200 and 62 GeV Cu-Cu Minbias 2 particle correlations Duncan Prindle For the STAR Collaboration October 26, 2008

Prindle2 Dealing with pileup 2-particle correlation fit function Correlations Fit Model Parameter dependence on pileup, Summary Agenda

Prindle3 measures number of correlated pairs per final state particle Event 1 Event 2 ρ sibling ( p 1,p 2 ) ρ reference ( p 1,p 2 ) Correlation Measure normalized ratio of 2D binned histograms; acceptance cancellation; two-track ineff. corrections sqrt of  ref ; projected onto (a,b) Fill 2D histograms (a,b), e.g. (  1,  2 ), (  1,  2 ), (  1 -  2,  1 -  2 ), (p t1,p t2 ), etc. ρ( p 1,p 2 ) = 2 particle density Correlation Measure

Prindle4 Trigger/Vertex finding problems in peripheral events HBT peak wider Final state  -K,  -p interactions more important exclude bins near (     ) = (0,0) for now Pileup is an issue with 200 GeV Cu-Cu Track merging/crossing not a serious problem Multiplicity dependence at large   not a big problem 200 GeV Cu-Cu Issues compare to Au-Au - +

Prindle5 STAR TPC takes 40  s to read out. If a collision occurs before or after trigger some of those tracks can contribute to triggered vertex. For 200 GeV Cu-Cu data this happened for about 5% of events. We can reject about 75% of those. Subtract (or isolate) pileup. Estimate rejection factor f = 0.75 CI STAR Preliminary Dealing with pileup

Prindle6 Same fit function as Au-Au, except Proton-Proton fit function =+ STAR Preliminary longitudinal fragmentation 1D gaussian HBT, e+e- 2D exponential ηΔηΔ φΔφΔ ηΔηΔ φΔφΔ ηΔηΔ φΔφΔ Note: from this point on we’ll include entire momentum range instead of using soft/hard cuts ηΔηΔ φΔφΔ dipole quadrupole cos(2φ Δ ) Same-side “Minijet” Peak, 2D gaussian Away-side -cos(φ) “soft”“hard” Exclude 4 bins at origin Fit Function

Prindle7 Cu-Cu data histograms 8.1M minbias events; p t > 0.15 GeV/c, |η| < 1, full φ 200 GeV Cu-Cu Data peripheral central (maximum scale cuts off e + e - peak) STAR Preliminary 3.5M minbias events; p t > 0.15 GeV/c, |η| < 1, full φ 62 GeV Cu-Cu Data (maximum scale cuts off e + e - peak) peripheral central STAR Preliminary

Prindle8 Cu-Cu model fits peripheral central STAR Preliminary 62 GeV Cu-Cu Fits peripheral central STAR Preliminary 200 GeV Cu-Cu Fits

Prindle9 Cu-Cu minijet structure Remove fit cos(2  ), Gaussian on   terms Qualitative change for most central bin peripheral central 200 GeV Cu-Cu Data STAR Preliminary peripheral central 62 GeV Cu-Cu Data STAR Preliminary Small evolution with centrality

Prindle GeV Cu-Cu Data, split two central bins Finer centrality binning STAR Preliminary Remove fit cos(2  ), Gaussian on   terms central

Prindle GeV: Effects of pileup and its rejection f=0.25 f=0 f=0.4 STAR Preliminary 200 GeV Cu-Cu Fit Parameters Little sensitivity except for   Gaussian

Prindle12 62 GeV Fit parameters STAR Preliminary 200 GeV Data 62 GeV Data 200 GeV Hijing 62 GeV Hijing Cu-Cu Fit Parameters, 200 and 62 GeV

Prindle13 plot against transverse energy 200 GeV Cu-Cu 62 GeV Cu-Cu

Prindle14 Summary Pileup only affects Gaussian on   component Central 5% 200 GeV Cu-Cu just reaches the transition No evidence of transition in 62 GeV Cu-Cu collisions