Chemical Equilibrium in Collisions of Small Systems

The Statistical Model is recognized as a powerful approach to describe particle yields established at chemical decoupling. It has to be formulated in the canonical ensemble with respect to strangeness conservation if the number of strange particles becomes small. However, in some cases canonical suppression under the assumption of strangeness chemical equilibrium in the whole fireball volume was found not to be sufficient to reproduce the observed yields. It has been proposed to introduce a non-equilibrium factor γS as additional fit parameter. Here we focus on an alternative approach: Strangeness conservation happens in subvolumes. Canonical suppression which refers to small systems is demonstrated in Fig 1. The left hand panel illustrates ratios of various particles with different strangeness. It shows that the grand-canonical limit is reached at radii of a few fm and that the suppression becomes stronger with increasing strangeness content. On the right hand side, the suppression of ratios of particles with a difference of ΔS = 1 shows that it is similar but not identical, it depends on the strangeness content of both hadrons, not only on the difference. The latter is true for the suppression caused by the undersaturation parameter γS .