Thermal Hadron Production in Central 4.2 A GeV/c C+C And C+Ta Collisions

The transverse mass and rapidity spectra of participant protons and negative pions in central C+C and C+Ta collisions at 4.2A GeV/c, have been interpreted, in the full phase space, on the basis of thermal model with assumption that several fireballs in relative motion are formed, and that each fireball represents a mixture of ideal relativistic gases of protons, pions and Δ resonances in thermal equilibrium. For adequate description of the particle spectra, the stopping and symmetry/asymmetry of the colliding system require three fireballs (two fragmentation and one central) in the case of C+C and two fireballs (fragmentation and central) in the ease of C+Ta. By using the classical regime, we find that regardless of the colliding system, the freeze-out temperature of the fragmentation (central) fireballs is in the range 65-75 MeV (170-180 MeV), while the average relative velocity of the fragmentation fireball(s), with respect to the central one, is (0.55-0.60)c. Also, we find that (20-30)% of protons and (50-60)% of negative pions originate from o resonance decays. (author)