Space-time analysis: HBT at SPS and RHIC

The CERN SPS lead beam program has covered for the first time with high statistics all HBT radius parameters of the full three-dimensional parametrization over a wide region in rapidity and transverse momentum (for data, see Ref. [1]). Theory provides by now a detailed data analysis strategy and interpretation (for a review, see Ref. [2]) . In this talk, I summarize the conclusions which we can draw from the recent CERN SPS data, and I identify questions which can be settled by the upcoming RHIC experiments. The main goal of HBT is to determine the geometry and dynamics of the heavy ion collision at freeze-out. This information is becoming increasingly important for the discussion of other observables. For example: i) In studies of anisotropic flow [3,4], one aims at exploiting the HBT space-time picture to disentangle geometrical and dynamical components of anisotropy [5–8]. These cannot be separated on the basis of one-particle spectra alone. ii) In the search for event by event multiplicity fluctuations of qualitatively new origin (e.g. DCCs), fluctuations due to Bose-Einstein correlations are an important background source [9]. Their calculation depends strongly on the phase-space density attained in the last stage of the collision. The HBT analysis of experimental data now provides estimates of this phase space density for a large variety of collision systems [10], and work on its consequences for multiplicity fluctuations is in progress [11]. iii) In event generator studies, HBT measurements provide constraints on the simulated space-time structure [12], and the question how microscopic dynamics can account for the observed final state emission region becomes of the utmost importance. In the light of these rapidly developing applications, one should not forget the fundamental limitations of any particle interferometric analysis. HBT extracts the phase space density S(x,K) from a combination of hadronic one-particle and identical two-particle spectra of the abundant hadron species: