Increasing the neutralino relic abundance with slepton coannihilations: consequences for indirect dark matter detection

We point out that if the lightest supersymmetric particle (LSP) is a higgsino- or wino-like neutralino, the net effect of coannihilations with sleptons is to increase the relic abundance, rather than producing the usual suppression, which takes place if the LSP is bino-like. The reason for the enhancement lies in the effective thermally averaged cross section at freeze-out: sleptons annihilate (and coannihilate) less efficiently than the neutralino(s)-chargino system, and therefore slepton coannihilations effectively act as parasite degrees of freedom at freeze-out. Henceforth, the thermal relic abundance of LSPs corresponds to the cold dark matter abundance for smaller values of the LSP mass, and larger values of the neutralino pair annihilation cross section. In turn, at a given thermal neutralino relic abundance, this implies larger indirect detection rates, as a result of an increase in the fluxes of antimatter, gamma rays and neutrinos from the Sun originating from neutralino pair annihilations.