1. At time $t = 0$ they touch; subsequently they interpenetrate, forming hot and dense matter in the region of overlap. During this stage, additional matter streams into the hot zone even as this zone is expanding along the beam axis. The nucleons streaming in undergo scatterings with the hot matter already present, degrading their longitudinal momentum and producing baryonic isobars and/or mesons. Finally, at time $t_0 = L/2v\gamma$, all the cold nuclear matter has streamed into the region of overlap, and expansion and cooling begins. Here, $L$ is the nuclear thickness, $v$ is the velocity in the center-of-momentum frame, and $\gamma$ is the associated Lorentz contraction factor. This is a very simplified picture of the early stage of the collision, but it seems to semi-quantitatively represent the outcome of both the ARC and RQMD simulations [@rqmd; @arc; @us].

We are interested in the possibility that an incoming projectile nucleon suffers little or no energy loss during its passage to the longitudinal point $z$ inside the hot and dense zone where it encounters a target nucleon which also has suffered little or no energy loss. The energy available in the ensuing nucleon-nucleon collision, $\sqrt{s}$, can go into meson production. Suppose 