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Explore the methods, results, and outlook of the HBT interferometry analysis in proton-proton collisions at RHIC by the STAR Collaboration. Learn about source geometry information extracted, data fits, and comparisons with heavy-ion results. Investigate the intriguing soft-physics regime and event multiplicities in this preliminary study.
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Pion HBT from pp Collisions at STAR Thomas D. Gutierrez For the STAR Collaboration University of California, Davis APS DNP Meeting Lansing, MI October 12, 2002 • HBT • Motivation • Results • Outlook
HBT Interferomertry • An intensity interferometry technique using two-particle correlations. • Compares relative momenta of identical particles to determine information about space-time geometry of source. • Experimentally, 1D Qinv correlation functions are created by comparing relative 4-momenta of pairs from a “real” event signal to pairs from “mixed” events. The mixed background presumably has no HBT signal. STAR Preliminary STAR Preliminary
HBT • The correlation function, “C2”, is created by dividing the “real” pairs by “mixed” pairs. The histogram is then normalized to the baseline. • The data are fit to a Gaussian C2g = 1 + λexp(-qinv2Rinv2) or an exponential C2e = 1 + λexp(-qinvRinv) to extract fit parameters Rinv and λ. lambda_g=0.397 +/- 0.013; R_g=1.16 fm +/- 0.032; lambda_e=0.749 +/- 0.030; R_e=1.94 fm +/- 0.071 STAR Preliminary ~λ The Coulomb repulsion experienced by charged pairs tends to deplete the correlation function at low Q -- this can be corrected ~1/R Both fits are to the Coulomb corrected data (dark blue)
π+/p NA22 e+/e- AMY E735 UA1 OPAL Why study HBT in pp collisions? • There is a long history of doing Bose-Einstein pion correlations in elementary particle collisions • In the context of RHIC, it provides a baseline for the heavy ion results Just a sampling • Dowell., Proc. Of the VII Topical Workshop on Proton-AntiProton Collider Physics, p115, Word Scientific 1989. • Lindsey. “Results from E735 at the Tevetron Proton-AntiProton Collider with root s= 1.8TeV”, Presented at the Quarkmatter 1991, Gatlinberg, Tennessee, Nov 11-15, 1991. • OPAL Collaboration. Physics Letters B. Vol 267 #1, 5 September, 1991. • NA22 Collaboration “Estimation of Hydrodynamical model parameters from the invariant spectrum and the Bose-Einstein Corrilations…”, Nijmegen preprint, HEN-405, Dec. 97. fm p/pbar
HBT study of the pp system is interesting in its own right • HBT studies in pp interactions provide a peek into the fascinating soft-physics regime of hadronic collisions • We can ask questions (some old, some new) of the pp data coming out of RHIC. For example: • What do the regions of homogeneity look like? • Combining HBT with spectra, what does the phase space density look like? • How do the HBT parameters depend on event multiplicity? This analysis is a first step in answering some of these questions
A look at the STAR data • 3.44 million full field, minimum bias pp events at 200GeV from RHIC using the STAR detector • For negative pions at mid-rapidity (-0.5<y<0.5), two kt ranges were analyzed (0.2<kt<0.4 and 0.4<kt<0.6) • kt is the average transverse momentum of the pair. • Particle identification was done by taking a one sigma cut around the pion bethe-bloch curve while excluding other particles at the two sigma level. dEdx vs. P (GeV/c) STAR Preliminary
Details • Some additional cuts used for this analysis • Event level (~2M events accepted after cuts): • vertices accepted 3m across the STAR TPC • Event Multiplicity < 30 • Track level (~2M tracks accepted after cuts) • -0.5 < y < 0.5 (mid rapidity) • PID cuts as discussed • Primary tracks only • Pair level • Two kt bins between: 0.2 GeV < kt <0.6GeV • anti-merging and anti-splitting cuts STAR Preliminary STAR Preliminary
1D Correlation Functions All fits are to the Coulomb corrected data: STAR Preliminary STAR Preliminary lambda_g=0.397 +/- 0.013; R_g=1.16 fm +/- 0.032; lambda_e=0.749 +/- 0.030; R_e=1.94 fm +/- 0.071 lambda_g=0.345 +/- 0.025; R_g=1.01 fm +/- 0.065; lambda_e=0.667 +/- 0.056; R_e=1.62 fm +/- 0.141
Comparison to Au-Au 200 GeV STAR Preliminary Central Au-Au 200 GeV (closed dots) • pp has a smaller source size than AuAu (not a shock) • Within statistical error bars, pp has a weaker kt dependence than AuAu Peripheral p-p 200 GeV (open boxes) For pp: Red=exp fit Black=gaussian fit Preliminary 200GeV AuAu data from Mercedes Lopez Noriega: AuAu data analyzed with pt cuts ; data point placed at <kt> fort Q->0 For AuAu all fits are Gaussian Error bars on kt axis represent bin width, not error
3D correlation function: BP parameterization for 0.2<kt<0.4GeV STAR Preliminary Causes “out hole” STAR Preliminary lambda=0.450 +/- 0.014; R_out=0.73 fm +/- 0.034; R_side=0.79 +/- 0.025; R_long=1.25 fm +/- 0.041 Black=Coulomb corrected Red=no CC STAR Preliminary Fits and correlations projected 120MeV in the “other” directions C2 = 1 + λexp(-qout2Rout2 -qside2Rside2 -qlong2Rlong2)
Outlook • This was a first look at preliminary pp 200 GeV pi-minus HBT from STAR at RHIC • Found: • 1D Gaussian radii are consistent with systematics of previous elementary particle results (~1fm) • Smaller source sizes than AuAu • kt dependence appears weaker than AuAu • 3D correlation functions show promise • Next-Step Plans for 200 GeV pp data • Analyze more statistics • Look at Pi+ results • Refine 3D correlations • Refine kt analysis • Study event multiplicity dependence of HBT parameters