Effect of a late decaying scalar on the neutralino relic density

Abstract

If the energy density of the Universe before nucleosynthesis is dominated by a scalar field φ that decays and reheats the plasma to a temperature TRH smaller than the standard neutralino freeze out temperature, the neutralino relic density differs from its standard value. In this case, the relic density depends on two additional parameters: TRH, and the number of neutralinos produced per φ decay per unit mass of the φ field. In this paper, we numerically study the neutralino relic density as a function of these reheating parameters within minimal supersymmetric standard models and show that the dark matter constraint can almost always be satisfied. I. MOTIVATIONThe neutralino is considered a good dark matter candidate, one that naturally yields the required relic density. Recently, however, it has been recognized that, owing to the experimental constraints and to the increased precision in the determination of the dark matter content of the Universe, the agreement between the observed relic density, Ω cdm h 2 = 0.113 ± 0.009 [1] 1 and the relic density predicted with standard cosmological assumptions, is far from being a generic feature of supersymmetric models. In fact, models with bino-like neutralinos tend to overproduce them, and special mechanisms such as coannihilations or resonant annihilations are required to suppress the relic density down to the observed range. In contrast, models with higgsino-or wino-like neutralinos usually give too small a relic density and, to compensate for it, large neutralino masses (m χ > 1 TeV) are needed. Non-standard cosmologies and in particular models with low reheating temperatures provide a plausible solution to this problem. These include models with moduli decay [3], Q-ball decay [4], and thermal inflation [5]. In all of these models there is a late episode of entropy production and non-thermal production of the LSP in particle decays is possible.We concentrate on cosmological models in which the early Universe is dominated by the energy density of a scalar field that after some time decays giving rise to the radiation dominated era. The decay of the scalar field into light degrees of freedom and their subsequent thermalization -the reheating process-leaves the Universe at a temperature T RH known as the reheating temperature. If, as assumed in the standard scenario, T RH is larger than the neutralino freeze out temperature (T f.o. ≃ m χ /20), the neutralino relic density is insensitive to its value. But, because we have no physical evidence of the radiation dominated Universe before big-bang nucleosynthesis, T RH should be considered as a cosmological parameter that can take any value above a few MeV [6,7].The existence of a weakly coupled scalar field that dominates the Universe during the process of neutralino production and freeze out may affect the relic density in several ways. It modifies the temperature-scale factor and the temperature-expansion rate relations [8,9,10] that determine the freeze out condition. It dilutes the neutralino thermal abundance by incr...

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