Relativistic $ $ in the calculation of relics abundances: a closer look

In many fields of physics and astrophysics one has to study reaction rates in a system that can be considered, to a good approximation, a classical non-relativistic gas in equilibrium. In the gas can be present different species of particles. We consider two species with masses m1 and m2 and number densities n1,2, the number of particles per unit volume. For a given process with total cross section σ, the number of reactions per unit time per unit volume, the reaction rate, is given by R = n1n2σvr, where