Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 1

– OUTLINE –

motivationanalysisresults

summary

Fuqiang WangPurdue University

for the Collaboration

STAR Measurement of Jet Modification in Au+Au

Collisions

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 2

Physics motivationThe goal of RHIC is to create QGP –

a state of deconfined, thermalized quarks and gluons

Lattice QCD prediction:

F. Karsch, Nucl. Phys. A698, 199c (2002)

TC ~ 170 15 MeV

C ~ 0.5 GeV/fm3

Two prerequisites:(1) High enough energy

density(2) Parton thermalization

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 3

(1) energy density: Bjorken estimate

Cold nuclear matter:0 ~ 0.16 GeV/fm3

30.5 GeV/fmC

Boost invariant hydrodynamics: Bjorken Estimate of Initial Energy Density

d

dNp

Rdy

dE

Rch

TT

2

31122

30x0

nucl-ex/0311017

PRL 87 (01) 52301

~1 fm/c

Low bound: likely is smaller at RHIC.

ET drops with time.

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 4

(1) energy density: “measuring” it?

Jet quenching:• Large pT partons/jets are generated early (initial hard-scatterings) d+Au ~ p+p: hard-scatterings are similar in Au+Au and p+p• Partons / jets need time to escape the collision zone, during which a QGP (or whatever medium) is formed.• Partons/jets lose energy when traversing and interacting with the medium (final state interactions) modifications to jets

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 5

(1) energy density: inferred from models

pQCD calculations:

x30 gluon density x100 energy densityin central Au+Au collisions~ Bjorken estimate

X.-N. Wang, PLB 579 (04) 299

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 6

Measuring the lost energy?

How is energy distributed?

amount of energy loss?

contribution from medium?

possible… by going to low pT.S. Pal, S. Pratt, PLB574 (2003) 21.X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036.C.A. Salgado, U.A. Wiedemann, hep-ph/0310079.M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/0302077.……

Pal, Pratt, PLB 574 (2003) 21

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 7

(2) thermalizationThermalization in the final state:

• Final state low pT hadron distributions look thermal.• Event-by-event <pT> variation is small:

every event looks thermal.• Hadron compositions described by thermal models.• TChemical ~ 160 MeV ~ TC.Necessary but not sufficient condition for early thermalization.

Early state thermalization?

• First time at RHIC elliptic flow at low pT described by hydro:zero mean free path max. possible v2.

• SPS v2 lower than hydro, however energy density may be not much lower.

NA49, PRC 68 (2003) 34903

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 8

(2) thermalization at work?

yes… by putting two sources of particles together:• one from jet fragmentation that are initially hard.• the other from bulk medium that are soft.

going to low pT.

medium

jet

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 9

Are leading particles from jets?

PHENIX, PRL 91, 172301 (2003)

p / ~ 0.9 in central

p / ~ 0.3 in peripheral

non-frag. p / ~ 0.6non-frag. p / Nch ~ 0.3

pT=3-4 GeV/c: ~30% are probably from other sources.

B. Alper, NPB 87 (1975) 41

p+p @ ISRQM’04

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 10

Coalescence / recombination models

Fries et al, PRC 68 (03) 44902 Greco et al, PRC 68 (03) 34904

Hwa et al, nucl-th/0401001Coalescence / recombination models

predict a range of non-fragmentation contributions.

All predict a rapid drop of non-fragmentation contribution above 4 GeV/c.

pT>4 GeV/c: may mainly come from jets, or related to jets.

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 11

Reconstructing low pT associated particles

High pT particle

p+p

High pT particle

Au+Au

• Select a leading particle 4<pT<6 GeV/c, ||<0.75.

• Associate other particles (0.15<pT<4 GeV/c,||<1.1) with the leading particle. Form correlation.

• Background from mix- events. v2 modulation on background. Normalize in 0.9<||<1.3.

• Efficiency corrections are applied to associated particles.

• Take difference and normalize per trigger.

(1/N

trig)

dN

/d(

)

STAR Preliminary

background

Signal

p+pJet-like structures

Au+Au top 5%

4 6 GeV/c, 0.15 4 GeV/ctrig assocT Tp p

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 12

Azimuth angular correlations

M.G. Albrow et al. NPB145, 305 (1978)High pT:1/

Ntr

igge

r dN

/d()

(radians)

Low pT:

near side: ||<1.1, ||<1.4

away side: ||<2, ||<1.1

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 13

(1/N

trig)

dN/d

() bkgd subt.

(1/N

trig)

dN/d

()

Au+Au top 5%

near

“Jet” sizes

With increasing centrality:• Near side broadens in but not .• Away side modest increase in size.

STAR Preliminary

4 / 12

/ 12

near: ||<1.1, ||<1.4away: ||<2, ||<1.1

4 6 GeV/c, 0.15 4 GeV/ctrig assocT Tp p

RMS

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 14

assoc

away

1.5 Tp

trig assoc

near

1.5T Tp p

“Jet” charge multiplicity and “energy”

With the same final leading particle,we are selecting a larger energy jet

in central AA than in pp.

AA pp

For the same final leading particle

(4 < < 6 GeV/c):

near side "jet" energy difference:

1.4 0.2 0.2 GeV

away diff. in TPC 2.2 0.2 0.3 GeV

trigTp

E E

STAR Preliminary

p+p

Leading particle: 4 6 GeV/c

4.5 GeV/c

Associated particle: 0.15 4 GeV/c

trigT

trigT

T

p

p

p

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 15

Medium contribution?

points: lines:T Tp p

away near

T Tp p

TPC acceptance of away side partner?

Total scalar pT:Initial parton energy + medium contribution?

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 16

Jet quenching model

X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036

with energy loss

without energy loss

E = 1.4 – 2.2 GeV}Caution: cannot be readily compared to data yet.

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 17

Thermal-shower recombination

Hwa, Yang, nucl-th/0401001

In this model, the thermal-shower recombination is the largest contributionto high pT particles.

One mechanism for energycontribution from medium.

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 18

Away

syst. error

Associated particles pT distributions

STAR Preliminary

Near side:

overallenhancementfrom pp to AA

larger initialparton energy(and modestenergy loss)?

4 6 GeV/ctrigTp

Away side:

energy from theinitial parton has been converted

to lower pT

particles

energy loss inmedium!

Near

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 19

Away side <pT>

away side associated particle <pT> decreases

with centrality, approachingmedium hadron <pT> in

central collisions

equilibration between the two sources of particles

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 20

(1/N

trig)

dN/d

()

STAR Preliminary

p+p Au+Au 5%trig4 6 GeV/c

0.15 4 GeV/cT

T

p

p

Fit to near side: const. + gaussian + Borghini-cos(fixed)

stat. mom. conserv.Borghini et al.

free fit

stat. mom. conserv.Borghini et al.

Broadened distribution and thermalization

HIJINGall

STAR|| < 0.5

p+p 0.23 0.26

Au+Au 5% 0.31 0.50

2 2 [GeV/c]Tp1 2

2all all

2pT T T

T

p pC

N p

ZZZZZZZZZZZZZZZZZZZZZZZZZZZZ

Borghini et al. PRC 62, 034902 (2000):

jet .2

.

cos( )

( )Tacc

TTall

pdNP

d p

the away side excess approaching equilibrium with the medium!

Cannot distinguish: (1) the full event participates in momentum balance. (2) Only a handful particles: e.g. jet-jet production.

the away excess has a similar shape to a stat. distr. from momentum conservation.

Fuqiang Wang – Users’ meeting workshop – Jets, jT, kT 21

Summary

(1) high enough energy density?

Models require x30 normal nuclear gluon density to describe suppression data.

Statistical reconstruction of jets in pp and AA collisions.Potential possibility of experimental measure of energy loss.

Same pT leading particles come from larger jet energy in central AA than in pp.

• Near side: overall increase in multiplicity.

• Away side: increase in multiplicity and softening in pT.

(2) parton thermalization?

• Away side: towards thermalization in more central collisions.

• May imply high degree of thermalization in medium itself.