Pion interferometry in E814 -- Toward equilibrium at the AGS

The ultimate goal of ultra-relativistic heavy-ion program at the BNL AGS and CERN SPS is to study highly excited nuclear matter and the transition from hadronic matter to quark gluon plasma. In the QGP state, quarks and gluons are no longer confined but they move freely in a fairly large volume. According to their knowledge, the de-confined state can be approached via either compression or heating of nuclear matter in heavy-ion collisions. At AGS energies, the most relevant process is believed to be compression. However, before the authors try to identify any exotic events such as, for example QGP phase transition or chiral symmetry restoration, they have to answer the following questions: (1) Were the densities(due to the compression) high enough? (2) Was the high density region large enough? (3) Was the system thermalized? The authors report recent pion interferometry results from AGS experiment E814. By comparing to the results of RQMD calculations, a freeze-out size Rrms = 8.3 fm is found in Si + Pb central collisions. A consistent thermal equilibration picture is established by comparing experimental data with the results of both hydrodynamic and cascade model calculations