Jet fragmentation in A+A collisionsAndrew AdareUniversity of Coloradofor the PHENIX Collaboration
2Energy loss and modified jet fragmentation
Example of quenched vs. unquenched frag. function
Experimental observation:
Hadronic jet fragmentation products suffer suppressed yields in A+A collisions.
This suppression reflects energy loss by hard-scattered partons, and a softened fragmentation function.
Z
3Probing E-loss at high pT: RAA vs. correlations
RAA suggests large energy loss, but can’t get to P(E).
Study of correlated particles adds spatial and kinematic information on high-pT jet quenching.
High-pT jet correlation observables characterizeWidth Shape Yield
Vacuum D(Z)E-loss prob.Hard-scattering x-section
Suppressed hadron cross-section (~RAA)
T. Renk
Phys.Rev.C77:017901,2008.
How do these quantities depend on energy and system size?
Au+Au di-hadron 1/Ntrig dNpair/d()
4pT evolution of dihadron correlations
Trends: As pT increases,
– Double-peak single peak
– Away-side yield drops
– Head region yield begins to dominate over shoulder region
arXiv:0801.4545 [nucl-ex]
See poster P54 by J. Jia
SR jet
HR jet
HS Yd
YdR
)()(
)()(
(yields normalized over )
5RHS in di-hadron correlations
Would shoulder-dominated head-dominated transition still occur at higher trigger pT?
arXiv:0801.4545 [nucl-ex]
For high-pT triggers, Au+Au shape approaches p+p with growing pT
6Pushing to high pT in Au+Au0-h
“Zooming in” with 7-9 GeV 0s– Good reconstruction
efficiency– Good 0 S/B (> 5)– 3x statistics in Run 7 vs.
Run 4
Measured RMS width and RHS
*Not corrected for h± efficiency
PHENIX preliminaryHigher pT /
more peripheral
7RHS vs. partner pT
10
1
200 GeV di-hadron RHS
partner pT [GeV/c]
RHS > 1 in all bins
similar to p+p
Dip/rise trend in RHS not observed at higher pT
5-10 GeV trigger pT bin dominated by low range Onset of vacuum fragmentation at trigger pT > ~7 GeV
8Near side h-h correlations:p+p / Au+Au widths
p+p and Au+Au near-side widths agree closely at high pT
arXiv:0801.4545 [nucl-ex]
9
Near-side RMS width
No significant centrality dependence
0-h at high pT: near side
10
p+p 0-h:
6.5-8 x 1.4-5
away width = 0.350 ± 0.03 (Phys Rev D 74 072002)
Away side RMS width:Comparable to p+p; again, centrality-independent
0-h at high pT: Away side
11But broadening was predicted
Ejet = 7 GeV
Phys. Lett. B 630 (2005)Vitev: Large angle gluon radiationCollinear gluon emission is suppressedSignificant enhancement of peak width
Polosa and Salgado:Radiation-inducedshoulder enhancement
J.Phys.G34:S675-S678,2007
Wicks et al:Collisional E-loss also non-negligible!Multi-scattering leads to broadening
Nucl.Phys.A783:493-496,2007
12STAR hasn’t reported broadening either
8 < pT(trig) < 15 GeV/cpT(assoc)>6 GeV
M. Van Leeuwen, QM06
nucl-ex/0604018
13Understanding the width discrepancySome possibilities:
1. Theorists are overpredicting E-loss2. High pT dijets don’t probe the medium
Sizable P(E) fluctuations we observe mainly punch-thruGeometric bias we observe primarily surface emission
PRL 98 212301, 2007
tangential emmision
punch- through
reaction
plane
14
Gluon radiation also affects yields:R(2)
AA = IAA = (A+A / p+p) integrated jet yield ratio
Suppression predicted at high partner pT, enhancement at low pT
What about the jet pair yields?
I. Vitev,
Phys. Lett. B 630 (2005)
15Au+Au Away sideyield modificationIAA for head and head/shoulder regions
Strong partner pT dependence:High pT suppression / low pT enhancement occurs as predicted.Jet energy redistribution via jet-medium interactions:SR more enhanced than HR. Why? Could beWidening of head component (incoherent rad. / collisional E-loss)Mach/cherenkov energy transfer (coherent rad.)
arXiv:0801.4545 [nucl-ex]
16An additional probe:Fragmentation photons
NLO ’s - bremss. and jet fragmentationMay be enhanced in A+A from medium-induced bremsstrahlungNear-side integrated yield of fragmentation photons:Brand new measurement in p+p baseline
Near-side frag yields in A+A will add new information on fragmentation modification.
17Other prospects for tighter constraintsHigher Au+Au stats now available for analysis finer
binning to pin down shape evolution
2+1 Correlations Increase prob. that away side parton traverses medium. See H. Pei’s talk
+jet challenging, but nice progress, see M. Nguyen’s talk
trigger
additional away-side particle
partner
18What did we learn?
At trigger pT > 7 GeV, jet shapes and widths are p+p-like– suggests vacuum fragmentation– Challenges models predicting jet broadening
BUT:– We may mostly observe the hadronized partons that
passed through the medium with little/no interaction,– or didn’t pass through the bulk of the medium at all.
Jet yield measurements suggest energy transfer through the medium, in agreement with E-loss predictions.
Dijets are an important step towards QGP tomography– Shape/yield studies augment RAA
New data, new analysis technology inspires optimism!
19Extras
207-9 x 4-5 0-h per-trigger yields
21ZYAM/flow systematicsZYAM subtraction applied to measured, high, and low v2 curvesResulting variation taken as systematic error
Examples
Top: High/low CF fit curves shown after ZYAM applied
Bottom: Error boxes from v2 and ZYAM uncertainty.
22Mass peaks