X iv : h ep - p h / 05 06 23 3 v 2 8 F eb 2 00 6 Coincidence probability as a measure of the average phase - space density at freeze - out

It is pointed out that the average semi-inclusive particle phase-space density at freeze-out can be determined from the coincidence probability of the events observed in multiparticle production. The method of measurement is described and its accuracy examined. 1. Recently, several methods were proposed which allow to estimate the average of the single-particle inclusive phase-space density produced in ultra-relativistic heavy ion collisions [1]-[5]. This quantity is useful in discussions of the equilibrated systems and therefore such measurements open possibilities to verify the expected presence of the thermal phase-space distribution at freeze-out [6] and/or search for more exotic phenomena [7]. In the present paper we propose an extension of these studies by including , in addition, the exclusive and semi-inclusive M-particle phase-space densities. We show that their averages can be estimated from measured coincidence probabilities of the multiparticle events observed in high-energy collisions. The information one may gain from this approach is complementary to that obtained from single-particle inclusive measurements. In particular , it gives an insight into the correlation structure of the final state of the