Dilution of Cosmological Densities by Saxino Decay

Abstract

Saxino decay can generate significant cosmological entropy, and hence dilute theoretical estimates of the present mass density of a given particle species. The dilution factor depends on the saxino and axion masses, and is constrained by the requirement that saxino decay should not affect nucleosynthesis, as well as by the usual requirement that the axion density be less than the critical density. The latter constraint is evaluated carefully, under both the Harari-Sikivie and Davis proposals about the emission spectrum from axionic strings. Uncertainties are carefully evaluated, points of principle are addressed, and with an eye to future numerical simulation the spacing and typical oscillation wavelength of the strings are represented by parameters varying in the range 1 to 3. Within the constraints, the entropy dilution varies from 1 to $10^{-4}$. Only saxinos originating from thermal equilibrium are considered, so that more dilution might arise from non-thermal saxinos.