Role of the equation of state in the hydrodynamical model

The influence of the dependence on energy density (temperature) of the velocity of sound $c_0$ on the hydrodynamical expansion is considered for the first time. A numerical solution of the one-dimensional Landau model is given for this more general case. The result can be parametrized by an effective constant velocity of sound which is very close to the initial value of $c_0$. Using for this initial value the canonical value 1/ √3 corresponding to an unconfined quark-gluon plasma and taking into account in an analytic approximation the later three-dimensional expansion, the pseudorapidity distributions of secondaries in collisions of pp at √s =63 GeV and p¯p at √s =540 GeV are found to be in good agreement with data.