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Three-Particle Azimuthal Correlations From STAR

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. Motivation. 4.0<P T Trig<6.0 GeV/c 0.15<P T Assoc<4.0 GeV/c.

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Three-Particle Azimuthal Correlations From STAR

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  1. 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

  2. Motivation 4.0<PTTrig<6.0 GeV/c 0.15<PTAssoc<4.0 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.

  3. Analysis Procedure Trigger Δ1 Δ2 • Trigger particle selected with transverse momentum 3<pT<4 GeV/c. • Look at Δ=Assoc-Trigger for all pairs of associated particles with 1<pT<2 GeV/c. • Plot Δ1 vs Δ2 for each pair of associated particles. • Particles are assumed to be jet-like or background. • Raw signal contains (Jet+Bkgd) x (Jet+Bkgd). • To obtain Jet x Jet we must subtract Bkgd x Bkgd and Jet x Bkgd (and Bkgd x Jet.) Δ2 (radians) Δ1 (radians)

  4. 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. Δ Δ2 Δ1

  5. 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). Δ2 Δ1

  6. Flow Δ2 Δ1 Δ2 Δ1 • 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 v2Trigger*v2Associated. • Bottom plot contains terms of v4Trigger*v4Associated and v2*v2*v4 with v4 = 1.15*v22. • v2 is taken as average of reaction plane and 4-particle measurements.

  7. Background Subtraction Soft-Soft Hard-Soft + Flow - - = Raw Signal

  8. Conical Flow vs Deflected Jets near near near Medium Medium Medium away away π away di-jets 0 π 0 deflected jets mach cone

  9. Centrality Dependence Au+Au Central 0-12% Triggered pp d+Au Au+Au 50-80% Au+Au 30-50% Au+Au 10-30% Au+Au 0-10%

  10. Projections d+Au Au+Au 10-30% Au+Au 0-12% (1+2)/2 (1-2)/2

  11. Centrality Dependence of the Signal Away Cone Cone + Deflected • Cone and cone + deflected at 1.45 radians from . • Positive conical emission signal seen in central Au+Au collisions. Average Signal in 0.7x0.7 Squares

  12. Associated PT Dependence Au+Au 0-12% 3<pTTrig<4 GeV/c 0.5<pTAssoc<0.75 GeV/c 0.75<pTAssoc<1.0 GeV/c 1.0<pTAssoc<1.5 GeV/c 1.5<pTAssoc<2.0 GeV/c • Mach cone signals should display no pT dependence of the angle. • Current Cerenkov gluon radiation models predict decreasing angle with pT.

  13. 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% Au+Au and 1.46 for 0-50% Au+Au from minimum bias.

  14. 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

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

  16. Extreme Systematics • 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. No Jet Flow No v2Triggerv2Associated (1-2)/2

  17. Hard-Soft Plus Flow • Flow contributions from v2Trigger*v2Associated and v4Trigger*v4Associated cancel to first order. • Robust with respect to variations in flow. Δ2 Δ1

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