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