Two-pion Bose-Einstein correlations at SPS energies and a novel interpretation of freeze-out

by H. Tilsner (University of Heidelberg)

Tuesday 27 Aug 2002, 14:00 → 14:30 Europe/Zurich

160/1-009 (CERN)

The addition of a radial TPC to the CERES experiment at the CERN-SPS improved significantly the sensitivity of the spectrometer to hadronic observables near midrapidity and allowed for a systematic study of the centrality dependence of two-pion Bose-Einstein correlations in Pb+Au collisions at 40, 80, and 158 AGeV. This range of beam energies at SPS provides an important link between the nucleon dominated domain at lower beam energies and the pion dominated domain at the colliders. A multi-dimensional analysis of the correlation function as function of the pair transverse momentum $k_t$ provides access to quantities characterizing the space-time evolution, such as the lifetime of the system, the transverse expansion velocity and the duration of pion emission. A detailed comparison of the $k_t$ dependencies of the HBT parameters from all available beam energies reveals a smooth transition from nucleon to pion dominated freeze-out between AGS and RHIC energies and explains the apparently weak overall beam energy dependence. The transition region is characterized by a minimum of the effective freeze-out volume. This non-monotonic behaviour rules out that freeze-out happens at constant density. We will show that the minimum appears naturally if different cross sections of pions with the surrounding medium due to a change of the fireball composition with beam energy are considered. This finally leads to a universal pion freeze-out condition in terms of their mean free path from AGS to RHIC.

Organiser(s): Yiota Foka, Guy Paic and Urs Wiedemann