Transverse momentum bounds and scaling in the hydrodynamical model

It is shown that the equation of state of an ideal relativistic gas, as applied in the hydrodynamical model, leads not only to deviations from scaling in longitudinal rapidity distributions, but also to an average transverse momentum increasing asymptotically as a power of the incident energy. To prevent such an increase, one must use the equation of state of an interacting gas, in which the velocity of sound becomes asymptotically equal to that of light. This then also restores scaling (up to logarithmic terms) in longitudinal rapidity. (25 refs).