Constraints on a system of two neutral fermions from cosmology

Abstract

Using the standard model of cosmology we study the evolution of the population of a coupled system of two neutral fermions in which the lighter one is stable. During the expansion their population can be frozen at a certain level which makes them contribute to the mass density of the universe. The details of the freezing depend crucially on the couplings and on the masses of these two fermions, so that, comparison with the measured mass density in the universe gives constraints on the parameters of the physical system we examine. We discuss in detail different configurations for the couplings among these fermions; in particular in the case of large mixing we obtain restrictive bounds on both masses. Our study is relevant to supersymmetric grand unified models which predict the occurance of light interacting neutral fermions, particularly higgsinos.