Two-particle correlation measurements with STAR detector at RHIC Vitaly A. Okorokov

1. XXXII International Symposium on Multiparticle DynamicsSeptember 7-13, 2002, Alushta, Crimea, Ukraine Two-particle correlation measurements with STAR detector at RHIC Vitaly A. Okorokov (for the STAR Collaboration) Moscow Engineering Physics Institute (State University), Kashirskoe Ave.31, Moscow, 115409, Russian Federation Vitaly Okorokov

2. Outline • Introduction • STAR detector • Physics motivation • HBT methods and techniques • STAR HBT experimental results • HBT correlations of identical particles • Various dependences of HBT parameters • Status of the “RHIC HBT puzzle” • Correlations of non-identical particles • Kp correlations • Kp, pp and Kp HBT results and model predictions • Conclusions Vitaly Okorokov

3. STAR experiment at RHIC First Au-Au collision events at RHIC @ 100+100 GeV/c per beam recorded by STAR Vitaly Okorokov

4. Physics motivation Ultra-relativistic heavy ion physics is entering the new era of collider experiments with the start-up of RHIC at BNL. The basic questions and central in the goals of RHIC today are, “What is the nature of nuclear matter at energy densities comparable to those of the early Universe?” “What are the new phenomena and physics associated with the simultaneous collisions of hundreds of nucleons at relativistic energies?” The study of small relative momentum correlations, a technique also known as HBT interferometry, is one of the most powerful tools at our disposal to study complicated space-time dynamics of heavy ion collisi- ons. It provides crucial information which helps to improve our under- standing of the reaction mechanisms and to constrain theoretical mo- dels of heavy ion collisions. It is also considered to be a promising signature of theQuark Gluon Plasma (QGP). Vitaly Okorokov

5. Two-particle correlations Single particle spectrum is sensitive to momentum distribution only Relative momentum distribution of particle pairs is sensitive to space-time information FSI Source function Intensity interferometry, HBT technique, etc…. Vitaly Okorokov

6. 1 = + K ( P P ) T T1 T2 2 Q Q O T Q S p p 2 1 Q beam direction Q T Q L p 1 p 2 beam direction Decomposition of the pair relative momentum (measured in the longitudinal co-moving source –LCMS- frame; (p1+ p2)z=0) Pratt-Bertsch parameterization • Information: • geometrical source size: Rside • lifetime • (for simple sources!) • Rout2=Rside2+(bpairt)2 Vitaly Okorokov

7. In search of the QGP. Naïve expectations QGP has more degrees of freedom than pion gas Entropy should be conserved during fireball evolution Hence: Look in hadronic phase for signs of: Large size, Large lifetime, Expansion…… Vitaly Okorokov

8. In search of the QGP: Expectations • “Naïve” picture (no space-momentum correlations): • Rout2=Rside2+(bpairt)2 • One step further: • Hydro calculation of Rischke & Gyulassy expects Rout/Rside ~ 2->4 @ kt = 350 MeV. • Looking for a “soft spot” Rside Rout Vitaly Okorokov

9. Centrality and transverse mass dependences PRL 87 (8) 2001 Centrality dependence: Larger initial size->Larger final size. Significant expansion ! STAR Final s=130 GeV/A mT dependence: Generally behavior is consistent with flow and observations at AGS, SPS Vitaly Okorokov

10. HBT Parameters: mT dependence Au+Au (data @ S=200 GeV are preliminary) 200 GeV HBT radii increase with centrality (including RL). HBT radii decrease with increasing <mT> Experimental data at 130 GeV/A consistent with results at 200 GeV/A According to Sinyukov fits, evolution duration: t (central) = 10.08 fm t (midcentral) = 9.30 fm t (peripheral) = 7.59 fm Vitaly Okorokov

11. Excitation function of the HBT parameters • ~10% Central AuAu(PbPb) events • y ~ 0 • kT0.17 GeV/c • no significant rise in spatio-temporal size of the  emitting source at RHIC • RO/RS ~ 1 Note ~100 GeV gap between SPS and RHIC ! Vitaly Okorokov

12. Pion HBT: STAR, PHENIX Results @ 130 GeV C. Adler et al. (STAR Col.), PRL 87 (2001) 082301. K. Adcox et al. (PHENIX Col.), nucl-ex/0201008, January 2002. S.C. Johnson, nucl-ex/0205001, May 2002. The top panel shows the measured Rside from identical pions for STAR and PHENIX. Lines are fits of analytical equations (1) and (2) for boost-invariant, hydrodynamically expanding source to the STAR and PHENIX data.The bottom panel shows the ratio Rout/Rside as a function of kT overlaid with theoretical predictions for a phase transition for two critical temperatures. Fit (1) - dashed line Fit (2) - solid line Fit (2) - dot-dashed line f=0.69-the boost velocity, f=0.85-transverse rapidity boost, T=125 MeV is the temperature. STAR and PHENIX agree All theoretical calculations show only Rout/Rside ratio to be greater than unity due to system lifetime effects which cause Rout to be larger than Rside. They also predict that the ratio increases with kT.Such an increase seems to be a generic feature of the models based on the Bjorken-type, boost-invariant expansion scenario. Model does not reproduce the data D.H.Rischke, Nucl.Phys. A610 (1996) 88c; D.H.Rischker, M.Gyulassy, Nucl.Phys. A608 (1996) 479. S.A. Bass, A. Dumitry, S. Soff PRL 86, nucl-th/0012085, December 2000 (Hydro+UrQMD). Vitaly Okorokov

13. Possible solutions for RHIC HBT puzzle One of the most dramatic results from RHIC are the STAR results for interferometry. There are at least two outstanding issues in two-particle HBT pion results that have avoided easy physical descriptions: (i)   the absolute magnitude of the radii and their shape in kT at RHIC is strikingly similar to lower energy measurements; (ii) the ratio of the transverse radii Rout/Rside is close to 1 over all kT. It is not yet clear what physics leads to the kind of dependences reported by STAR Collaboration above. • Something exotic • Freeze out at critical point ?! • Sudden hadronization ?! • Non-Bjorken expansion scenario (Landau?) • ……… • 3d hydro seems to be needed • In any case solution will require some sort of paradigm shift Vitaly Okorokov

14. Correlations of non-identical particles • Correlations due to the final state interactions • Coulomb or strong • No symmetrization requirement ! => • Pair wave function has odd terms => • Sensitivity to source asymmetries • Source asymmetries can be due to: • different emmision times • Collective flow • Since particles, in general, have different mass => • Correlations in small relative velocities not momentum ! Vitaly Okorokov

15. Catching up: cosY  0 • small mean separation • strong correlation • Moving away: cosY  0 • large mean separation • weak correlation • Ratio of both scenarios allow quantitative study of the emission asymmetry Non-identical particle correlations How to reveal the asymmetries? See talk by R. Lednicky purple K emitted first green p is faster purple K emitted first green p is slower Crucial point: kaon begins farther in “out” direction (in this case due to time-ordering) Vitaly Okorokov

16. We can assume that pions, kaons, and protons sources have different size and may be their are shifted in source frame We measure only size and shift in pair rest frame 1D relativistic view. What can be probed? Source of particle 1 (pion) Source of particle 2 (kaon) Separation between source 1 and 2 in pair rest frame Dr Separation between particle 1 and particle 2 and boost to pair rest frame r (fm) • 2 parameters • Mean shift (<Dr*>) • Sigma (sr*) Dr* = gpair (Dr– bpair Dt) Dr* separation in pair rest frame Function of gpair(bpair) which depend on the pair acceptance Vitaly Okorokov

17. Correlation functions andratiosK @ 130 GeV/A • Positive correlations for • unlike sign pairs • Negative correlations for • like sign pairs Good agreement for like-sign and unlike-sign pairs points to similar emission process for K+ and K- CF Shape of correlation functions different for different cuts! Out Clear sign of emission asymmetry Two other ratios done as a double check – expected to be flat Side a), b) pion kaon correlation functions c), d) ratio of correlation function C-/C+ with respect to the sign of k*out e), f) ratio of correlation function C-/C+ with respect to the sign of k*side g), h) ratio of correlation function C-/C+ with respect to the sign of k*long Long Vitaly Okorokov

18. Blast wave consistent with data However, systematic errors need to be reduced to conclude Parameters of source Correlations K @ 130 GeV/A p @130 GeV/A Kp @ 200 GeV/A Fit in pair rest frame <r1* - r2*>, fm (t1 - t2), fm/c 6.4 - - (r1 - r2), fm <4.6 - - Kp, pp and Kp HBT results combined Table 1. Fit parameters for HBT correlations of non-identical particle pairs (Preliminary STAR data) STAR Preliminary Vitaly Okorokov

19. Conclusions The HBT interferometry measurements have been performed at RHIC energies and hereby extended the HBT excitation function into the new energy domain. A various dependences of the particle-emitting source parameters can be measured with high statistics in STAR. Pion HBT results from Au-Au interactions at 130 GeV/A and 200 Gev/A are presented. The anomalously large source size or source lifetimes predicted for a long-lived mixed phase have not been observed in this study. The one of the most intriguing feature of the (preliminary) HBT results from RHIC is the KT dependence of the ROut/RSide ratio observed by the STAR Collaboration. Evidence of space-time shift between p, K, and p sources are obtained. Protons are ahead of kaons, kaons are ahead of pions (in the radial direction). Qualitative agreement is observed between experimental results and a Blast wave scenario which describes other STAR data. Vitaly Okorokov