Femtoscopy at RHIC

1. Femtoscopy at RHIC Sergey Panitkin Brookhaven National Lab Sergey Panitkin

2. 99.5% Importance of soft physics at RHIC • We want to create/study a new type of matter (= bulk system) • large-scale (soft??) deconfinement • Collective properties (T,p, flow?) • Jets/hard probes are probes of this system • Crucial to understand bulk properties and dynamics in their own right • Soft sector gives information about EoS Sergey Panitkin

3. Collective behavior at RHIC P. Kolb, J. Sollfrank, U. Heinz • Hydrodynamics seems to reproduce p-spaceaspects (spectra and elliptical flow) of particle emission up to pT~2GeV/c • Bulk system?! Perfect Liquid?Note: Hydro provides complete space-time evolution. Can and should be tested! Heinz & Kolb, hep-th/0204061 Sergey Panitkin

4. General Comment Single particle spectrum is sensitive to momentum distribution only Relative momentum distribution of particle pairs is sensitive to space-time information FSI Source function • Basis for Identical and Non-identical particle femtoscopy • EoS determines S(r,q) Sergey Panitkin

5. Two Particle Interferometry: Idealized Case For non-interacting identical bosons: emission function Sergey Panitkin

6. Au+Au R ~ 6 fm p+p R ~ 1 fm d+Au R ~ 2 fm Correlation functions for different colliding systems STAR preliminary C2(Qinv) Qinv (GeV/c) Correlations have more information One can use more sophisticated analysis to extract it Sergey Panitkin

7. “Standard” Pratt-Bertsch coordinate system Sergey Panitkin

8. Pratt-Bertsch parameterization Decompose q into components: qLong: in beam direction qOut : in direction of transverse momentum KT qSide:  qLong & qOut Radii are related to source variances: Sensitive to emission time Sensitive to transverse extent Sensitive to longitudinal extent In Longitudinally Co-Moving System (LCMS) bl =0 Sergey Panitkin

9. In Search of the QGP. Generic Expectations. “Energy density” • Hydro calculation of Rischke & Gyulassy expects Rout/Rside ~ 2->4 @ Kt = 350 MeV. • Looking for a softest point in EoS Sergey Panitkin

10. RHIC Energy Scan • Measurements at 200, 130, 62 GeV • No significant change with energy • from AGS to RHIC • Ro/Rs ~1 Puzzle #1 Where are signs of softest point?! Lower energies? Smaller systems? See talk by D. d’Enterria Is HBT sensitive to geometry at all?! PHOBOS nucl-ex/0409001 Sergey Panitkin

11. More Experimental Systematics b≠0 STAR PRL 93:12301 (2004) PHENIX nucl-ex/0401003 Centrality dependence asHBT Clear sensitivity to source geometry ! Sergey Panitkin

12. Ry Rx Source expansion at RHIC STAR Collaboration, nucl-ex/0312009 initial = final STAR preliminary Expansion at low Pt Change in eccentricity of the source Sergey Panitkin

13. the puzzle Model Comparisons. Puzzle #2 • Good agreement between experiments • Subset of models shown • Broad range of physics scenarios explored • Good description of p-space (Pt, V2) • Poor description of HBT data Sergey Panitkin

14. General observations • Each theory paper has explored systematic changes in at least one important variable • Tf = Freeze-out Temp/profile • Tc = Critical Temp. • sel = parton scattering cross-section • DE = Latent Heat/transition order • Tch = chem. Freeze-out temp • n = viscoscity (-20% Rout, -60% Rlong) D. Teany, nucl-th/0301099 • Femtoscopic observables are sensitive to a variety of parameters, at least in the models! • Sensitivity to physics at early stages of reaction. • Still no compelling solution from dynamical models • Open questions about late stages effects • Hydro inspired parameterization can fit the data, but not hydro • More theoretical work is needed Sergey Panitkin

15. Non identical particle correlations • No symmetrization or antisymmetrization requirement • Pair wave function is of general form • Existence of odd terms in w.f. provides sensitivity to space-time asymmetries f(x) ≠ f(-x) • Now it matters (for example) what was emitted earlier and what was emitted later • Asymmetries can exist due to a variety of physics phenomena: flow, sequence of emission, early decoupling, strangeness distillation, etc Sergey Panitkin

16. Effect of space-momentum correlation STAR PRL 91 (2003) • Evidence of a space – time asymmetry • Qualitatively consistent with “default” blast wave calculation • T=110 MeV, • <r> = 0.6, t=9,R=13fm • No need for extra time shift Never compared with hydro, AFAIK More measurements to come! Pion <pt> = 0.12 GeV/c Kaon <pt> = 0.42 GeV/c Sergey Panitkin

17. “traditional” femtoscopy axis STAR Femtoscopy Matrix Analysis in progress published submitted Not shown: 3p Correlations asHBT Phase space density Correlations with Cascades dAu, pp Gamma-Gamma HBT Sergey Panitkin

18. RHIC as a QCD Machine • Strong interaction for many particle combinations that we want to study is poorly known • Non-identical correlations can shed light on that • Sometimes the only way to study certain systems • Fixing space time parameters from well known correlations and models • Determine scattering lengths • Not the main goal of two particle correlations, but a nice additional topic • Interesting for theory of hadron interactions, hypernuclear community, exotic hadrons H0,ppK-,.. Sergey Panitkin

19. What about RHIC II ? • Is there any soft physics left to do at RHIC II ? • “RHIC is a worst enemy of RHIC II” - Anonymous • Many things are and will be done at RHIC • What is RHIC II in general terms ? – • High luminosity, high rates • New detector capabilities (extended/improved pid to higher Pt…) • Rare probes • Femtoscopy with rare probes • Photons, (electrons?) • HBT in Jets • Non-identical correlations with hyperons • Non-identical correlations with charm ?! Charm thermalization and flow Sergey Panitkin

20. Photon HBT • Prospects • Photons are penetrating probes • Photons are emitted at all stages of collision • Can carry information from the earliest stages of reaction (QGP?,PC) • Photons from different Pt can come from different stages of collision ( QGP, Hadronic gas, resonances,…) • Very few experimental results • Challenges • Relatively small yields • Large background (pi0 decays, eta decays, etc) • Statistics is dominated by background Same applies for electrons ! Sergey Panitkin

21. First measurements at SPS WA98 PRL Sergey Panitkin

22. Photon HBT at RHIC • Lots of theoretical activity: • Different approaches (just to mention a few): • VNI/BMS+hydro+HG • S. Bass, B. Muller, D. Srivastava PRL 94(2005) • Hydro inspired fireball • T.Renk, hep-ph/050382 ; 0408218 • HG • J. Alam, et al - Phys. Rev. C67 (2003) • Still large uncertainties about contributions from different sources • Pt>2 GeV may be a window to early stages • Experimental studies under way (see talk by J. Sandweiss today) Sergey Panitkin

23. Photon correlation function predictions • Bass et al -PRL 94(2005) VNI/BMS No background. Realistic calculations are needed Sergey Panitkin

24. Conclusions • Rich set of femtoscopic results already exists at RHIC • Evidence that matter at RHIC exhibits collective bulk properties • Most models still can not reproduce simultaneously experimental observables at low Pt: v2, spectra, correlations • We are gaining better understanding of model failures • Some promising solutions are offered • Ball is in theory corner • More detailed correlation measurements will be available soon: non-identical correlations, HBT Pt>1 GeV/c, Kaons, protons, etc • Lower energy scan? • RHIC II will offer new opportunities for femtoscopy with rear probes • Will shed light on detailed properties of bulk matter at RHIC Sergey Panitkin

25. Femtoscopy at RHIC Workshop At RHIC/AGS Users Meeting- June 21, 2005 Sergey Panitkin