Kirill Filimonov

1
Anisotropy and high pT hadrons in AuAu
collisions at RHIC

• Kirill Filimonov
• Lawrence Berkeley National Laboratory

• Introduction
• Inclusive spectra
• Jets and dijets
• Azimuthal anisotropy at high pT

2
Jets in heavy ion collisions
AuAu ???? (STAR_at_RHIC)
ee- ?qqbar (OPAL_at_LEP)
pp ?jetjet (STAR_at_RHIC)
3
Leading Particle Suppression
Jets modified in heavy ion collisions? -Leading
particle suppression already observed in AuAu at
130 GeV
leading particle
Partonic Energy loss in high density matter
Nuclear Modification Factor
Glauber Model
4
High pT hadrons AuAu at RHIC

• Excellent quality data from the experiments

5
Hadron suppression AuAu at 200 GeV
PHENIX p0 peripheral and central over measured
pp
STAR charged hadrons central/peripheral
Preliminary ?sNN 200 GeV
PHENIX preliminary
QM02 prelim. data suppression of factor 4-5
persists to pT12 GeV/c
6
Two-particle correlations at high pT

• Jet core Df x Dh 0.5 x 0.5
• look at near-side correlations (Df0) of high pT
  hadron pairs
• Complication elliptic flow
• high pT hadrons correlated with the reaction
  plane orientation also correlated with each other
  (v22)
• but elliptic flow has long range correlation (Dh
  gtgt 0.5)
• Solution compare azimuthal correlation
  functions for Dhlt0.5 and Dhgt0.5

7
Direct evidence for jets

• Azimuthal correlation function
• Trigger particle pT triggt 4 GeV/c
• Associate tracks 2 lt pT lt pTtrig

Central AuAu
N.B. Away-side jet contribution subtracted by
construction, needs different method
Near-side correlation shows jet-like signal in
central AuAu
8
Suppression of back-to-back pairs
Peripheral Au Au

• Near-side well-described
• Away-side suppression in central collisions

STAR Preliminary
STAR Preliminary
near side
Central Au Au
STAR Preliminary
away side
Away side jets are suppressed!
9
Azimuthal anisotropy v2
Coordinate space initial asymmetry
Momentum space final asymmetry
py
multiple collisions (pressure)
px
x
10
v2 predictions at high pT
pQCD inelastic energy loss parameterized hydro
component
M. Gyulassy, I. Vitev and X.N. Wang
PRL 86 (2001) 2537

• The value of v2 at high pt sensitive to the
  initial gluon density
• Saturation and decrease of v2 as a function of pt
  at higher pt

11
Comparison v2 (pT) with models (130 GeV)
Adler et al., nucl-ex/0206006, STAR

• qualitative agreement with jet-quenching
  scenario

12
?s dependence at high pT (200/130)
STAR preliminary
Ratio of spectra 200/130
Ratio of v2 200/130

• Inclusive spectra follow pQCD prediction (XN
  Wang)
• minbias v2 has no energy dependence at pTgt2 GeV
• ? surface emission at high pT? (i.e. strong
  absorption)

13
Centrality dependence of v2(pT)
PHENIX, preliminary
STAR, preliminary
()

• v2 saturates for pTgt3 GeV/c for all centralities
• Indication of geometric origin?

14
Comparison with parton cascade
D. Molnar 5x minijet multiplicity from
HIJING 13x pQCD gg?gg cross section
Preliminary

1. Does v2 saturate for all species?
2. Will v2 decrease beyond pT6 GeV/c?

15
v2(pt) proposed scenario flavor dependence
Baryon production by a non-perturbative mechanism
(junctions or hydro)
M. Gyulassy, I. Vitev, X.N. Wang and P. Huovinen,
Phys. Lett. B 526 (2002) 301-308
16
v2 of identified hadrons
STAR Preliminary
STAR Preliminary

• Mesons and baryons show
• different trend at higher pT
• Qualitative agreement with the predictions

Negatives pi-K-,pbar
PHENIX Preliminary
17
v2(pT) up to 12 GeV/c (STAR)

• Finite v2 up to 12 GeV/c in mid-peripheral bin
• Decrease from saturation level for more central
  collisions

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Summary

• Jet fragmentation observed, agreement with pQCD
• Strong suppression of inclusive yields
• Suppression of back-to-back hadron pairs
• Azimuthal anisotropy at high pT
• large parton energy loss and surface emission?
• What would large energy loss imply?
• Partonic or hadronic energy loss?
• Thermalization?