Spatial information on the pion-emitting source from pion interferometry

We discuss the physical meaning of the spatial source parameters obtained from two-pion interferometry. We propose that the average radius left-angle r right-angle of the source is a suitable quantity to describe the spatial extent of the source under various assumptions about the source density distribution, and demonstrate that the average radius is more appropriate than the root-mean-square radius proposed by Bartke and Kowalski. The mean square radius left-angle r2 right-angle of the source can be estimated using a correlation function for small relative momenta, and more detailed information about the source density distribution can be inferred from the value of k=left-angle r2 right-angle/left-angle r right-angle 2. Studies of central collisions of 1.5A GeV Ar+KCl and 1.8A GeV Ar+Pb at the Bevalac streamer chamber show that the data are consistent with a phenomenological Gaussian model for the density distribution at the present level of experimental accuracy