# Quark Matter 2014 - XXIV International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

19-24 May 2014
Europe/Zurich timezone

## On Chemical Freeze-outs of Strange and Nonstrange Hadrons

20 May 2014, 16:30
2h

Board: J-04

### Speaker

Prof. Kyrill Bugaev (Bogolyubov Institute for Theoretical Physics of NAS of Ukraine, Kiev, Ukraine)

### Description

\noindent We performed a thorough analysis of two alternative approaches to treat the chemical freeze-out of strange particles in hadron resonance gas model with the multicomponent hard-core repulsion [1]. The first approach accounts for their chemical non-equilibrium via the usual $\gamma_s$ factor and such a model describes the hadron multiplicities measured in nucleus-nucleus collisions at AGS, SPS and RHIC energies with $\chi^2/dof = 63.4/55 \simeq$ 1.15. In contrast to earlier beliefs, we find that strangeness rather tends to be enhanced than suppressed at low collision energies, i.e. $\gamma_s > 1$, at $\sqrt{s_{NN}} =$ 2.7, 3.3, 3.8, 4.9, 6.3, 9.2 GeV. Second approach is to treat strange particle freeze-out separately from non-strange particle freeze-out. Conservation laws allow us to connect the freeze-outs of strange and non-strange hadrons and end up with the same number of free parameters as for $\gamma_s$ approach. We show that this approach works not worse than the $\gamma_s$ approach with $\chi^2/dof = 58.5/55 \simeq$ 1.06 and for $\sqrt{s_{NN}} =$ 6.3, 12 and 17 GeV it significantly improves the fit quality. For all considered collision energies we see that $\bar p/\pi^-$, $\bar \Lambda/\Lambda$, $\bar \Xi^-/\Xi^-$ and $\bar \Omega/\Omega$ ratios are described better than within the traditional $\gamma_s$ approach, since a separation of chemical freeze-outs relaxes the strong connection between the non-strange and strange baryons. The novel concept of strange particle freeze-out allows us to describe 111 independent hadron ratios measured at 14 different energies with the highest quality ever achieved. Basing on these results we conjecture that an apparent strangeness enhancement is due to the separate strangeness chemical freeze-out. \\ \noindent {\bf References:}\\ 1) K. A. Bugaev, D. R. Oliinychenko, J. Cleymans, A. I. Ivanytskyi, I. N. Mishustin, E. G. Nikonov and V. V. Sagun, Europhys. Lett. 104, 22002 (2013)

### Primary author

Prof. Kyrill Bugaev (Bogolyubov Institute for Theoretical Physics of NAS of Ukraine, Kiev, Ukraine)

### Co-authors

Mr Aleksei Ivanytskyi (Bogolyubov Institute for Theoretical Physics of NAS of Ukraine, Kiev, Ukraine) Dmytro Oliinychenko Prof. Edward Nikonov (Laboratory for Information Technologies, JINR, Dubna, Russia) Prof. Igor Mishustin (Frankfurt Institute for Advanced Studies, Frankfurt upon Main, Germany) Jean Cleymans (University of Cape Town (ZA)) Ms Violetta Sagun (Bogolyubov Institute for Theoretical Physics of NAS of Ukraine, Kiev, Ukraine)

 Slides