LargepTfrom the fragmentation of 1.2-GeV/nucleonLa139nuclei

The first measurements of the transverse momenta ($p_T$) of heavy projectile fragments produced by fragmentation of 1.2-GeV/nucleon La are presented. The momentum distributions of the fragments (Z≃26–56) are Gaussians and broader than predicted by models based on internal momenta of the projectile nucleons. The larger $p_T$ observed do not appear to be due solely to Coulomb effects, but are consistent with a simple model whereby nucleons from the (hot) overlap region recoil collectively into the (colder) projectile fragments. Thus, $p_T$ and the fragment mass may provide a good indication of the vector impact parameter b.