Sergey Panitkin Femtoscopy at RHIC Sergey Panitkin Brookhaven National Lab

Sergey Panitkin Importance of soft physics at RHIC 99.5% 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 Collective behavior at RHIC P. Kolb, J. Sollfrank, U. Heinz Heinz & Kolb, hep-th/ Hydrodynamics seems to reproduce p-space aspects (spectra and elliptical flow) of particle emission up to p T ~2GeV/c Bulk system?! Perfect Liquid? Note: Hydro provides complete space-time evolution. Can and should be tested!

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

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

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

Sergey Panitkin “Standard” Pratt-Bertsch coordinate system

Sergey Panitkin Pratt-Bertsch parameterization Decompose q into components: q Long : in beam direction q Out : in direction of transverse momentum K T q Side :  q Long & q Out Radii are related to source variances: In Longitudinally Co-Moving System (LCMS)  l =0 Sensitive to emission time Sensitive to transverse extent Sensitive to longitudinal extent

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

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

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

Sergey Panitkin Source expansion at RHIC  initial =  final RxRx RyRy STAR preliminary STAR Collaboration, nucl-ex/ Expansion at low PtChange in eccentricity of the source

Sergey Panitkin 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 the puzzle

Sergey Panitkin General observations  Each theory paper has explored systematic changes in at least one important variable – T f = Freeze-out Temp/profile – T c = Critical Temp. –  el = parton scattering cross-section – E = Latent Heat/transition order – T ch = chem. Freeze-out temp – n = viscoscity (-20% R out, -60% R long ) D. Teany, nucl-th/ 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 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 Effect of space-momentum correlation  Evidence of a space – time asymmetry –Qualitatively consistent with “default” blast wave calculation T=110 MeV, = 0.6, t=9,R=13fm –No need for extra time shift Kaon = 0.42 GeV/c Pion = 0.12 GeV/c STAR PRL 91 (2003) Never compared with hydro, AFAIK More measurements to come!

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

Sergey Panitkin 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 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 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 First measurements at SPS WA98 PRL

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

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