Separate freeze-out of strange particles and the quark-hadron phase transition

¹ Bogolyubov Institute for Theoretical Physics, Metrologichna str. 14 b, 03143 Kyiv, Ukraine
² Centro de Astrofísica e Gravitação - CENTRA, Departamento de Física, Instituto Superior Técnico - IST, Universidade de Lisboa - UL, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
³ Laboratory for Information Technologies, JINR, Joliot-Curie str. 6, 141980 Dubna, Russia
⁴ Department of Physics, University of Cape Town, Rondebosch 7701, South Africa
⁵ FIAS, Goethe-University, Ruth-Moufang Str. 1, 60438 Frankfurt upon Main, Germany
⁶ Kurchatov Institute, Russian Research Center, Kurchatov Sqr., Moscow, 123182, Russia
⁷ Department of Physics, University of Oslo, PB 1048 Blindern, N-0316 Oslo, Norway
⁸ Skobeltzyn Institute of Nuclear Physics, Moscow State University, 119899 Moscow, Russia
⁹ National Research Nuclear University “MEPhI” (Moscow Engineering Physics Institute), Kashirskoe Shosse 31, 115409 Moscow, Russia

^a e-mail: bugaev@fias.uni-frankfurt.de

Published online: 3 August 2018

Abstract

The scenario of the independent chemical freeze-outs for strange and nonstrange particles is discussed. Within such a scenario an apparent in-equilibrium of strangeness is naturally explained by a separation of chemical freeze-out of strange hadrons from the one of non-strange hadrons, which, nevertheless, are connected by the conservation laws of entropy, baryonic charge and third isospin projection. An interplay between the separate freeze-out of strangeness and its residual non-equilibrium is studied within an elaborate version of the hadron resonance gas model. The developed model enables us to perform a high-quality fit of the hadron multiplicity ratios measured at AGS, SPS and RHIC with an overall fit quality ϰ²/dof = 0:93. A special attention is paid to a description of the Strangeness Horn and to the well-known problem of selective suppression of Δ^- and ж hyperons. It is remarkable that for all collision energies the strangeness suppression factor γ_s is about 1 within the error bars. The only exception is found in the vicinity of the center-of-mass collision energy 7.6 GeV, at which a residual enhancement of strangeness of about 20 % is observed.