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

1
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
2
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 AuAu 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.
3
Reconstructing associated particles

• Select a leading particle 4ltpTlt6 GeV/c,
  hlt0.75.
• Associate all other particles (0.15ltpTlt4
  GeV/c, hlt1.1) with the leading particle.
  Form Df,Dh correlation.
• Background from mix- events. v2 modulation
  on background. Normalize in 0.9ltDflt1.3.
• Efficiency corrections are applied to
  associated particles.
• Take difference and normalize per trigger.

4
Jet sizes
near Dflt1.1, Dhlt1.4 away Df-plt2, hlt1.1
bkgd subt.
AuAu 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
5
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.
6
Associated particles pT distributions
STAR Preliminary
7
Energy loss and thermalization
STAR Preliminary

• Away side ltpTgt 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 ltpTgt is still larger than that from the
  inclusive result.

? Towards thermalization in more central
collisions!
8
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.
HIJING all h STAR h lt 0.5
pp 0.23 0.26
AuAu 5 0.31 0.50

• 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 lt pTtrig lt 10 GeV/c.

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

9
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.

10
---Backup slides---
Backup Slides
11
What we know from previous measurements
Suppression phenomena in central AuAu are due to
final-state interactions. Parton-parton
hard-scatterings are initially present.
pT gt 4 GeV/c region is mainly from jet
fragmentation.
12
Baryon/meson
13
Analysis Technique II
Leading particle hlt0.75, 4ltpTlt6 GeV/c
Associated particle hlt1.1, 0.5ltpTlt4 GeV/c
STAR Preliminary
Background with v2 modulation. Normalized to
0.9ltDflt1.3
Near side
Away side
Central Ntrig103K, S/B2/200
Peripheral Ntrig5K, S/B1/13
Near side Dhlt1.4, Dflt1.1 Away side
hlt1.1, Dfgt1.1
Df
14
AA/pp vs IAA
Peripheral AA/pp1.01 IAA0.84
- broadening in h - pp reference - v2 - model
difference
Central AA/pp1.66 IAA1.25
Df
Df
15
IAA lt 1 in peripheral ?
STAR Preliminary
pp reference
16
Compare to UA1
STAR Preliminary
dN/dpT2 a.u.
pT (GeV/c)
17
Existing pp, pA results
G. Boca et al. ZPC49, 543 (1991)
M.G. Albrow et al. NPB145, 305 (1978)
18
Higher trigger pT
STAR Preliminary
19
Cos-coefficient vs centrality
pp
60-40
80-60
20-10
40-30
30-20
10-5
5-0
cos coeff.
Nch
20
Hijing (no quenching)
4 lt pTtrig lt 6 GeV/c
0.15 lt pT lt 4 GeV/c
2 lt pT lt 4 GeV/c
Df
Df
21
Like-sign and unlike-sign
STAR Preliminary
22
Unlike vs like (top 10)
Unlike-sign
Like-sign
Back-to-back paper
Dhlt0.5
Dhgt0.5
1.2
Dhlt0.5 Dhgt0.5
1.0
0.2-0.3
(Unlike-like)/unlike
0.1
Dhlt0.5 - Dhgt0.5
1-1.5
0
Df
23
pp (left) AuAu top 5
(right)
Unlike-sign
Like-sign
Unlike-sign
Like-sign
0.7
0.15
Dhlt0.5
-0.03
0.5
0.7
0.15
Dhgt0.5
-0.03
0.5
Dhlt0.5 Dhgt0.5
0.15
1.3
-0.03
1.0
Dhlt0.5 - Dhgt0.5
0.15
0.1
-0.03
-0.05
Df
Df