Strange hard probes to Strange hard probes to characterize the partonic characterize the partonic

medium at RHICmedium at RHIC

Rene BellwiedWayne State University

Phase Transition In Strongly Interacting Matter, NPDC18

Prague, Czech Republic, August 23- 29, 2004

dNg/dy ~ 200 (HIJING)

dNg/dy ~ 1000 (CGC)

Gluon density in proper modelEquals final state hadron density:dNch/dy ~ 1000 (measured)

Parton – hadron duality ??

Signs of partonic ‘hydro’ matterSigns of partonic ‘hydro’ matter

Constituent quark scaling

First time in Heavy-Ion Collisions a system created which, at low pt ,is in quantitative agreement with ideal hydrodynamic model (for mid-central to central collisions)

How do we determine medium properties ?How do we determine medium properties ?

(by producing probe and medium in the same collision)(by producing probe and medium in the same collision)

We are producing ‘soft’ and ‘hard’ matter. An arbitrary distinction is coming from the applicability of pQCD which is generally set to pT > 2 GeV/c (hard). Below 2 GeV/c we expect thermal bulk matter production.– Medium: The bulk of the particles; dominantly soft production and

possibly exhibiting some phase.– Probe: Particles whose production is calculable, measurable, and

thermally incompatible with (distinct from) the medium (hard production)

Measure bulk matter properties to determine global properties (collectivity, equilibration, timescales)(Talks by Boris Hippolyte and Magali Estienne)

Measure the modification of high pt probes to determine specific properties of the matter produced (jet tomography)

Understanding ‘jet and bulk’ properties in Understanding ‘jet and bulk’ properties in the same experimentthe same experiment

99.5%

Dominant feature: order of magnitude increase at high pT

Behavior of hard probes when Behavior of hard probes when traversing an opaque mediumtraversing an opaque medium

coneRFragmentation:

z hadron

parton

p

p

Jets from hard scattered quarks observed via fast leading particles orazimuthal correlations between the leadingparticles

However, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium: decreases their momentum (fewer high pT particles) “kills” jet partner on other side

pp is well described by fragmentationpp is well described by fragmentation

Ingredients:– pQCD– Parton distribution functions– Fragmentation functions– Next-to-leading (NLO)

calculations

p+p->0 + X

Hard

Scattering

Thermally-shaped Soft Production

hep-ex/0305013 S.S. Adler et al.

“Well Calibrated”

We measure two predicted QGP signaturesWe measure two predicted QGP signatures

The ‘quenching’ of high pt particles due to radiative partonic energy loss

The disappearance of the away-side jet in dijet events traversing the apparently opaque medium

?

Test the matter with high pt probesTest the matter with high pt probes

Is jet quenching an initial or final state effect ? Measure dA

Nuclear suppression factors (AA/pp) vs (dA/pp)Nuclear suppression factors (AA/pp) vs (dA/pp)

Striking difference of d+Au and Au+Au results. Enhancement vs. suppression. (Cronin effect in cold nuclear matter).

Final state effect confirmed by back-to-back correlations Energy loss depends on the size of medium traversed

Cronin Effect:

Multiple Collisions broaden high PT

spectrumPedestal&flow subtracted

AA quenching: where is the energy ?AA quenching: where is the energy ?On the away side: energy loss in medium has been converted to lower pt particles

Leading hadrons

Medium

Away

syst. error

Near

STAR Preliminary

pTrigger = 4 -6GeV/c

<pt> in cone is still higher than in medium but is approaching equilibration with medium

Statistical distribution of momentum conservation describes the correlation function at all centralities

Summary of measured Summary of measured experimental observationsexperimental observations

At RHIC we showed that Au+Au collisions create a medium that is dense, dissipative and exhibits strong collective behavior– We observe suppression phenomena in single

particle observables and very importantly also in the correlations (large acceptance)

– We observe constituent quark scaling in v2 and Rcp at ~ 2-5 GeV/c and gluon density scaling in the energy production

– We observe strong collective behavior (flow) in all bulk matter observables

(nucl-th/0403032)

What else can be measured ?What else can be measured ?We have the unique opportunity to measure the

fragmentation of a parton into hadrons in the vacuum and in the medium.

We can learn about hadronization, and therefore learn how particles acquire mass, by measuring medium modifications to the fragmentation process in an

opaque medium and compare to the behavior in the vacuum.

This is a fundamental question of physics that also connects Nuclear Physics to Elementary

Particle Physics !

Modification of fragmentation functions Modification of fragmentation functions (e.g.Gyulassy et al.,nucl-th/0302077)(e.g.Gyulassy et al.,nucl-th/0302077)

Induced Gluon Radiation ~collinear gluons in cone “Softened” fragmentation

in je

i j t

t

n e

: increases

z : decreases

chn

(quite generic, but attributable to radiative rather than collisional energy)

Different partons lose different Different partons lose different amounts of energyamounts of energy

Examples:

1.) dead cone effect for heavy quarks:Heavy quarks in the vacuum and in the medium (Dokshitzer

and Kharzeev (PLB 519 (2001) 199)) the radiation at small angles is suppressed

2.) gluon vs. quark energy loss:Gluons should lose more energy and have higher particle

multiplicities due to the color factor effect.

Quark vs. gluon jet measurementsQuark vs. gluon jet measurements Gluon bremsstrahlung is expected to be higher in gluon than in quark jets by CA/CF (= 2.25). Jet multiplicities in elementary collisions already higher by color factor due to softer

fragmentation function. Measurement shows ratio slightly lower at lower pt and expected value at higher pt (higher order corrections ?)

1.) anti-s softer than anti-light at low x (hep-ph/9303255) 2.) s to anti-s asymmetry in the q to fragmentation ? (hep-ph/0005210)

Flavor Dependence of Parton DistributionsFlavor Dependence of Parton Distributions

Fragmentation Fragmentation functions for functions for different baryonsdifferent baryons

Bourelly & Soffer (hep-ph/0305070)

Statistical approachbased on productioncross section measurements in e+e-

Do we understand fragmentation ?Do we understand fragmentation ?statistical approach based on measured inclusive cross sections of unpolarizedoctet baryons in e+e- annihilation: Du

~ 0.07 Ds ,and Du

/ Ds ratio about constant as a function of x

but: de Florian et al., (1998): u,d,s contribution to is about the same J.J.Yang (2001,2002): Du

/ Ds drops by factor 5 with increasing X.

Is RHIC the right place to study Is RHIC the right place to study fragmentation as a function of x ?fragmentation as a function of x ?

There couldn’t be a better place !!

The goal of particle identified The goal of particle identified fragmentation in the mediumfragmentation in the medium

1.) we need to understand fragmentation (hadronization) in the elementary binary process

2.) In addition to the statistical approach we can use the medium modified fragmentation functions in AA collisions

3.) the claim is that by having different contributions to the elementary fragmentation function at different x and by

having these different contributions lose different amounts of energy (z) in the opaque medium, we learn about the basic

hadronization process by measuring particle identified fragmentation and correlation functions

Identified particles at intermediate to high-pIdentified particles at intermediate to high-p tt

Two groups, baryons and mesons, which seem to approach each otheraround 5 GeV/c

Suggesting relevance of constituent quarks for hadron production Coalescence/recombination provides a description ~1.5 - 5 GeV/c

The ‘intermediate’ pt regionThe ‘intermediate’ pt region

pT

pQCDHydro

2-3 GeV/c 6-7 GeV/c ?

SoftSoftFragmentation Fragmentation and quenchingand quenching of jets of jets

0

pT independence of pbar/p ratio.

p/ and /K ratio increases with pT to > 1 at pT ~ 3-4 GeV/c in central collisions.

Suppression factors of p, different to that of , K0

s in the intermediate pT region.

Parton Parton recombinationrecombination

andandcoalescencecoalescence

Recombination + Fragmentation at mid ptRecombination + Fragmentation at mid pt Recombination at moderate PT

Parton pt shifts to higher

hadron pT.

Fragmentation at high PT:

Parton pt shifts to lower

hadron pT

recombining partons:p1+p2=ph

fragmenting parton:ph = z p, z<1

Recomb.

Frag.

-charged hadron correlations in pp and AA-charged hadron correlations in pp and AA

0-5% 10-30%5-10%

30-50% 50-70%

pp

Two jets or Two jets or a monojet plus momentum conservation ?a monojet plus momentum conservation ?

bgaaF

)*

)(exp(*)

*

)(exp(*)(

22

22

21

21

22

21

))*2cos(0.21()*2

)(exp(*2)

*2

)(exp(*1)( ,2,22

2

22

21

21

flow

assoflowtriggervvbgaaF

pp:

Au + Au:

))*cos(.)cos(()*

)(exp(*)( ,,cos

2021

21 222

1

21 flow

assoflowtriggervvCbgaF

Gaussian Fit of back side:

Cosine Fit of back side (momentum balance):

Same Side

Back Side

Background

Parameters are compared for different fits for two different pT cuts as a function of centralities

2T

associatedtriggerassociatedtrigger

P

PN

)cos(ΔPP2)P,(PC T

Nicolas Borghini et al. Phys. Rev. C 62, 034902(2000).

Λ +h correlations AuAu (cosine fit)

Conclusion: fit quality is equally good.

1.5<pT,trigger<3.0, 1.5<pT,asso<3.0

Width and associated particle yields(Width and associated particle yields(ΛΛ+h)+h)G

auss

ian

fitC

osin

e fit

Comparison of STAR/PHENIX for same Comparison of STAR/PHENIX for same side difference for baryon and meson side difference for baryon and meson

trigger particles as a function of centralitytrigger particles as a function of centrality

Different methods, similar result: STAR integrated Gaussian fit, PHENIX bin counting in fixed bin

Comparison of STAR/PHENIX for away Comparison of STAR/PHENIX for away side difference for baryon and meson side difference for baryon and meson

trigger particles as a function of centralitytrigger particles as a function of centrality

Different methods, similar result: STAR integrated cos-fit, PHENIX bin counting in fixed bin

STAR comparison for different particle speciesSTAR comparison for different particle species

No significant difference for different trigger particle species as a function of centrality at intermediate trigger pt.

Asymmetry seems to develop as a function Asymmetry seems to develop as a function of trigger pt (increased ‘jettiness’)of trigger pt (increased ‘jettiness’)

(Same side – away side) two particle correlation strength incentral Au-Au Collisions at RHIC

Strange trigger, charged associated particles

(associated pt > 2 GeV/c)

trigger pt (GeV/c)STAR preliminary

But it mayprovide answersto 3 out of the11 greatest unansweredquestions ofPhysics !!

A few thoughts for your way homeA few thoughts for your way home

The fundamental question of parton to hadron conversion can be tackled though

through systematic studies of particle identified fragmentation processes inside and outside the produced medium. This

topic is far reaching, it is challenging, it will require systematic studies, and it might

require more dedicated equipment than the existing RHIC/LHC detectors.

From my point of view that in itself is tantalizing, but it does not lead to a larger physics payoff per se.

Many wise men, in particular theorists, claim that the evidence for QGP formation is overwhelming and indeed the signatures for the creation of strongly interacting, collective

partonic matter formation is strong (sQGP).

A few thoughts for your way homeA few thoughts for your way home

The matter produced is an almost perfect fluid ! A strongly interacting parton liquid is not what we expected. (sQGP is the new theory label)

Maximum opacity (Gyulassy 01) Navier-Stokes (Teaney 03)

Where is the weakly interacting Where is the weakly interacting plasma ?plasma ?

Shuryak, QM04

deconfinement

restoration

partonfluid(pre-hadrons)

Cassing, priv.comm.

England: University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

India:Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC

Netherlands:

NIKHEFPoland:

Warsaw University of TechnologyRussia:

MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino

Switzerland:University of Bern

U.S. Labs: Argonne, Lawrence Berkeley, and Brookhaven National Labs

U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan, USTC,Tsinghua, SINR, IMP Lanzhou

Croatia: Zagreb University

Czech Republic: Nuclear Physics Institute

STAR: 51 Institutions, ~ 500 STAR: 51 Institutions, ~ 500 PeoplePeople

Consequences of a strong vConsequences of a strong v22 and and

final state jet quenching at RHICfinal state jet quenching at RHIC

1.) v2 is strong and has to come from very early time after collision. Hadronic v2 is not sufficient in terms of magnitude and timescale.2.) v2 is very well described by hydrodynamics (fluid dynamics). 3.) if the phase producing the flow is partonic then we have partonic fluid (dissipative, strongly interacting, small correlation length) rather than a plasma (large correlation length, weakly interacting quasi-particle gas).

v2 in Au+Au 62GeVv2 in Au+Au 62GeV

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-0.1

0.0

0.1

0.2

0.3 Au+Au 62GeV (TOFr) k(TOFr) p(TOFr)

K*0

V2

PT(GeV/c)

Elliptic flow vElliptic flow v22 scaling at intermediate to high-p scaling at intermediate to high-ptt

two groups, baryons and mesons

suggesting relevance of constituent quarks in hadron production

S.A. Voloshin, Nucl. Phys. A715, 379 (2003).

D. Molnar and S.A. Voloshin, PRL 91, 092301(2003).

Further tests: , 0, K*, pentaquarks

scaling could be seen as a signature of deconfinement !

Hadron suppression prevails at Hadron suppression prevails at 62 GeV62 GeV

2 bins, driven by p+p– = 0: pT <~6 GeV– = 0.7: pT <~10

GeV Significant

suppression seen at 62 and 200 GeV

1/3 of dataset: quantitative treatment awaits full analysis

RC

P

Charged particle correlations:Charged particle correlations: AA/pp ratio for Gaussian fit to same side peak AA/pp ratio for Gaussian fit to same side peak

(trig-pt =1.5-3.0 GeV/c , assoc-pt = 1.5-3.0 Gev/c )(trig-pt =1.5-3.0 GeV/c , assoc-pt = 1.5-3.0 Gev/c )

+h vs. Anti-+h vs. Anti-+h (most central)+h (most central)

The compelling global The compelling global questionsquestions

Could there be evidence for a different phase of matter at even lower x ?

Are the quarks and gluons weakly interacting, as expected from a plasma, or strongly interacting as

expected from an ideal fluid description ?

Is this phase thermally and chemically equilibrated ?

Is there evidence for a phase transition to a deconfined and chirally symmetric phase of quarks and gluons at

high T ?

Modification of fragmentation functionsModification of fragmentation functions (e.g.hep-ph/0005044)(e.g.hep-ph/0005044)

What is there to measure ?What is there to measure ?

The series of measurements is very big, but at a minimum any particle identified measurement at high pt will lead to a

quantification of the energy loss process.

STAR and PHENIX have started to measure identified particle yields and azimuthal two particle correlations at high pt.

Particle species, trigger pt and associated pt cuts can be varied.

The problem is to distinguish between jet properties and bulk matter background and the intermediate pt coalescence

production.

Time scales according to STAR dataTime scales according to STAR data

hadronization

initial state

pre-equilibrium

QGP andhydrodynamic expansion

hadronic phaseand freeze-out

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

dN/dt

1 fm/c 5 fm/c 10 fm/c 20 fm/ctimeChemical freeze out

Kinetic freeze out

Balance function (require flow)Resonance survival

Rlong (and HBT wrt reaction plane)

Rout, Rside

AA/pp for (AA/pp for (ΛΛ+h) and (h+h)+h) and (h+h)

1.5<pT,trigger<3.0, 1.5<pT,asso<3.0

Gau

ssia

n fit

Cos

ine

fit Large AA/pp ratio for the same side Trigger Bias? X N Wang, nucl-th/0405017

Is suppression of high pt particles in RHIC AA Is suppression of high pt particles in RHIC AA collisions an initial state (due to gluon saturation) collisions an initial state (due to gluon saturation)

or final state (due to jet quenching) effect?or final state (due to jet quenching) effect? Initial state?

Final state?

partonic energy loss in dense medium generated in collision

strong modification of Au wavefunction (gluon saturation)

Ultimate test: dA collisions