evidence for the production of the quark-gluon plasma at rhic

1

T. Hallman SC MTG Jan 2005

Evidence for the Production of the Quark-Gluon Plasma at RHICEvidence for the Production of the Quark-Gluon Plasma at RHIC

Tim Hallman

Scientific Council Meeting

Dubna, Russia January 20-21, 2005

2


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

This definition is consistent within the community and over time

3


• The overlap region in peripheral collisions is not symmetric in coordinate space

– Almond shaped overlap region• Easier for particles to emerge in the

direction of x-z plane• Larger area shines to the side

– Spatial anisotropy Momentum anisotropy• Interactions among constituents generates

a pressure gradient which transforms the initial spatial anisotropy into the observed momentum anisotropy

• Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane

• v2 is the 2nd harmonic Fourier coefficient of the

distribution of particles with respect to the reaction plane

2cos2 v

Anisotropic Flow

x

yz

px

py

Anisotropic (Elliptic) Transverse Flow

Elliptic Flow at RHIC

Peripheral Collisions

x

y

p

patan

4


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

Best agreement with v2 and spectra for therm < 1 fm/c and soft (mixed-phase- dominated) EOS ~ consistent with LQCD expectations for QGP hadron

Hydro calculations: Kolb, Heinz and Huovinen

5

T. Hallman SC MTG Jan 2005How Unique & Robust is Hydro Account in Detail?How Unique & Robust is Hydro Account in Detail?

P. Kolb, J. Sollfrank, and U. Heinz, Phys. Rev. C. C62 054909 (2000).

Sharp freezeout dip

Hydro+RQMD no dip?

Teaney, Lauret & Shuryak

Hydro vs. STAR HBT Rout/Rside

Are we sure that observed v2 doesn’t result alternatively from harder EOS (no transition) and late thermalization?

How does sensitivity to EOS in hydro calcs. compare quantitatively to sensitivity to other unknown features: e.g., freezeout treatment (compare figures at right), thermaliz’n time, longitudinal boost non-invariance, viscosity?

What has to be changed to understand HBT (below), and what effect will that change have on soft EOS conclusion?

6


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

Self-Analyzing (High pT) Probes of the Matter at RHIC

7


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

8


Questions for Parton Energy Loss ModelsQuestions for Parton Energy Loss Models

Can pQCD models account for orientation- dependence of di-hadron correlation? Should be sensitive to both path length and matter expansion rate variation with (R).

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 to: assumptions of factorization in-medium and vacuum fragmentation following degradation; treatments of expansion and initial-state cold energy loss preceding hard collision?

9

T. Hallman SC MTG Jan 2005Soft Sector: Hadron Yield RatiosSoft Sector: Hadron Yield Ratios

Strangeness Enhancement Resonances

STARPHENIX

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 .

Does result point to thermodynamic and chemical equilibration, and not just phase-space dominance?

10

T. Hallman SC MTG Jan 2005Intermediate pIntermediate pTT: Hints of : Hints of Relevant Degrees of Relevant Degrees of

FreedomFreedom

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 suggestive of constituent-quark scaling. If better established exp’tally, would give direct evidence of degrees of freedom relevant at hadronization, and suggest collective flow @ constituent quark level.

N.B. Constituent quarks partons! Constituent quark flow does not prove QGP

11

T. Hallman SC MTG Jan 2005Questions for Coalescence ModelsQuestions for Coalescence Models

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 fragment-ation treatments compatible? Double-counting, mixing d.o.f., etc.?

Do coalescence models have predictive power? E.g., can they predict centrality-dependences?

Duke-model recomb. calcs.

Duke-model recomb. calcs.

12

T. Hallman SC MTG Jan 2005Gluon Saturation: a QCD Scale for Initial Gluon Density + Gluon Saturation: a QCD Scale for Initial Gluon Density +

Early Thermaliz’n Mechanism?Early Thermaliz’n Mechanism?

sNN = 130 GeV Au+Au

Saturation model curves use optical Glauber

Does the high initial gluon density inferred from parton E loss fits demand a deconfined initial state? Can QCD illuminate the initial conditions?

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

How robust is agreement, given optical vs. MC Glauber ambiguity in calcu -lating Npart , and assumption of ~one charged hadron per gluon?

CGC applies @ SPS too? If not, why is measured dNch/d(sNN) so smooth?

13


Lattice QCD Predicts Some Sort of RAPID Transition!Lattice QCD Predicts Some Sort of RAPID Transition!

in entropy density, hence pressure

in heavy-quark screening mass

in chiral condensate

The most realistic calcs. no discontinuities in thermodynamic proper-ties @ RHIC conditions (i.e., no 1st- or 2nd-order phase transition), but still crossover transition with rapid evolution vs. temperature near Tc 160 – 170 MeV.

14

T. Hallman SC MTG Jan 2005But What We Observe (at least in the soft sector) But What We Observe (at least in the soft sector) Appears Smooth :Appears Smooth :

No exp’tal smoking gun! Rely on theory-exp’t comparison

Need critical evaluation of both! Theory must eventually explain the smooth energy- and centrality-dependences.

Charged particle pseudo-rapidity density

HBT parameters

pT-integrated elliptic flow

pT-integrated elliptic flow, scaled by initial spatial eccentricity

15


…suggest appealing QGP-based picture of RHIC collision evolu-tion, BUT invoke 5 distinct models, each with own ambigu-ities, to get there. pQCD parton E loss

The Five Pillars of RHIC WisdomThe Five Pillars of RHIC WisdomIdeal hydro

Quark recombination constituent q d.o.f.

CGC

Statistical model

Early thermalization + soft EOS

Very high inferred initial gluon density

Very high anticipated initial gluon density

u, d, s equil-ibration near Tcrit

16


Summary on QGP SearchSummary on QGP Search

All indications are that a qualitatively new form of matter is being produced in central AuAu collisions at RHIC

1) The extended reach in energy density at RHIC appears to reach simplifying conditions in central collisions -- ~ideal fluid expansion; approx. local thermal equilibrium.

2) The Extended reach in pT at RHIC gives probes for behavior inaccessible at lower energies – jet quenching; ~constituent quark scaling.

But: In the absence of a direct signal of deconfinement revealed by experiment alone, a QGP discovery claim must rest on the comparison with a theoretical framework. In this circumstance, further work to establish clear predictive power and provide quantitative assessments of theoretical uncertainties is necessary for the present appealing picture to survive as a lasting one.

In order to rely on theory for compelling QGP discovery claim, we need: greater coherence; fewer adjusted parameters; quantitative

estimates of theoretical uncertainties

17


Backup SlidesBackup Slides

18

T. Hallman SC MTG Jan 2005Critical Future Exp’t Needs: Short-Term (some data Critical Future Exp’t Needs: Short-Term (some data already in the bag from run 4)already in the bag from run 4)

Establish v2 scaling more definitively: better statistics, more particles (incl. , , resonances), include correlations in recomb.-model fits.

Establish that jet quenching is an indicator of parton, not hadron, E loss: higher pT; better statistics dihadron correlations vs. reaction plane; away-side punchthrough? charmed meson suppression?

Extend RHIC Au+Au meas’ments down toward SPS energy, search for possible indicators of a rapid transition in measured properties: determine turn-on of jet suppression vs. s; pp reference data crucial.

Measure charmonium yields + open charm yields and flow, to search for signatures of color screening and partonic collectivity: charmed hadrons in chem. equil.? Coalescence vs. frag-mentation? D-meson flow; J/ sup-pression? (eventually , other “onia”)

Measure hadron correlations with far forward high-energy hadrons in d+Au: search for monojet signature of interaction with classical gluon field.

19


Some Critical Future Exp’t Needs: Longer-TermSome Critical Future Exp’t Needs: Longer-Term

Develop thermometers for the early stage of the collision, when thermal equilibrium is first established: direct photons ( HBT for low E), thermal dileptons.

Quantify parton E loss by measurement of mid-rapidity jet fragments tagged by hard direct photon, a heavy-quark hadron, or a far forward energetic hadron: constrain E loss of light quarks vs. heavy quarks vs. gluons in bulk matter.

Test quantitative predictions for elliptic flow in U+U collisions:

Considerable extrapolation away from Au+Au significant test for hydro predictive power @ RHIC.

Measure hadron multiplicities, yields, correlations and flow at LHC & GSI, and compare to quantitative predictions based on models adjusted to work at RHIC: test viability and falsifiability of QGP-based theoretical framework.

Devise tests for the fate of fundamental QCD symmetries in RHIC collision matter: chiral & UA(1) restoration? CP violation? Look especially at the strongly affected particles opposite a high-pT hadron tag.

20

T. Hallman SC MTG Jan 2005Soft-Hard Correlations: Partial Approach Soft-Hard Correlations: Partial Approach Toward Thermalization?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.

21


pQCD parton E loss

Five Pieces of Important EvidenceFive Pieces of Important Evidence

Ideal hydroQuark recombination constituent q d.o.f.

CGC

Statistical modelEarly thermalization + soft EOS

Very high inferred initial gluon density

Very high anticipated initial gluon density

u, d, s equilibration near Tcrit

evidence for the production of the quark-gluon plasma at rhic

Documents