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标题：Is Chemical Equilibrium achieved in Collisions of Small Systems at the SPS?
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作者：I. Kraus, H.,Oeschler ; K. Redlich
期刊名称：PoS - Proceedings of Science
印刷版ISSN：1824-8039
出版年度：2005
卷号：2005
出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Ultrarelativistic nucleus-nucleus collisions are investigated with the goal to study the properties of strongly interacting matter under extreme conditions of high energy density. Hadron multiplicities can provide information on the nature of the medium from which they are originating. The Statistical Model was recognised as a powerful approach to describe particle yields established at chemical decoupling. In general, this model assumes that at freeze-out the collision fireball appears as a statistical system in thermal and chemical equilibrium. The ensembles are constrained by charge conservation laws. The Statistical Model has to be formulated in the canonical ensemble with respect to strangeness conservation if the number of strange particles becomes small. However, the canonical suppression under the assumption of strangeness chemical equilibrium in the whole fireball volume was found to be not sufficient to reproduce observed yields. Two approaches have been proposed to modify the model. First, a non-equilibrium factor gS was introduced in canonical and grandcanonical ensembles as the additional fit parameter to account for the suppressed strange particle phase-space. Here we focus on the second method: The model is extended by correlation volumes which restrict the strangeness chemical equilibrium only to certain subvolumes of the system. In this work we report on the analysis of experimental data on particle production from p+p and central C+C, Si+Si and Pb+Pb collisions at the top SPS energy within the Statistical Model. The abundances of strange particles, in particular in the small systems, are found to be below the expectation of the Statistical Model formulated in the canonical ensemble. Therefore, we introduce strangeness equilibrated subvolumes. The canonical strangeness suppression in these correlated clusters accounts successfully for the smaller production of strange particles. The system size dependence of the correlation volume and of the thermal parameters are presented.