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Transverse radius of an expanding hadronic system at freeze-out time.

I construct a simple, analytical model for both the longitudinal and transverse expansion of an interacting hadronic system. This model explicitly assumes that the particle rapidity distribution is not boost invariant, and specifically includes an initial transverse dimension. It employs standard assumptions about the delayed formation of produced particles and the correlation between longitudinal position and rapidity. No particular dynamical scenario is assumed for the early stages of the system's expansion, but rather the last stage, freeze-out, is tied to the particle rapidity distribution. The observed multiplicity and width of the rapidity distribution control the transverse size and time when the system freezes out. The model predicts a transverse radius at freeze-out and its dependences on rapidity and multiplicity, which are very similar to those obtained by pion interferometry. This analysis can be used with different event classes to correlate changes in the multiplicity and width of the rapidity distribution with changes in the size, shape, and lifetime of the system at freeze-out.