Axinos as dark matter

Supersymmetric extensions of the Standard Model that incorporate the axionsolution to the strong CP problem necessarily contain also the axino, thefermionic partner of the axion. In contrast to the neutralino and thegravitino, the axino mass is generically not of the order of thesupersymmetry-breaking scale and can be much smaller. The axino is therefore anintriguing candidate for a stable superpartner. In a previous Letter [1] it wasshown that axinos are a natural candidate for cold dark matter in the Universewhen they are generated non-thermally through out-of-equilibrium neutralinodecays. Here, we extend the study of non-thermal production and include acompeting thermal production mechanism through scatterings and decays ofparticles in the plasma. We identify axino masses in the range of tens of MeVto several GeV (depending on the scenario) as corresponding to cold axinorelics if the reheating temperature $\treh$ is less than about$5\times10^4\gev$. At higher $\treh$ and lower mass, axinos could constitutewarm dark matter. In the scenario with axinos as relics, the gravitino problemfinds a natural solution. The lightest superpartner of the Standard Modelspectrum remains effectively stable in high-energy detectors but may be eitherneutral or charged. The usual highly restrictive constraint $\abundchi\lsim1$on the relic abundance of the lightest neutralino becomes void.