Medium properties and jet-medium interaction from STAR

1
Medium properties and jet-medium
interaction from STAR
Jana Bielcikova
for the STAR
Collaboration NPI ASCR
and Center for physics of ultra-relativistic
heavy-ion collisions,
Prague, Czech Republic


XLIII Rencontres de Moriond QCD and High Energy
Interactions
2

• Outline
• Introduction
• Jet-medium interaction via 2- and 3-particle
  correlations
• - conical emission ?
• - long range pseudo-rapidity
  correlations (ridge)
• Summary

3
Probing QCD matter with jets
pp
AuAu
What happens to high-pT particles/jets which
pass through the medium? Are they similar to pp
or modified by the medium?
4
Jet-like correlations at RHIC
4 ltpT(trig)lt6 GeV/c
2 GeV/c ltpT(assoc)ltpT(trig)
STAR, PRL 91 (2003) 072304

• central AuAu collisions
  _at_ 200 GeV
• disappearance of away-side correlations observed
  at intermediate pT
• dAu and pp similar -gt jet suppression is a
  final state effect
• lowering associated pT resurrects correlated
  yield
• - enhanced yield at near and away side
  shape modification

5
Conical emission in AA collisions?
2.5 lt pTtriglt 4 GeV/c and 1lt pTassoc lt 2.5 GeV/c

• Mach cone in heavy-ion physics introduced
• in1970s (Hofmann, Stöcker, Heinz, Scheid,
  Greiner)
• a supersonic parton creates shock waves
• - hydrodynamics
• Stöcker et al.,
  NPA750 (2005) 121
• Casalderrey-Solana et
  al., NPA774 (2006) 577
• Renk, Ruppert, PRC73
  (2006) 011901
• - colored plasma
• Ruppert, Mueller, PLB618
  (2005) 123
• - AdS/CFT
• Gubser, Pufu, Yarom,
  PRL100, (2008) 012301

M. Horner (STAR), J.Phys.G34, S995,2007
To distinguish from other mechanisms
3-particle correlation studies needed
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Conical flow or deflected jets? (I)
cartoons of 3-particle azimuthal correlations (1
trigger 2 associated particles)
di-jets
conical emission
deflected jets (by the collective movement,
flow, of the expanding medium)
Armesto , Salgado, Wiedemann, PRL 93, (2004)
242301
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Conical flow or deflected jets? (II)

• STAR uses 2 methods
• 1. Jetflow background
• - model dependent analysis
• evidence for conical emission
• 2. Cumulant method
• C. Pruneau (STAR),J.Phys.G34 (667),2007
• C. Pruneau, PRC 74 (2006) 064910
• unambiguous evidence
• for 3-particle correlations
• strength and shape of away-side
• structures depend on magnitude
• of flow coefficients
• no conclusive evidence for

dAu
central AuAu
STAR Preliminary
STAR Preliminary
Note Large and complicated backgrounds
J. Ulery (STAR), arXiv0704.0224
3 lt pT,trig lt 4 GeV/c 1 lt pT,assoc lt 2 GeV/c
STAR Preliminary
??
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Mach cone or Cerenkov gluons?

• Mach cone
• cone angle independent of pTassoc
• Cerenkov gluon radiation
• cone angle decreases with pTassoc

pT dependence of cone angle favors Mach cone
over Cerenkov gluon emission
Koch, Majumder, Wang, PRL96 172302 (2006)
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Momentum conservation in correlation analyses
Calculation for ltpT,trig gt 3.2 GeV/c ltpT,assoc
gt 1.2 GeV/c
N. Borghini, PRC 75 (2007) 021904
Calculation by N. Borghini momentum conservation
? sizable correlation between pairs or triplets
of high-pT particles in central AuAu collisions
at RHIC ? C3(pT) C3(flow)
? jet distorts the event needs
to be evaluated in the data
2-particle correlation after v2 subtraction

N. Borghini, arXiv0710.2588
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A closer look at the near-side peak
pTtrig3-6 GeV/c, 2 GeV/c ltpTassoclt pTtrig
dAu, 200 GeV
AuAu, 200 GeV
STAR preliminary
jet
ridge
Additional near-side correlation in ?? observed
in central AuAu collisions

What is the ridge? 1) Medium heating and parton
recombination Chiu Hwa PRC 72 (2005)
034903 2) Radial flow high-pT trigger
particle Shuryak, Phys.Rev.C76 (2007)
047901 Voloshin, nucl-th/0312065 NPA 749,
(2005) 287 3) Parton radiation and its coupling
to the longitudinal flow Armesto,
Salgado, Wiedemann, PRL 93 (2004) 242301
4) Momentum broadening in anisotropic QGP
Romatschke, PRC 75 (2007) 014901 5) Longitudinal
broadening of quenched jets in turbulent
color fields Majumder, Mueller, Bass, PRL
99 (2007) 042301 6) Momentum kick imparted on
partons in medium Wong, PRC PRC 76 (2007)
054908
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Centrality and system size dependence of
near-side yield

• jet yield independent of colliding system,
  
• Npart and trigger particle type
• ridge yield increases with Npart

AuAu J. B. (STAR), QM 2006 CuCu C. Nattrass
(STAR), QM2008
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Jet and ridge pT dependence
pT assoc gt 2 GeV/c
Ridge solid symbols Jet open symbols
J. Putschke (STAR), J.Phys.G34S679 (2007)
J. Putschke (STAR), J.Phys.G34S679 (2007)
Jet T(jet) gt T(bulk) T(jet)
increases with pTtrig Ridge
T(ridge) T(bulk) T(ridge) independent
of pTtrig ridge persists up to pTtrig 10
GeV/c
jet slope ridge slope inclusive slope
h-h correlations pTassociatedgt2GeV/c
jet slope ridge slope inclusive slope
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Particle composition in jet and ridge

• Baryon/meson ratios
• jet smaller than inclusive
• ridge similar to inclusive

CuCu C. Nattrass (STAR), QM2008 AuAu J.B.
(STAR), WWND07
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Near-side jet and ridge w.r.t. event plane
3lt pTtrig lt 4 GeV/c, pTassoc 1.0- 1.5 GeV/c
A.Feng (STAR), QM2008
ridge decreases its magnitude
with fS jet
slight increase with fS magnitude
consistent with dAu
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3-particle DhxDh correlations
1) Jet fragmentation in vacuum
P. Netrakanti (STAR), QM 2008
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Summary

• Strong modification of correlation patterns (not
  present in dAu collisions) observed in central
  AA collisions at RHIC
• broadening of away-side peak with angular
  substructure
• inconsistent with Cerenkov gluon radiation
• possible evidence for conical emission ?
• (need to evaluate effects of momentum
  conservation)
• medium responds through ridge formation in
  pseudo-rapidity
• bulk-like properties (spectra, particle
  composition)
• medium density/path length effects
  (dominated in the event plane)
• further studies of 3-particle
  correlations to understand ridge underway
• THANK
  YOU!

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BACKUP
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Di-hadron correlations w.r.t. event plane
3lt pTtrig lt 4 GeV/c, pTassoc1.0-1.5 GeV/c
RMS
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Di-hadron correlations pathlength effects
3lt pTtrig lt 4 GeV/c, pTassoc1.0-1.5 GeV/c
AuAu 200 GeV
in-plane similar to dAu in 20-60 broader than
dAu in central collisions out-of-plane small
difference between the two centralities
A.Feng (STAR), QM2008
Away-side features reveal pathlength
effects
20-60
0-5
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Extracting near-side jet and ridge yields
J jet, R ridge
pTtrig3-4 GeV/c, pTassocgt2 GeV/c

J. Putschke (STAR),QM2006
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What is the origin of the ridge? (I)

• 1) Medium heating and parton recombination
• Chiu Hwa Phys. Rev.
  C72034903,2005
• hard parton enhances thermal parton distribution
• (?T15 MeV)
• ? recombination of thermal partons forms
• a pedestal (ridge)
• enhanced baryon/meson ratio
• 2) Parton radiation and its coupling
• to the longitudinal flow
• gluon bremsstrahlung of hard-scattered parton
• radiated gluon contributes to broadening

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What is the origin of the ridge? (II)

• 3) Longitudinal broadening of quenched
• jets in turbulent color fields
• A. Majumder, B. Mueller, S.A.Bass,
  hep-ph/0611135
• plasma instabilities in expanding medium
• ? non-thermal color fields
• ? broadening of jet cone
• ? wide ridge in rapidity at low
  pTassoc

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What is the origin of the ridge? (III)

• 4) Correlations between jet and radial flow
• S. Voloshin, nucl-th/0312065, Nucl.Phys.
  A749, 287 (2005)
• E. Shuryak, arXiv 0706.3531 nucl-th
• radial expansion jet quenching ? correlation
• ridge is independent of jet
• particle spectra in ridge
• points of origin are biased towards surface
  
• ? a bit stiffer slope than that of bulk
• 5) Momentum kick
• C.-Y. Wong , arXiv 0707.2385,
  arXiv0712.3282
• medium partons acquire kick from propagating
  jet
• T470 MeV, q1 (mom. kick) and sy (rapidity
  distribution)
• narrow peak in Df depends mainly on momentum
  kick
• ridge in Dh depends on initial parton y
  distribution

More quantitative theoretical predictions are
needed!