Three Particle Correlations Jason Glyndwr Ulery, Purdue University For the STAR Collaboration 21 October 2005 Midwest Critical Mass

2 Motivation Interested in probing the medium. Using jets as the probe Can be calculated in pQCD and thus are well calibrated in a vacuum. Are expected to be modified by the medium and can therefore give information on the properties of the medium. Use three particle correlations to gain more information than provided by two particle correlations.

3 Is there conical flow? away near Medium mach cone Medium away near deflected jets hard-soft-corr + soft-bkgd. soft-soft-corr in underlying event. trigger soft bkgd assoc 3-4 GeV/c1-2 GeV/c flow modulated background: 3-particle correlation Medium away near di-jets  1  2  1  2

4 Hard-Soft-Soft Correlation Analysis Trigger particle chosen. Distributions of pairs of associated particles Δ  1 = φ 1 - φ trigger and Δφ 2 =φ 2 -φ trigger Mixed events modified by flow and subtracted 1 +2*v2 1 v2 trigger cos[2(φ 1 -φ trigger )] +2*v2 2 v2 trigger cos[2(φ 2 -φ trigger )] +2*v2 1 v2 2 cos[2(φ 1 -φ 2 )] Cross terms (hard-soft and soft- soft in background) is subtracted. 3<P T Trigger <4 1<P T Associated <2 AuAu 0-10% 1/N Trigger (dN/d(Δφ)) 2 Δφ STAR Preliminary Raw Signal Mixed Events With Flow Hard-Soft Soft-Soft Final Signal

5 Closer Look at the Terms Raw signal is constructed by choosing a trigger particle between 3 and 4 GeV/c then looking at Δφ 1 vs Δφ 2 for all n(n-1) pairs of particles between 1 and 2 GeV/c. Random mixed events do the same process for trigger particles from 1 event and all n 1 *n 2 from 2 other events. The flow modulation is also added in. 1 +2*v2 1 v2 trigger cos[2(φ 1 -φ trigger )] +2*v2 2 v2 trigger cos[2(φ 2 -φ trigger )] +2*v2 1 v2 2 cos[2(φ 1 -φ 2 )] STAR Preliminary 1/N Trigger (dN/d(Δφ)) 2 Δφ

6 Closer Look at the Terms II Hard-Soft term created by taking one particle from the background- subtracted 2 particle correlation and another from the background with flow. STAR Preliminary 1/N Trigger (dN/d(Δφ)) 2 Δφ

7 Closer Look at Terms III Soft-soft term found by taking the trigger particle from one event and the two associated particles from another event. Background for this term is done by taking trigger from one event and associated particle from two other events and adding in the flow between the two associated particles. STAR Preliminary Δφ 1/N Trigger (dN/d(Δφ)) 2

8 pp and d+Au Appears elongated along the diagonal axis in d+Au Possibly k T broadening STAR Preliminary d+Au 2<P T Trigger <4 1<P T Associated <2 Δφ ΔΔ STAR Preliminary 1/N Trigger (dN/d(Δφ)) 2 pp 2<P T Trigger <4 1<P T Associated <2 d+Au 3<p T Trigger <4 1<P T Associated <2 away   /2 = 0.89 ± 0.11   = 0.69 ± 0.05 near   = 0.66 ± 0.07

9 Conical Flow? 3-Particle Correlation d+Au and Au+Au elongated along diagonal: k T effect, and deflected jets? Distinctive features of conical flow are not seen in present data with these p T windows. Three regions on away side: center =( ,  ) ±0.4 corner =(  +1,  +1) ±0.4 x2 cone =(  +1,  - 1) ±0.4 x2  1  2   0 0  1  2  0 0  p T trig =3-4, p T assoc =1-2 GeV/c 2-particle corr, bg, v2 subtracted  φ 2 = φ 2 - φ trig d+Au min-bias dN 2 /d Δφ 1 d Δφ 2 /N trig  φ 1 = φ 1 - φ trig  φ 2 = φ 2 - φ trig Au+Au 10% difference in Au+Au average signal per radian 2 : center – corner = 0.3 ± 0.3 (stat) ± 0.4 (syst) center – cone = 2.6 ± 0.3 (stat) ± 0.8 (syst)

10 3 Particle V2 Previous plots used average of 4 particle cumulant and reaction plane v2. Correlation is robust with v2. Reaction Plane V2 4 Particle Cumulant V2 3<P T Trigger <4 1<P T Associated <2 0-10% STAR Preliminary Δφ STAR Preliminary

11 Conclusions Three particle correlations can provide us with additional information over two particle correlations. Elongation along Δφ 1 = Δφ 2 in d+Au possibly due to k T broadening. Elongation also seen along Δφ 1 = Δφ 2 in Au+Au possibly due to the combination of k T broadening and away side jets deflected by radial flow. No indication of conical flow seen in the present data.