of products of the cross section of the nucleon with the density of objects it can collide with. $$\lambda^{-1} = \sum_i \, n_i \sigma_i$$ Average particle densities, including baryons and mesons, were computed in ref. [@us] for the hot and dense matter under consideration. A plot of the density as a function of beam energy is shown in Figure 2. Assuming an average hadron-nucleon cross section of 25 mb, we find $\lambda$ = 0.4 fm at a laboratory beam energy of 11.6 GeV/nucleon. This is very short, and just emphasizes the physics we discussed in the introduction concerning hadronic matter versus quark-gluon plasma.

We assume that the collisions suffered by the nucleons are independent and can be characterized by a Poisson distribution. $$S(N,d) = \frac{1}{N!} \left( \frac{d}{\lambda}\right)^N
\exp{\left(-\frac{d}{\lambda}\right)}$$ Here $d/\lambda$ is the average number of scatterings in a distance $d$.

The invariant distribution function $H(x,N)$ describes the momentum degradation of a nucleon propagating through the hot zone. This distribution function was introduced in the evolution model of Hwa [@hwa]. In this model the nucleon propagates on a straight line trajectory and interacts with target particles contained within a tube with area given by the elementary nucleon–nucleon cross section $\sigma_{NN}$. Csernai 