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 r 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 r2 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 kr2r2. 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.