Three-Particle Azimuthal Correlations From STAR

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Three-Particle Azimuthal Correlations From STAR

• School of Collective Dynamics in High Energy
  Collisions
• Berkeley, California
• Jason Glyndwr Ulery
• For the STAR Collaboration
• 21 May 2007

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Motivation
4.0ltPTTriglt6.0 GeV/c 0.15ltPTAssoclt4.0 GeV/c

• Jets are expected to be modified by the medium we
  create and therefore can be used to probe the
  medium.
• 2-Particle correlations show broadened or double
  humped away-side.
• Mach-cone
• Cerenkov gluon radiation
• Jets deflected by radial flow or path length
  dependent energy loss.
• Large angle gluon radiation
• 3-particle correlations can distinguish conical
  emission from other mechanisms.

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Analysis Procedure
Trigger

• Trigger particle selected with transverse
  momentum 3ltpTlt4 GeV/c.
• Look at ???Assoc-?Trigger for all pairs of
  associated particles with 1ltpTlt2 GeV/c.
• Plot ??1 vs ??2 for each pair of associated
  particles.
• Particles are assumed to be jet-like or
  background.
• Raw signal contains (JetBkgd) x (JetBkgd).
• To obtain Jet x Jet we must subtract Bkgd x Bkgd
  and Jet x Bkgd (and Bkgd x Jet.)

??1
??2
??2 (radians)
??1 (radians)
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Jet x Background (Hard-Soft)

• Top plot is 2-particle correlation.
• Red is Jet Background
• Black is Background (from mixed events with v2
  and v4 added) and blue is scaled background (such
  that Red - blue is zero around 1.)
• Mini panel is background subtracted signal.
• Jet x Background term is created by folding
  2-particle jet-like signal (mini panel) with
  2-particle background.

??
??2
??1
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Background x Background (Soft-Soft)

• Term is constructed by mixing a trigger particle
  from one event with pairs of background particles
  from another event of the same centrality.
• Contains correlations between associated
  particles that are not associated with a trigger
  particle (including the flow between the 2
  associated particles).

??2
??1
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Flow

• Soft-soft term contains from between the
  associated particles irrespective of the trigger.
• Other flow terms must still be subtracted.
• Top plot contains terms of v2Triggerv2Associated.
  
• Bottom plot contains terms of v4Triggerv4Associat
  ed and v2v2v4 with v4 1.15v22.
• v2 is taken as average of reaction plane and
  4-particle measurements.

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Background Subtraction
Raw Signal
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Conical Flow vs Deflected Jets
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Centrality Dependence
pp
dAu
AuAu 50-80
AuAu 30-50
AuAu 10-30
AuAu 0-10
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Projections
AuAu 0-12
AuAu 10-30
dAu
(??1??2)/2
(??1-??2)/2


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Centrality Dependence of the Signal
Away
Cone
Cone Deflected
Average Signal in 0.7x0.7 Squares

• Cone and cone deflected at 1.45 radians from ?.
• Positive conical emission signal seen in central
  AuAu collisions.

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Associated PT Dependence

• Mach cone signals should display no pT dependence
  of the angle.
• Current Cerenkov gluon radiation models predict
  decreasing angle with pT.

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Angle from Fits

• Fit of off-diagonal projections to Gaussians to
  extract conical emission angle.
• Shaded errors are systematic and solid are
  statistical.
• Fitting angle from different centralities to a
  constant gives an angle of 1.47.
• Angle consistent with flat or increasing with
  associated pT.
• Inconsistent with current Cerenkov radiation
  models.
• Fitting points to a constant gives angles for
  1.41 for ZDC triggered 0-12 AuAu and 1.46 for
  0-50 AuAu from minimum bias.

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Systematics

• Major sources of systematic error are from the
  elliptic flow measurement and the normalization.
  
• Off-diagonal signal robust with respect to
  variations in v2 and normalization.
• Other sources include
• effect on the trigger particle flow from
  requiring a correlated particle (20 on trigger
  particle v2)
• uncertainty in the v4 parameterization
• multiplicity bias effects on the soft-soft
  background

Reaction Plane v2
4-Particle v2
Wide Normalization
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Conclusions

• Within the jet-like background model, we
  conclude
• On-diagonal elongation is seen in pp, dAu
  consistent with kT broadening.
• Additional elongation in AuAu is consistent
  with additional contribution from deflected jets.
  
• Evidence for conical flow.
• Consistent with Mach cone, inconsistent with
  simple Cerenkov radiation.

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Extreme Systematics
No Jet Flow
No v2Triggerv2Associated

• No jet flow systematic has the jet not flowing
  with the medium.
• No v2Triggerv2Associated has no subtraction of
  the v2 terms.
• Signal persists even in these extreme cases.

(??1-??2)/2
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Hard-Soft Plus Flow

• Flow contributions from v2Triggerv2Associated
  and v4Triggerv4Associated cancel to first order.
• Robust with respect to variations in flow.

??2
??1