Effect of resonance decays on the extracted kinetic freeze-out parameters

1. Effect of resonance decays on the extracted kinetic freeze-out parameters Levente Molnar, Purdue University For the STAR Collaboration • Outline: • Physics Motivation • Measurements • Model description • Results • Summary

2. Physics Motivation I. Tchem Tkin Tc T Elastic scattering and kinetic freeze-out Hadronic interactionand chemical freeze-out Initial state QGP and Hydro. expansion Hadronization Pre-equilibrium by S. Bass. t • Measured particle ratios infer chemical freeze-out close to phase transition • boundary. • Evolution after chemical freeze-out are explained differently by models: • Single freeze-out models: Tc~ Tchem = Tkin • Two distinct freeze-out models: Tchem≠ Tkin

3. STAR Preliminary ΔT ~ 70 MeV Physics Motivation II. Freeze-out parameters are extracted from bulk particle (π±, K±, p/pbar) spectra • Tchem is extracted from thermal model • (N. Xu and M. Kaneta, Nucl. Phys. A 698, 306, 2002) • Tkin is extracted from blast wave parameterization, assuming primordial spectra shape based on MC calculations. (E. Schnedermann et. al. PRC48 (1993) 2462) • Significant cooling and expansion Question: If one includes resonances in the blast wave parameterization, is the Tchem= Tkin? How do the extracted kinetic freeze-out parameters change?

4. Model description • Our model is built from thermal model and blast wave parameterization with resonances • (based on code from ref.: U.A.Wiedemann, U.Heinz, Phys.Rev. C56 (1997) 3265-3286 ) • Improvements and modifications: • A more complete list of resonances (measured by STAR, AuAu 200GeV 0-5%) • , , ’, , K*0, K*±, , , , 1520, , 1385 , ,  • Implementation of two freeze-out temperatures: • Thermal model fit to measured particle ratios: • Extracted parameters: Tchem= 160 MeV, μB = 22 MeV, μS = 1.4 MeV and γ = 0.98. • Primordial particle and resonance yields are calculated by thermal model. • All primordial spectra are calculated at kinetic freeze-out temperature • Treatment of 2 and 3 body decays, as well as consecutive decays, eg. η’→ηπ+π- • η→ π+π-

5. More model details … • Compared to the Wiedemann/Heinz code: • Instead of Gaussian, we used box profile for the flow velocity: β= βS(r/R)n • Assumed flat rapidity distribution instead of Gaussian. • Compared to experimental measurements: • Decay daughters are combined through multiple decays: η’ →η→π • η’→π • And the fits: • Inclusive π±, K±, p/pbar spectra are obtained combining primordial and decay daughter spectra with proper BR × isospin • Free parameters: Tkin, β, n • .

6. K spectra from model • Inclusive calculated spectrum • shape is not significantly altered • with respect to primordial. • Main contribution: K*

7. P spectra from model • Inclusive calculated spectrum • shape is not significantly altered • with respect to primordial. • Main contribution:, , 

8. Pi spectra from model • Calculated pion spectra do not • include contribution from weak • decays (similarly to the measured • spectra). • Low pT enhancement: , , ’,Δ • Higher pT:  dominates • Inclusive pion spectrum shape is • modified in the measured pT range.

9. It is an open question what flow velocity and temperature should be assigned to short-lived resonances such as:  (c =1.3 fm),  (c =1.6 fm), … Three cases are considered for : (i.)  participates in flow just like other particles, and then decays into pions at the end (at kinetic freeze-out). This implies no regeneration of  and the decay pions have the strongest flow because  efficiently gains flow due to its large mass. (ii.)  decays instantly and is regenerated continuously from the pions in the thermal bath. In this case  does not pick up flow during its lifetime. The decay pions are as same as primordial pions in terms of spectral shapes. In this case the decay pion flow is underestimated. (iii.) Half of the ‘s are treated as in (i.) and the other half as in (ii.).  decays are still included but their contribution is small Short lived resonances

10. Parameter space of BW fit without resonances STAR Preliminary • Parameter space is scanned to map out systematics: • Well defined minimum in β- n and Tkin- n • β – T are strongly anti correlated

11. Parameter space of BW fit with resonances (100% ρ) STAR Preliminary • Coarse binning, but well defined minimum

12. Fit to central Au-Au at 200GeV (I.) • Fits performed at n fixed to be 0.82. • For pions all cases are plotted: (no resonances, 0% • , 50% , 100% ) • For kaons and protons: spectra are plotted with and • without resonances. • In case of pions the fit without resonances • seem to give the best description.

13. Fit to central Au-Au at 200GeV (II.) ● Primordial ● With resonances • As expected from spectra, • no significant change is • observed in the inclusive • spectra shapes.

14. Fit to central Au-Au at 200GeV (III.) • Note: χ2/ndf is small due to point to point systematic errors are included in the fits. • Data seem to favor 0%  case, i.e.  decay pions as same as primordial pions, • implies significant  regeneration, and the  's that experiment can see are from last • minute of evolution. • However, all scenarios are consistent with Tkin ±10 MeV syst. error.

15. Fit with a single freeze-out temperature Tkin=160MeV, n=0.82, resonances are included STAR Preliminary β=0.1 STAR Preliminary β =0.5 ● Data ○ Calc • Small radial flow: pions are described • but not kaons and protons • Larger radial flow would “describe” kaons • and protons but pions are overestimated

16. Effect of resonance decays on extracted kinetic freeze-out properties are investigated in central Au-Au collision at 200GeV. Two freeze-out model: chemical freeze-out parameters are obtained from thermal model fit; particle spectra are calculated by blast wave parameterization including resonances. Model gives good description of particle spectra. Resonances seem to have small effect on the extracted parameters (parameters are within systematic errors. 10%), due to the similar shape of primordial and inclusive spectra in the measured pT range Model fits seems to favor small  contribution; hint for  regeneration. Summary