High pT Probes of QCD Matter

Huan Zhong Huang黄焕中

Department of Physics and AstronomyUniversity of California, Los Angeles

Department of Engineering PhysicsTsinghua University

Characteristics of Interactions

Non-Abelian Nature of QCD

QED QCD

Salient Feature of Strong Interaction

Asymptotic Freedom: Quark Confinement:

庄子天下篇 ~ 300 B.C. 一尺之棰，日取其半，万世不竭

Take half from a foot long stick each day,You will never exhaust it in million years.

QCD q q

q qq q

Quark pairs can be produced from vacuumNo free quark can be observedMomentum Transfer

Co

up

lin

g S

tren

gth

Shorter distance

(GeV)

e+e- qq (OPAL@LEP)

p+p jet+jet (STAR@RHIC)

Quarks are Real !

Discovery of the Gluons ?!

Geometry of Nucleus-Nucleus Collisions

Number of Participants

Impact Parameter

Npart – No of participant nucleonsNbinary – No of binary nucleon-nucleon collisions cannot be directly measured at RHIC estimated from Woods-Saxon geometry

Expectations for High pT from Au+Au

ddp

dN

collddpdN

TAA

T

2pp

T

2AA N/

)p(R

Use number of binary nucleon-nucleon collisions to gauge the colliding parton flux:

N-Binary Scaling RAA = 1

N-Binary Scaling works for very rare processes i.g., Drell-Yan and direct photon production with some caveats (parton F2 and G change in A).

RAA can also be measured using central/peripheral ratios !

Collision Dynamics

(high pT processes !)

Collisions at high pCollisions at high pT T (pQCD)(pQCD)

)ˆˆˆ(),(

ˆˆˆ

),(),( 22

2/

2/3

3

utszDz

s

td

dxfxfdxdx

pd

dE hhc

h

cdab

bpbapaabcd

bah

h

At sufficiently large transverse momentum, let us consider the process:

p + p hadron + x

1) f(x,2) – parton structure function

2) ab->cd – pQCD calculable at large 2

3) D(zh,2) – Fragmentation function

Kinematic Variables in 22 process

)ˆˆ()(ˆ

)1()(ˆ

)(ˆ

).(

),(

232

2231

212

21

2

1

43

43

43

tsppu

epppt

sxxpps

ees

px

ees

px

yyT

yyT

yyT

Parton Distribution Function

Fragmentation Function from e+e collisionscharged hadron

PQCD LO Parton-Parton Cross Sections

Jet prominent at high energy collisions !

Jet total energy Avoid Fragmentations !

PQCD Works for p+p at RHIC

You can ‘see’ jets clearly!!

High Energy p+p collisions!

Four Jets event possible !

Jet Energy Reconstruction NOT at RHIC !

Hard Scattering and Jet Quenching

back-to-back jets disappear

leading particle suppressed

Hard Scattering in p+p Parton Energy Loss in A+A

Reduction of high pT particlesDisappearance of back-to-back high pT particle correlations

Disappearanceof back-to-back correlation !

Disappearance of back-to-back angular correlations

x

y ptrigpss

pos

Ptrig – pss same side correlation

Ptrig – pos opposite side corr.

ptrig> 4 GeV/c, pss pos 2<pT<ptrig

Suppression of high pT particles

pT Spectra Au+Au and p+p

p+p

Au+Au 0-5%

RAA=(Au+Au)/[Nbinaryx(p+p)]

Strong high pT suppression by a factor of 4-5 in central Au+Au collisions !The suppression sets in gradually from peripheral to central Au+Au collisions !

Two Explanations for High pT Observations

Energy Loss: Particles lose energy while traversing high density medium after the hard scattering. Energy loss quenches back-to-back angular correlations. J. Bjorken, M. Gyulassy, X-N Wang et al….

Parton Saturation: The parton (gluon) structure function in the relevant region (saturation scale) is modified. Not enough partons available to produce high pT particles. Parton fusion produces mono-jet with no back-to-back angular correlations. D. Kharzeev, L. McLerran, R. Venugopalan et al…..

d+Au Collisions

qq

qq

Au+Au Geometry d+Au Geometry

d+Au collisions: Little energy loss from the dense medium created, But Parton saturation from Au nuclei persists!

Data from d+Au collisions

No high pT suppression ! No disappearance of back-to-back correlations!

High pT Phenomena at RHIC

Very dense matter has been created in central Au+Au collisions!

This dense matter is responsible for the disappearance of back-to-back correlation and the suppression of high pT particles !

The Suppression is the Same for and – parton level effect

No suppression for direct photons – photons do not participant !

No Significant Difference BetweenQuarks and Gluons at High pT

Baryons more likely from gluon fragmentations in the pQCD region

STAR PRELIMINARY

Energy Loss and Soft Particle Production

Leading hadrons

Medium

High pT Physics 1) Energy Balance in Jet Production and Trigger pT Effect

-- Mach Shockwave Phenomenon-- and Correlations

2) Heavy Quark Energy Loss

3) -jet Correlations

4) Di-jet Correlations-- kT Smearing-- Nuclear A Dependence of kT Scale

5) Can High pT Probes Be Sensitive to the DOF of the Dense Medium?

6) Color Glass Condensate

STAR data motivated sonic-boom prediction

F. Wang (STAR), QM’04 talk, nucl-ex/0404010.Now published: STAR, PRL 95, 152301 (2005).

pTtrig=4-6 GeV/c, pT

assoc=0.15-4 GeV/c

Many recent studies:H. Stoecker, nucl-th/0406018.Muller, Ruppert, nucl-th/0507043.Chaudhuri, Heinz, nucl-th/0503028.Y.G. Ma, et al. nucl-th/0601012.

Casalderrey-Solana, Shuryak, Teaney, hep-ph/0411315

Actually sonic-boom was first predicted in the 70’s by the Frankfurt school.

Sonic Boom

cos M sc

M Trigger

Casalderrey-Solana, Shuryak and Teaney

Linearize disturbance

(1) Can hydro equation be applied to a few particles?(2) Transverse expansion?

Dumitru

Jet-Medium InteractionsJet-Medium Interactions

• how does a fast moving color charge influence the medium it is traversing?

• can Mach-shockwaves be created?

information on plasma’s properties is contained in longitudinal and transverse components of the dielectricity tensor

two scenarios of interest:1. High temperature pQCD plasma2. Strongly coupled quantum liquid

(sQGP)

• H. Stoecker, Nucl. Phys. A750 (2005) 121• J. Ruppert & B. Mueller, Phys. Lett. B618 (2005) 123• J. Casalderrey-Solana, E.V. Shuryak, D. Teaney, hep-ph/0411315

1. High temperature pQCD plasma:• Calculation in HTL approximation• color charge density wake is a co-moving screening

cloud

2. Strongly coupled quantum liquid (sQGP):• subsonic jet: analogous results to pQCD plasma case• supersonic jet: emission of plasma oscillations with Mach

cone emission angle: ΔΦ=arccos(u/v) [v: parton velocity, u: plasmon propag. velocity]

Wakes in the QCD MediumWakes in the QCD Medium

J. Ruppert & B. Mueller, Phys. Lett. B618 (2005)

123

High pT

Low pT

The Structure at Correlations in Central Au+Au

At Low pT !

In order to discriminate Mach-cone from deflected jets, one needs three-particle correlation.

away

near

Medium

mach cone

Mediumaway

near

deflected jets 1

2

0

0

1

2

0

0

01

221 2

0

1

21

2

pp Au+Au 80-50% Au+Au 30-10%

d+Au Au+Au 50-30% Au+Au 10-0%

STAR preliminary 3-particle correlation results

Au+Au ZDC central (12%)data: x10 more statistics.

: System, Centrality Dependence at 200 GeV

3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig)

|| < 0.5

STAR preliminary

Au+Au: peak broadens, height drops with centrality

: System, Centrality Dependence at 200 GeV

increases from p+p to central Au+Au at lower pT(trig)

– Higher pT(trig) flat across all centralities

– Systematic error not assigned (fit range, projection window)

2 < pT(assoc) < pT(trig)|| < 0.5

6 < pT(trig) < 12 GeV

3 < pT(trig) < 6 GeV

Observed baryon to meson ratio is higher for away-side jetsObserved baryon to meson ratio is higher for away-side jetsObserved baryon to meson ratio is higher for away-side jetsObserved baryon to meson ratio is higher for away-side jets

Centrality Dependence Jet-yield Ratios for baryons and mesons

Partonic Matter Hadronization

1) Definition of Nuclear Modification Factors and v2

2) pT Scale for Fragmentation Processes

3) Degree of Freedom at Hadron Formation

4) More Identified Particles and Higher pT

5) Model Dependence – Recombination/Coalescence

The The Field & FeynmanField & Feynman picture of cascade fragmentation picture of cascade fragmentation

Kretzer@ISMD04

Baryon Production from pQCD

K Kp p

e+e-jet fragmentation from SLD

Normal Fragmentation Cannot Produce the Large Baryon Yield

Too Many Baryons at Intermediate pT

pT Scales and Physical Processes

RCPThree PT Regions:

-- Fragmentation

-- multi-parton dynamics (recombination or coalescence or …)

-- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

Multi-Parton Dynamics for Bulk Matter Hadronization

Essential difference:Traditional fragmentation particle properties mostly determined by the leading quark !Emerging picture from RHIC data (RAA/RCP and v2) all

constituent quarks are almost equally important in determining particle properties !

v2 of hadron comes from v2 of all constituent quarks !

The fact that in order to explain the v2 of hadrons individual constituent quarks (n=2-meson,3-baryon) must have a collective elliptic flow v2 and the hadron v2 is the sum of quark v2 Strong Evidence for Deconfiement !

Recombination+Fragmentation ModelRecombination+Fragmentation Model

basic assumptions:

• at low pt, the quarks and antiquark spectrum is thermal and they recombine into hadrons locally “at an instant”:

features of the parton spectrum are shifted to higher pt in the hadron spectrum

• at high pt, the parton spectrum is given by a pQCD power law, partons suffer jet energy loss and hadrons are formed via fragmentation of quarks and gluons

qq M qqq B

• shape of parton spectrum determines if recombination is more effective than fragmentation• baryons are shifted to higher pt than mesons, for same quark distribution understand behavior of baryons!

Reco: Single Particle Observables Reco: Single Particle Observables

consistent description of spectra, ratios and RAA

Recombination model (Hwa+Yang)

p

Parton distribution

p1+p

2

p q

(recombine)(fragment)

hadron momentum

Mesons(2 quarks):

Baryons(3 quarks):

F: joint distribution of partons

T: thermal parton(low pt) S: shower parton(high pt)

The traditional hadronization:

high momentum partons fragment into hadrons

Recombination as a hadronization process:

lower momentum partons recombine to a hadron. May cause higher yield at some pT region.

Recombination model on d+Au data

proton

0-20%/60-90%

p

dE/dx at higher pT

Momentum: GeV/cdE/dx of K,p) separation: 2

10~

3 p

Log10(p)

Log

10(d

E/d

x)

Identified Particles at High pT

STAR:TPC dE/dx + TOF; Topological Identified Particles

Open Issues

1) Is there a MACH cone effect and how the energyloss of partons is distributed in the medium?

2) Light quark (u,d,s) versus heavy quark (c,b), and quarks versus gluons

3) Multi-parton correlations and coalescencerecombination models

The END

Comparison with calculations• Any of pQCD calculations

describe data well– Adding kT broadening

makes factor of ~2 difference• Around same factor as

E706– Calculation suggests that

slopes of the spectra at RHIC and E706 are same

• Jet Photon included calculation (Fries et al., PRL 90, 132301 (2003)) is also shown– Fits very well above 4GeV!– Assuming existence of hot

dense medium• Prompt partons scatter with

thermal partons– The line approaches to

simple pQCD calculation in high pT

Photon-hadron correlations

STA

R p

relim

inar

y

STA

R p

relim

inar

y

+jet correlation in Au+Au in run4?

More accurate determination of initial Et

Jet Photon overwhelms QGP?• Break-up of Fries prediction

• Jet Photons overwhelms all the other contributions below 7GeV/c

• Jet production rate calculated by LO pQCD with K factor compensation of 2.5

• pQCD photon calculation from LO with no K factor

• Fitting too good!– In Peripheral, the calculation

should fit the data as well• RAA and spectra themselves tell

you what happens– Calculation is assuming

existence of hot dense medium, which is not the case in peripheral!

Results for p-p

• NLO-pQCD calculation– CTEQ6M PDF.

– Gluon Compton scattering

+ fragmentation photon

– Set Renormalization scale

and factorization scale pT/2,pT,2pT

• Systematic Error: – 20(high pT)-45(low pT)%

The theory calculation shows a good agreement with our result.

(Subtraction)Bands represent systematic errors.

Errors on the backgrounds result in enlarged errors on the signal,especially at low-pT region.

Implication of the Experimental Observation

1) At the moment of hadronization in nucleus-nucleus collisions at RHIC the dominant degrees of freedom is related to number of constituent (valence) quarks.

2) These ‘constituent quarks’ exhibit an angular anisotropy resulting from collective interactions.

3) Hadrons seem to be formed from coalescence or recombination of the ‘constituent quarks’, and the hadron properties are determined by the sum of ‘constituent quarks’.

Is this picture consistent with recent LQCD on spectral function calculations near Tc ?