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Current issue   Ukr. J. Phys. 2016, Vol. 61, N 8, p.659-673
http://dx.doi.org/10.15407/ujpe61.08.0659    Paper

Bugaev K.A.1, Oliinychenko D.R.1,2, Ivanytskyi A.I.1, Cleymans J.3, Mironchuk E.S.4, Nikonov E.G.5, Taranenko A.V.6, Zinovjev G.M.1

1 Bogolyubov Institute for Theoretical Physics, Nat. Acad. of Sci. of Ukraine
(14b, Metrolohichna Str., Kyiv 03680, Ukraine; e-mail: bugaev@th.physik.uni-frankfurt.de)
2 FIAS, Goethe-University,
(Ruth-Moufang Str. 1, 60438 Frankfurt upon Main, Germany; e-mail: dimafopf@gmail.com)
3 Department of Physics, University of Cape Town
(Rondebosch 7701, South Africa; e-mail: jean.cleymans@uct.ac.za)
4 Moscow Institute of Physics and Technology
(Dolgoprudnyi, Moscow Region 141700, Russia)
5 Laboratory for Information Technologies, JINR
(Dubna 141980, Russia; e-mail: e.nikonov@jinr.ru)
6 National Research Nuclear University “Moscow Engineering Physics Institute”
(31, Kashirskoe Shosse, Moscow 115409, Russia)

Separate Chemical Freeze-Outs of Strange and Non-Strange Hadrons and Problem of Residual Chemical Non-Equilibrium of Strangeness in Relativistic Heavy Ion Collisions

Section: Fields and Elementary Particles
Original Author's Text: English

Abstract: We present an elaborate version of the hadron resonance gas model with the combined treatment of separate chemical freeze-outs for strange and non-strange hadrons and with an additional γs factor which accounts for the remaining strange particle non-equilibration. Within the suggested approach, the parameters of two chemical freeze-outs are connected by the conservation laws of entropy, baryonic charge, third isospin projection, and strangeness. The developed model enables us to perform a high-quality fit of the hadron multiplicity ratios measured at AGS, SPS, and RHIC with χ2/dof ~ 0.93. A special attention is paid to a successful description of the Strangeness Horn. The well-known problem of selective suppression of Λ and Ξ hyperons is also discussed. The main result is that, for all collision energies, the γs factor is about 1 within the error bars, except for the center-of-mass collision energy 7.6 GeV, at which we find about 20% enhancement of strangeness. Also we confirm the existence of strong jumps in the pressure, temperature, and effective number of degrees of freedom at the stage of strange particle chemical freeze-out, when the center-of-mass collision energy changes from 4.3 to 4.9 GeV. We argue that these irregularities may signal about the quark-hadron phase transition.

Key words: chemical freeze-out, strangeness enhancement/suppression factor γs, Strangeness Horn, hadron multiplicities.

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