# Presentation: 210724

## What can we learn from the energy dependence of (strangeness) production at RHIC ?

**Jeff Speltz**
- (for the                Collaboration)

**Hot Quarks 2006**
**May 15-20, 2006, Villasimius, Sardinia, Italy**

- Institut Pluridisciplinaire Hubert Curien, Strasbourg

-  Motivation
-  Chemical probes
-  Dynamic properties
-  Summary / Conclusions

- (Formerly IReS)

## Motivation

**Motivation**

- Why energy dependence?
- Onset of validity of models assuming equilibrium?
- ”Dramatic” variation in excitation function of variables?
- Statistical models
- Chemical equilibrium?
- Chemical freeze-out (inelastic scattering ends) conditions : Tch?
- Ideal hydrodynamic models
- Early, local thermalization (τ0)?
- Kinetic freeze-out (elastic scattering ends) conditions : Tdec (Tkin)?
- Equation of State (EoS)  QGP-like?

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## The data

- High statistics data at
-             	     = 62.4 GeV
- and
-                     = 200 GeV

**Motivation**** ****Chemistry Dynamics Summary**

**STAR Preliminary**

- Statistics sufficient to get significant elliptic flow measurements

**STAR Preliminary**

- All shown results are mid-rapidity |y| < 1

- Au+Au

- J. Speltz - Hot Quarks 2006

- 16th May 2006

##  Strange particle excitation functions

**Motivation**** ****Chemistry ****Dynamics Summary**

-    Ratio smoothly rising from SPS to RHIC
-    Evolution from a transport dominated regime to a dominance of quark pair production (baryon-free)

- No surprises!

- AGS : E896
- SPS : K0S,Λ (NA49); Ξ, Ω (NA57)
- RHIC : STAR

- Anti-baryon yield continuously rising
- Baryon yield ~ constant

**STAR Preliminary**

**STAR Preliminary**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Statistical model

- Ratio fits : data reasonably well reproduced at both energies
- Parameters Tch and γS (strangeness saturation factor)  and μB:
- same evolution with centrality
- same values at both energies
- μB at 62 GeV where it is expected to be

**Motivation**** Chemistry ****Dynamics Summary**

**STAR Preliminary**

- Model from : M.Kaneta and N.Xu, nucl-th/0405068

**STAR Preliminary**

- No surprises!

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Central 200 GeV : π-, K-, p 

**Motivation Chemistry**** Dynamics**** ****Summary**

- Best agreement for :  Tdec= 100 MeV                   α = 0.02 fm-1
- α ≠ 0 : importance of initial conditions
- τ0 = 0.6 fm/c

**T****dec**** = 165 MeV**
**T****dec**** = 100 MeV**

- α : initial (at τ0) transverse velocity : vT(r)=tanh(αr)

**Central Data**

- Model (plot) from P.F. Kolb and R. Rapp, Phys. Rev. **C 67** (2003) 044903

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Central 62 GeV : π-, K-, p 

**Motivation Chemistry**** Dynamics ****Summary**

- Same model as at 200 GeV
- Tuned to 62 GeV (sequ,...)
- Comparable (possibly slightly less for π-) agreement with data as at 200 GeV
- Small τ0 required at both energies
- Only at low-pT (pT < 1.5 – 2 GeV/c)
- Failing at higher pT (> 2 GeV/c) expected:
- Less rescattering

- P.F. Kolb, J. Sollfrank and U.Heinz, Phys. Rev. **C 62** (2000) 054909
- P.F. Kolb and R. Rapp, Phys. Rev. **C 67** (2003) 044903
- P.F. Kolb and U.Heinz, nucl-th/0305084

- Thermalization  validity limit

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Ω : central collisions 

**Motivation Chemistry**** Dynamics ****Summary**

- Data best reproduced with
- Tdec ≈ 100 MeV
- Same as for π-, K-, p
- Agreement holds for entire spectra!
- Same results at both energies!

- P.F. Kolb and U. Heinz, nucl-th/0305084

- Tdec ≈ 164 MeV (decoupling at hadronization): not enough radial flow

- pT = 2 GeV/c

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Blast - Wave

**Motivation Chemistry**** Dynamics ****Summary**

- Blast-Wave:                              hydro inspired parameterization:
- Parameter Tkin
- Parameter <βT>
- Direct fit (Χ2) on the data
- Blast-Wave gives slightly different results than hydro for Ω :
- Tkin ~ Tch > 100 MeV
- Large errors
- B-W fit on hydro :                Tkin ≠ Tdec                                                (up to 30 MeV difference)

**100 MeV**

**~ 160 MeV**

**~ 125 MeV**

- NA57 : C. Alt et al. Phys. Rev. Lett **94** (2005) 192301

- E. Schnedermann *et al.*, Phys. Rev. **C 48** (1993) 2462
- F. Retière and M. Lisa, Phys. Rev. **C 70** (2004) 044907

- Are Tdec(hydro) and Tkin(B-W) the same physical quantity?

- Blast-Wave :

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Ω : centrality dependence 

- good agreement : gets worse for more peripheral collisions
- Most peripheral not reproduced by hydro

**Motivation Chemistry**** Dynamics ****Summary**

- Failing for more peripheral collisions expected:
- Smaller system size
- Less rescattering

- Thermalization hypothesis less valid with less centrality

**central**

**peripheral**

- Characterize where “perfect fluid” ends?

- pT = 2 GeV/c

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Elliptic Flow : 200 GeV

**Motivation Chemistry**** Dynamics ****Summary**

- Mass hierarchy at low pT from hydro in data
- Qualitative agreement until pT ~ 2 GeV/c
- Same as for spectra
- pT > 2 GeV/c data deviates from hydro
- Description possible with dissipative effects

- Anisotropy in space

- Anisotropy in momentum           v2

- J. Adams *et al.*, Phys. Rev. **C 72** (2005) 014904

**Anisotropy parameter v****2**

- Early thermalization : account for large v2

**interactions**

- D. Molnar and M. Gyulassy, Nucl. Phys. **A 697** (2002) 495

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Elliptic Flow : 62 GeV

- Same features as at 200 GeV:
- Mass hierarchy
- Breakdown at pT ~ 2 GeV/c
- s quark flows
- Same parameters as for spectra
- Tdec ≈ 100 MeV
-  α ≠ 0 fm-1
- τ0 = 0.6 fm/c

**Motivation Chemistry**** Dynamics ****Summary**

**Ideal Hydrodynamics**

- M. Oldenburg, QM 2005

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Summary / Conclusions

- Excitation functions of strange baryon production don’t show any surprises
- Statistical models give good agreement with data and same results at 62 GeV and at 200 GeV
- Ideal hydrodynamic models give good agreement with (strangeness) (mid-rapidity) central data
- Spectra and Elliptic flow at all RHIC energies (62 GeV to 200 GeV)
- Using EoS including phase transition
- Indication for (early) thermalization
- Kinetic Freeze-out (Tdec) seem similar for all particles
- clarify on Blast-Wave (more precise measurements at lower pT,...)
- Peripheral data : help characterize the onset of ”perfect fluidity”?
- Considering the given strangeness observables:
- If a QGP is formed at 200 GeV it is most likely also the case at 62 GeV!

**Motivation Chemistry Dynamics**** Summary**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## 62 GeV : Λ , Ξ central

** ****Backup**

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Λ , Ξ centrality dependence

** ****Backup**

**Ideal Hydrodynamics**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

##  Blast-Wave with Ξ

** ****Backup**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Compilation of comparisons

- Use π, K, p B-W parameters on multi-strange baryons
- Tkin = 90 - 100 MeV
- <βT> = 0.57 c
- Ξ- and Ω- spectra not reproduced

- J. Speltz (for the STAR Collaboration), nucl-ex/0512037

- Differences between Ξ-, Ω- and π, K, p mainly due to <βT> (best constrained)

** ****Backup**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Strange particle v2 at 200 GeV

- All strange particles (including Ω(sss) and φ(ss)) flow (s quark flow as large as for light quarks)
- Further indication for thermalization
- Indication for collective flow in partonic phase (small hadronic cross-section for Ω,φ, meson/baryon difference)

- M. Oldenburg, QM 2005

- STAR preliminary

- Hydro: P. Huovinen, private communication

** ****Backup**

- J. Speltz - Hot Quarks 2006

- 16th May 2006

## Statistical Model : excitation function

- Cleymans et al., hep-ph/0511094

** ****Backup**

- J. Speltz - Hot Quarks 2006

- 16th May 2006