Kinetic freeze-out with resonances

1. Kinetic freeze-out with resonances Levente Molnar, Purdue University for the STAR Collaboration • Outline: • Motivation • Model description • Improvements • Fit method • Results • Calculated spectra • Short lived resonances • Fit results • Summary Levente Molnar, Purdue University

2. Motivation and introduction • Recent years, wealth of experimental data and theoretical models! • Discovery → characterization: quantification • Issues still unresolved (focus on spectra related): • Chemical freeze-out: Measured particle ratios infer chemical freeze-out close to phase transition boundary. Resonances? • What’s next? Single, two, …, N kinetic freeze-out(s)? Role of resonances? • How robust the blast wave description with and without resonances? • Investigate the ±, K±, p± mid-rapidity spectra measured in 5 % most central 200 GeV Au-Au collisions. Levente Molnar, Purdue University

3. Based on:N.Xu, M.Kaneta, Nucl. Phys. A698, 306 (2002). Particles included in the fit (0-5%, 200 GeV Au-Au): ±, K±, p±, K*0, K0S, ,  ,  Calculate primordial and resonance yields at chemical freeze-out temperature. (Tch = 160 MeV, μB = 22 MeV, μS = 1.4 MeV, S = 0.98) Based on code from ref.:U.A.Wiedemann, U.Heinz, Phys.Rev. C56, 3265-3286, (1997). Modifications: flat rapidity box source profile: β= βS(r/R)n Particles included: , K, p, K*, K0S, ω , η, η’, ρ, s, Σs, Ω Calculate primordial and resonancespectra at the same kinetic freeze-out temperature. Have resonance spectra decay; treat 2 body, 3 body and consecutive decays. Model description Thermal and Blast Wave model parameterization with resonances • Combine primordial and decay spectra to calculated ±, K±, p±, according to primordial yields, branching ratios, spin, isospin factors. • Fit calculated (±, K±, p± ) spectra to the measured ±, K±, p± spectra in central (0–5%), Au-Au collisions at 200 GeV → Extract Tkin, and <β>. Levente Molnar, Purdue University

4. K- spectra • Spectra are labeled after the • initial resonance particles, • combining the decay modes. • Main contribution: K* • Inclusive and primordial spectrum • shapes are similar • Small change at pT < 0.4 GeV/c Levente Molnar, Purdue University

5. spectra • Main contribution: , ,  • Inclusive and primordial spectrum shapes are similar • Gradual increase towards low pT Levente Molnar, Purdue University

6. - spectra • Main contribution: ρ, , , ’, Δ • At low pT: ρ,  contributes significantly • At higher pT: ρ dominates Levente Molnar, Purdue University

7. Short lived resonances (I.) ρ (II.) (III.) • 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.)  acquires flow as given by kinetic freeze-out temperature and transverse flow velocity, and the decay pions are calculated from decay kinematics. This implies no regeneration of  and the decay pions have the strongest flow because  efficiently gains flow due to its large mass. (100% ) • (II.) decay pions have the same pT spectral shape as the primordial pions. 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. (0% ) • (III.) Half of the ‘s are treated as in (I) and the other half as in (II). (50% ) •  decays are still included but their contribution is small Levente Molnar, Purdue University

8. Fit results • Data: Au-Au 200GeV, 0-5% • n = 0.82, fixed • (J. Adams et al. (STAR) , • Phys. Rev. Lett. 92, 112301 (2004)) • 2/NDF is small due to • point to point systematic • errors are included in the fits. Levente Molnar, Purdue University

9. Single freeze-out fit π- ● Data ─ Calc β=0.52 (best fit) β=0.3 (fixed) K- ─ p Au-Au 200GeV, 0-5% • Small radial flow: • pions are described well, • but not kaons and protons. • Larger radial flow would • “describe” kaons and • protons but pions are • overestimated. Levente Molnar, Purdue University

10. Summary • 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 the six particle spectra simultaneously. • 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? Levente Molnar, Purdue University