Measurement of Jet Fragmentation at RHIC
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Measurement of Jet Fragmentation at RHIC Spectra of charged hadrons associated with a large p T leading particle. Fuqiang Wang Purdue University for the Collaboration. – OUTLINE – Physics motivation Analysis techniques Preliminary results Summary and open questions.

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Fuqiang wang purdue university for the collaboration

Measurement of Jet Fragmentation at RHICSpectra of charged hadrons associated with a large pT leading particle

Fuqiang Wang

Purdue University

for the Collaboration

– OUTLINE –

Physics motivation

Analysis techniques

Preliminary results

Summary and open questions


Physics motivations
Physics motivations

The goal of RHIC is to create QGP – a thermalized partons state

RHIC AA + pp + dAu results:Hard-scatterings are initially present;

Suppression phenomena in central Au+Au

are due to final-state interactions.

(consistent with jet quenching)

…through interactions with what medium?

  • Study how the energy is distributed.

  • Investigate equilibration between the energy and the surrounding medium.

  • Measure the amount of energy loss.

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.

……

…by reconstructing hadrons associated with a large pT particle.

Fuqiang Wang – QM 2004 – High pT Jets


Reconstructing associated particles

STAR Preliminary

High pT particle

0.15<pT<4 GeV/c

High pT particle

p+p

Jet-like structures

Au+Au

p+p

(1/Ntrig) dN/d(Df)

Signal

Au+Au top 5%

background

Df

Reconstructing associated particles

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

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

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

  • Efficiency corrections are applied to associated particles.

  • Take difference and normalize per trigger.

Fuqiang Wang – QM 2004 – High pT Jets


Jet sizes

STAR Preliminary

s

RMS

“Jet” sizes

near: |Df|<1.1, |Dh|<1.4

away: |Df-p|<2, |h|<1.1

bkgd subt.

Au+Au top 5%

(1/Ntrig) dN/d(Df)

Df

near

(1/Ntrig) dN/d(Dh)

  • With increasing centrality:

  • Near side broadens in h but not f.

  • Away side modest increase in size.

Dh

Fuqiang Wang – QM 2004 – High pT Jets


Jet charge multiplicity and energy

p+p

“Jet” charge multiplicity and “energy”

STAR Preliminary

With the same final leading particle,

we are selecting a larger energy jet

in central AA than in pp.

Fuqiang Wang – QM 2004 – High pT Jets


Associated particles p t distributions

Near side:

Overall

enhancement

from pp to AA

larger initial

parton energy

(and modest

energy loss)?

Away side:

energy from the

initial parton has

been converted to

lower pT particles

energy loss in

medium!

syst. error

Near

Away

pT (GeV/c)

Associated particles pT distributions

STAR Preliminary

Fuqiang Wang – QM 2004 – High pT Jets


Energy loss and thermalization

STAR Preliminary

[a.u.]

near

away

away

Collimatedregion

away side:

whole: |Df-p|<2.0

collimated: |Df-p|<0.35

collimated/whole

Energy loss and thermalization

STAR Preliminary

  • Away side <pT> decreases with centrality.

  • Away side collimated region is harder in pp and peripheral AA – jet property. But the collimated region is softer in central AA! Effect appears gradually with centrality.

  • Away side <pT> is still larger than that from the inclusive result.

 Towards thermalization in more central collisions!

Fuqiang Wang – QM 2004 – High pT Jets


Broadened distribution and thermalization

STAR, PRL90, 082302 (2003)

STAR Preliminary: Y. Guo poster P25

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

STAR, PRL90, 082302 (2003)

STAR Preliminary: Ying Guo poster

(1/Ntrig) dN/d(Df)

(1/Ntrig) dN/d(Df)

Df

Df

STAR Preliminary

STAR Preliminary

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

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

free fit

p+p

Au+Au 5%

(1/Ntrig) dN/d(Df)

p+p

Au+Au 5%

free fit

Df

Df

stat. mom. conserv.Borghini et al.

stat. mom. conserv.Borghini et al.

Df

Df

(1/Ntrig) dN/d(Df)

cos strength per particle

cos strength per particle

stat. mom. conserv.Borghini et al.

stat. mom. conserv.Borghini et al.

Df

Df

assoc. particle pT (GeV/c)

trigger particle pT (GeV/c)

Broadened distribution and thermalization

e.g. a thermal fluctuated large pT particle (or a mono-jet) would produce an away side excess due to momentum conservation.

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

  • near side is mostly a jet, and initially no mono-jet at mid-rapidity.

No punch-through for 6 < pTtrig < 10 GeV/c.

  • the away side excess is approaching equilibration with the medium, consistent with the pT spectra results.

Fuqiang Wang – QM 2004 – High pT Jets


Summary and open questions
Summary and Open Questions

  • Statistical reconstruction of jets in pp and AA collisions.

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

  • Near side: overall increase in multiplicity.

  • Away side: increase in multiplicity as well as softening in pT.

  • Away side: towards thermalization in more central collisions.

  • How does jet lose energy? In what medium?How & to what extent are they equilibrated with the medium?

    Statistical jet reconstruction opens up opportunities to answer these questions experimentally.

Fuqiang Wang – QM 2004 – High pT Jets


Backup slides
---Backup slides---

Backup Slides

Fuqiang Wang – QM 2004 – High pT Jets


What we know from previous measurements

RCP

60-92%

10%

What we know from previous measurements…

Suppression phenomena in central Au+Au are due to final-state interactions. Parton-parton hard-scatterings are initially present.

pT > 4 GeV/c region is mainly from jet fragmentation.

Fuqiang Wang – QM 2004 – High pT Jets


Baryon meson
Baryon/meson

Fuqiang Wang – QM 2004 – High pT Jets


Analysis technique ii

Significance

of signal:

Analysis Technique II

Leading particle: |h|<0.75, 4<pT<6 GeV/c Associated particle: |h|<1.1, 0.5<pT<4 GeV/c

STAR Preliminary

Background with

v2 modulation.

Normalized to

0.9<|Df|<1.3

Near side

Away side

Central:

Ntrig=103K, S/B=2/200

Peripheral:

Ntrig=5K, S/B=1/13

Near side:

|Dh|<1.4, |Df|<1.1

Away side:

|h|<1.1, |Df|>1.1

Df

Fuqiang Wang – QM 2004 – High pT Jets


Aa pp vs i aa
AA/pp vs IAA

Peripheral:

AA/pp=1.01

IAA=0.84

- broadening in h

- pp reference

- v2

- model difference

Central:

AA/pp=1.66

IAA=1.25

Df

Df

Fuqiang Wang – QM 2004 – High pT Jets


I aa 1 in peripheral
IAA < 1 in peripheral ?

STAR Preliminary

pp reference

Fuqiang Wang – QM 2004 – High pT Jets


Compare to ua1
Compare to UA1

STAR Preliminary

dN/dpT2 [a.u.]

pT (GeV/c)

Fuqiang Wang – QM 2004 – High pT Jets


Existing pp pa results
Existing pp, pA results

M.G. Albrow et al. NPB145, 305 (1978)

G. Boca et al. ZPC49, 543 (1991)

Fuqiang Wang – QM 2004 – High pT Jets


Higher trigger p t
Higher trigger pT

STAR Preliminary

Fuqiang Wang – QM 2004 – High pT Jets


Cos coefficient vs centrality
Cos-coefficient vs centrality

p+p

60-40%

80-60%

20-10%

40-30%

30-20%

10-5%

5-0%

cos coeff.

Nch

Fuqiang Wang – QM 2004 – High pT Jets


Hijing no quenching
Hijing (no quenching)

4 < pTtrig < 6 GeV/c

0.15 < pT < 4 GeV/c

2 < pT < 4 GeV/c

Df

Df

Fuqiang Wang – QM 2004 – High pT Jets


Like sign and unlike sign
Like-sign and unlike-sign

STAR Preliminary

Fuqiang Wang – QM 2004 – High pT Jets


Unlike vs like top 10
Unlike vs like (top 10%)

Unlike-sign

Like-sign

Back-to-back paper

|Dh|<0.5

|Dh|>0.5

1.2

|Dh|<0.5

+

|Dh|>0.5

1.0

0.2-0.3

(Unlike-like)/unlike =

0.1

|Dh|<0.5

-

|Dh|>0.5

1-1.5

0

Df

Fuqiang Wang – QM 2004 – High pT Jets


Pp left auau top 5 right
pp (left) AuAu top 5% (right)

Unlike-sign

Like-sign

Unlike-sign

Like-sign

0.7

0.15

|Dh|<0.5

-0.03

0.5

0.7

0.15

|Dh|>0.5

-0.03

0.5

|Dh|<0.5

+

|Dh|>0.5

0.15

1.3

-0.03

1.0

|Dh|<0.5

-

|Dh|>0.5

0.15

0.1

-0.03

-0.05

Df

Df

Fuqiang Wang – QM 2004 – High pT Jets