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

• Does v2 saturate for all species?
• 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

18
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?