nq suggests constituent-quark scaling. If better established experimentally,

this would give direct evidence of degrees of freedom relevant at hadronization, and suggest collective flow at the constituent quark level (NB: constituent quarks ≠ partons).

What do the data indicate?

What do we need to understand better? Can one account simultaneously for spectra, v2 and di-hadron correlations at intermediate pT with mixture of quark recombination and fragmentation contributions?

Do observed jet-like near-side correlations arise from small vacuum fragmentation component, or from “fast-slow” recombination?

Are thermal recomb., “fast-slow” recomb. and vacuum fragmentation treatments compatible? Double-counting, mixing d.o.f., etc.?

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Where will some of the next insights come from?

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Direct photons from PHENIX

Good agreement between p+p baseline measurements and NLO pQCD

p+p

Photons scale as binary collisions while 0 are suppressed: consistent with energy loss picture

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Direct photons as a thermometer

• As yet, no measurement at RHIC directly sensitive to temperature at early times

• Intriguing possibility: direct photons, especially with low pT interferometric methods– Potentially sensitive to thermal

black-body radiation from the plasma

WA98: Phys. Rev. Lett. 93, 022301(2004)

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Shadowing vs absorption in d+Au collisions

• Data seem to show that shadowing /absorption plays a (small) role when comparing d-A to p-p

• How much is shadowing? No x2 scaling

• How much is nuclear absorption? Need more data

Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001

E866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983) hep-ph/0311048

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NA60: first resultspreliminary

• Anxiously waiting for the centrality dependence……• Should span the region across the onset of the anomalous suppression

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First Look at Centrality Dependence of Continuum

• Full systematic vs Npart

– Dimuon spectra normalized to

~ normalized to 1/Nch

similar to CERES

• Centrality dependence stronger than linear in Npart

– In the IM region– Between and – Below

• Also visible enhancement– J/ suppression

HP, Gianluca Usia

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First Look at Low Mass Pair region in In-In

• Low mass continuum – High statistics from 2m upward

– Low pair acceptnce for low M low pT

– Pair acceptance different from CERES

• Clearly visible hadron decays → → → BR: (5.8 +/- 0.8) 10-6)

HP, Gianluca Usia

NA50 158 GeV/u In-In

Statistical errors only

NA50 Pb-Pb• NA60 In-In158 GeV/nucleon

pT > 1.1 GeV/c

• Accurately measured yields, slopes, and centrality dependence

• No indication for medium modifications of

– mindependent of Npart within few MeV

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Discovery of Low Mass Dilepton Enhancement

Di-lepton excess at low and intermediate masses well established

HELIOS-3

NA38/NA50 S-U

NA38/NA50 E.Phys.J. C13 (2000) 69

HELIOS-3 E.Phys.J. C13 (2000) 433

CERES Phys.Lett.B422 (98) 405; Nucl.Phys.A661(99) 23

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Dileptons Measured at Low Energies

• DLS puzzle– Strong enhancement over

hadronic cocktail with “free” spectral function

– Enhancement not described by in-medium spectral function

• Verification expected to come soon from HADES

• Connection of enhancement to SPS results not clear

free spectral functioninmedium spectral fct.

DLS Ca-Ca 1 AGeV

preliminary

HADES C-C 1 AGEV

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Heavy quark energy loss: U. Wiedemann, M. Djordjevic, X.-N.Wang

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Kinetic Thermalization: Open Charm Flow

Test mechanism for thermalization: charm heavy, so needs many collisions to reach kinetic equilibrium.

Current measurements: indirect from electrons, and so suffer from large statistical and systematic errors; centrality dependence also needed.

Need direct open charm reconstruction (at low pT)!

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Soft-Hard Correlations: Partial Approach Toward Thermalization?Soft-Hard Correlations: Partial Approach Toward Thermalization?

Leading hadrons

Medium

STAR PRELIMINARY

s = 200 GeV Au+Au results:

NN Closed symbols 4 < pTtrig < 6 GeV/c

Open symbols 6 < pTtrig < 10 GeV/c {

{Assoc. particles: 0.15 < pT < 4 GeV/c

Away side not jet-like! In central Au+Au, the balancing hadrons are greater in number, softer in pT, and distributed ~statistically [~ cos()] in angle, relative to pp or peripheral Au+Au.

away-side products seem to approach equilibration with bulk medium traversed, making thermalization of the bulk itself quite plausible.

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What have we learned (so far)?

+ We know that the matter is extremely dense and it thermalizes very rapidly. First order estimates of the energy density from dET/d (a la Bjorken), Hydro, and jet suppression results are consistent and all well in excess of the density needed for a QGP predicted by LQCD (~ 10-15 GeV/fm3).

But– There is (so far) no direct (unequivocal) evidence that

• the matter is deconfined• the primary degree of freedom of the matter is that of quarks and gluons• the matter is at high temperature (T > 170 MeV)

– We need a better understanding of the real sensitivity of Hydro to the EOS, and to improve its consistency in describing spectra, v2, and HBT. At present we can not draw quantitative conclusions on the properties of the matter such as the equation of state and the presence of a mixed phase.

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What have we learned (so far)? The data appear to demand an explanation beyond a

purely hadronic scenario: – The lower limit of the energy densities derived from dET/d are

~ 4-5 GeV/fm3:The hydro-models require early thermalization (therm< 1fm/c) and high initial energy density > 10 GeV/fm3 . Their success implies the matter is well described as ideal relativistic fluid

– Initial gluon density dng/dy~1000 and initial energy density e~15 GeV/fm3 are obtained from GLV model of jet quenching. A similarly high initial energy density is obtained by other models.

All these estimates of energy density are well in excess of ~1 GeV/fm3 obtained in lattice QCD as the energy density needed to form a deconfined phase.

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ConclusionConclusion

A qualitatively new form of matter is produced in central relativistic nucleus-nucleus collisions! But: Further work is needed to prove this is the quark-gluon plasma according to our definition.

likely sources of insight on experimental side (in the near term): soft sector:

• open charm elliptic flow• v2 systematics (more particles, better statistics)• low mass di-leptons• low pT direct photons

jets and hard probes:• higher pt; search for away side punch-through• better statistics di-hadron correlations wrt reaction plane, • heavy quark suppression (energy loss)• search for forward mono-jets• study of suppression (?) for onium (screening in the plasma)

theory: provide quantitative assessments of sensitivity (e.g. to EOS) & theoretical uncertainties; incorporate higher level effects (e.g. correlations into coalescence)

Documents

Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Collisions