# Presentation: 722109

## Current Status and Future Femtoscopic Measurements at RHIC 

**Sergey Panitkin**
**Brookhaven National Lab**
**May 25, 2005**

## Outline

**Introduction and Motivation**
**Current results and open questions**
**(Near) Future measurements with existing detectors**
**RHIC II perspective**

## Soft Physics at RHIC

**Rich set of low Pt (< 2GeV/c) data exists at RHIC**
**Particle yields**
**Spectra**
**Azimuthal anisotropies (v2, etc)**
**Femtoscopy **
**Consistent with thermalization hypothesis and collective expansion**
** **

## Collective behavior at RHIC

- P. Kolb, J. Sollfrank, U. Heinz

- Heinz & Kolb, hep-th/0204061

- Hydrodynamics seems to reproduce **p-space**** **aspects (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!

## Two-particle Correlations

**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**

**Source function**

**FSI**** **

**System evolution encoded in S(r,q)**

## HBT for Gaussian sources

**HBT for Gaussian sources**

_**Decompose q into components:**_
- qLong	: in beam direction
- qOut 	: in direction of transverse momentum KT
- qSide 	:  qLong & qOut

_**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**

## Lattice QCD predictions for speed of sound

- G. Boyd et al., , Nucl.Phys. B469 (1996) 419** **

## Femtoscopic signature of QGP

- 3D 1-fluid Hydrodynamics

- “”

- Long-standing favorite signature of QGP:
- increase in , ROUT/RSIDE due to deconfinement  confinement transition
- hoped-for “turn on” as QGP threshold is reached

## Energy dependence of pion HBT

**Do we understand this?**

**AuAu (PbPb)**
**Y~0**
**<Kt>~170 MeV**
**Central ~10%**

**Where is the softest point ?**
**Lower energies?**
**Lighter systems?**

**RHIC HBT Puzzle #1**

## Model Comparisons at 200 GeV 

**Model Comparisons at 200 GeV **

**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**

**RHIC HBT Puzzle #2**

## What did we learn ?

**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/0301099**

-  Femtoscopic observables are sensitive to a variety of model assumptions
- Sensitivity to properties of early stages of evolution
- Sensitivity to final stages
- Femtoscopic observables are difficult to “mock up” in dynamical models

**We need consistent description of evolution**

## p+p → d+Au → Au+Au

**All three systems exhibit similar k****T**** dependence (?!) Puzzle #3 ?**
**Systematic study underway to assess “Gaussian-ness” of correlation**
**Need better understanding of pp system -> future measurements**

**T. Gutierrez for STAR Coll, poster**

- preliminary

**D. Magestro**
**QM04**

## Questions for Friday

**Do we have consistent description of low Pt physics at RHIC (Femtoscopy in particular)?**
**What leads to discrepancies between dynamical models and HBT data?**
**What does success of BW parameterizations tell us? How to arrive to BW solution in real hydro?**
**Do we understand similarity of Pt dependence in pp, dAu, AuAu?**
**Where is the “softest point” ?**
**Where is (phase transition) threshold behaviour?**
**Where are signs of chiral symmetry restoration?**

## Cramer-Miller Solution of the RHIC HBT Puzzle

**Relativistic quantum mechanical treatment of pion propagation through the medium**
**Explicit treatment of elastic and inelastic  scattering via optical potential**
**Fit to STAR pion spectrum and Kt dependence of HBT radii with 11 parameters**
**Fit values are interpreted as evidence of chiral symmetry restoration**

**nucl-th/0411031**

## Rout

- Rout

**U=0**

**Re[U]=0**

**No flow**

**Boltzmann**

**Full****Calculation**

- Prediction

**K****T**** (MeV/c)**

## Rside

- Rside

**U=0**

**Re[U]=0**

**No flow**

**Boltzmann**

**Full****Calculation**

**K****T**** (MeV/c)**

- Prediction

## Testing Cramer-Miller Model

- Rout

- Rside

- Spectrum

**Look at low Pt data (HBT and spectra)**
**PHOBOS ?**
**STAR measurements at quarter field?**

## Non identical particle correlations

**No symmetrization or anti-symmetrization requirement for pair wave function**
**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  **

## Effect of space-momentum correlations

**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 **
**<pt> = 0.42 GeV/c**

**Pion **
**<pt> = 0.12 GeV/c**

**STAR PRL 91 (2003)**

**Never compared with hydro, AFAIK**
**More measurements to come!**

## Non-identical correlations frontier 

**Correlations with hyperons p********,********,....**
**Test of expansion models**
**Extends systematics**
**Test/measurement of FSI**
**Non-identical correlations relative to the reaction plane**
**Non-identical correlationa in pp and dAu collisions **

## 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 reasonable way to study certain systems experimentally**
**Fix 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-,..**
**Perhaps will shed light on interaction properties of hyperons in hadron gas**
**See talk by F. Retiere**

## 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 measurements 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**
**Leptons**
**Femtoscopy in/with Jets**
**Non-identical correlations with hyperons**
**Non-identical correlations with charm ?! Charm thermalization and flow**

## Photon HBT

**Advantages**
**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 ( initial scattering, QGP, hadronic gas)**
**Very few experimental results**
**Challenges**
**Relatively small yields of direct photons**
**Large background**
**    ****(pi0 decays, eta decays, etc)**
**Statistics is dominated by background**
**Experimental issues **

**Same applies for electrons !**

## First measurements at SPS

** ****WA98 PRL**

## 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 at RHIC are under way**

## “Pure” photon correlation function

** ****Bass et al -PRL 94(2005)**

**VNI/BMS**

**No background !**

## Instead of Summary

**Impressive systematics already exists at RHIC**
**Number of puzzling results await theoretical explanation**
**The number of available datasets and possible analysis topics will be rapidly increasing**
**Analyses will be moving to rare particles**
**Hyperons, photons, etc**
**Non-identical correlations are becoming a mainstream measurements**
**Shall we continue with systematic approach? Energy scan, different systems, asHBT,...**
**Probably yes**
**If new results or theoretical predictions will suggest promising measurement - we will concentrate on it**

## Femtoscopy Workshop at BNL

**at RHIC/AGS Users Meeting. June 21, 2005**
**www.star.bnl.gov/~panitkin/UsersMeeting_05**

## Meaning of the Parameters

**Meaning of the Parameters**

**Temperature:  173 MeV -> Chiral PT predicted at ~ 170 MeV**
**Transverse flow rapidity: 1.3 -> v****max****=0.85 c,   v****av****=0.6 c**
**Mean expansion time: 8.2 fm/c -> system expansion at ~ 0.5 c**
**Pion emission between 5.4 fm/c and 11.1 fm/c -> soft EOS .**
**WS radius: 11.7 fm = R(Au) + 4.3 fm  > R @ SPS**
**WS diffuseness:  0.72 fm (similar to Low Energy NP experience)**
**Re(U):  0.137 + 0.582 p****2**** -> deep well -> strong attraction.**
**Im(U):  0.121 p****2**** ® l****mfp ****» 8 fm -> K****T****=1 fm****-1 ****-> strong  absorption ->  high density**
**Pion chemical potential:  m****p****=123 MeV » mass(p)**** **
- We have evidence for a *CHIRAL PHASE TRANSITION*!

**Slide by J. Cramer**

## Main ideas about non-identical particle correlation

**Non-identical particles interact after they freeze-out**
**Coulomb and strong interactions ****U=f(r)**
**In ********-K and ********-p correlations Coulomb interaction dominates**
**General pair wave function (no symmetrization requirement)**
**Odd terms in wave **
**	****function sensitive to **
**	****source asymmetries: **
**	****f(x)=-f(-x)**
**Small relative velocity **
**	****correlations ****m≠M********p≠P**
**C2 sensitive to ****spacetime extent*****and***** something****more... **

## STAR Femtoscopy Matrix

**Analysis**
**in progress**

**published**

**3******** Correlations**
**asHBT**
**Phase space density**
**Correlations with Cascades**
**dAu, pp**
**Gamma-Gamma HBT**

**submitted**

_**Not shown:**_

##  	Centrality dependance in 200GeV and 62.4GeV AuAu

**Combined unlike-sign pairs**
**Solid lines theory**** – using Blast Wave with parameters from pion-pion HBT and pion spectra**
**Significant ****centrality dependance**** of Ξ* peak.**
**Coulomb region affected by ****low statistics**
**Needs Y4 data****  **

**Petr Chaloupka**

**Corrected for purity and momentum resolution.**

## Two particle Interferometry: Idealized case

**emission function**

**For non-interacting identical bosons:**