Investigating HBT in pp Collisions at RHIC: Preliminary Results from STAR

1. 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

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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