Exploding quark-gluon plasma and correlation between energy density and mean transverse momentum

We argue that the energy per unit entropy and the initial energy density are related to the mean transverse momentum and to the rapidity density. A phenomenological analysis of cosmic ray data and pp collisions supports the conjecture that matter produced in such collisions undergoes a phase transition from a quark-gluon plasma to a hadron gas. Numerical hydrodynamical calculations which allow for both longitudinal and transverse expansion and which incorporate a first order phase transition are presented.