Angular distributions of relativistic alpha particles in heavy-ion collisions

Abstract

Angular distributions of relativistic $\alpha$ particles produced from Fe emulsion and Ar emulsion at about $2A$ GeV and Fe emulsion at $0.9A$ GeV have been studied. The angular distribution falls in either one or the other of two separate classes with different characteristics; hot for tail or cold for peak zones, and thus, a single Boltzmann distribution cannot fit the fragmentation peak and the tail simultaneously. Two zones are characterized by separate Boltzmann distributions in the projectile rest frame and the associated temperatures differ by a factor of 4 or more. The $\alpha$ particles with large angular distributions especially cannot be explained by a model which is based on the fragmentation from a clean-cut participants-spectator picture. Experimental results in the "hot" events give some direct evidence for the transfer of large "transverse" momenta in these collisions.