Neutralino dark matter from heavy axino decay

Abstract

We consider cosmological consequences of a heavy axino, decaying to the neutralino in R-parity conserving models. The importance and influence of the axino decay on the resultant abundance of neutralino dark matter depends on the lifetime and the energy density of axino. For a high reheating temperature after inflation, copiously produced axinos dominate the energy density of the universe and its decay produces a large amount of entropy. As a bonus, we obtain that the upper bound on the reheating temperature after inflation via gravitino decay can be moderated, because the entropy production by the axino decay more or less dilutes the gravitinos. I. AXINONeutralino, if it is the lightest supersymmetric particle (LSP) in R-parity conserving models, is a natural candidate for dark matter. Because of the TeV scale sparticle interactions, the thermal history of neutralinos allows the neutralino dark matter possibility. But, imposing a solution of the strong CP problem, the thermal history involves contributions from the additional sector.The strong CP problem is naturally solved by introducing a very light axion a. Most probably, it appears when the Peccei-Quinn (PQ) symmetry is broken at a scale of f a . Below the PQ scale, the effective axion interaction with gluons iswhere g s is the strong coupling constant [1]. The PQ scale is constrained by the astrophysical and cosmological considerations in the narrow window 10 10 GeV f a 10 12 GeV [2].TeV scale supersymmetry (SUSY) suggests axinoã, the superpartner of axion, around the electroweak scale in the gravity mediation scenario. Here, we consider the effects of heavy axinos in cosmology. The axino cosmology depends crucially on the axino decoupling temperature [3], The axion supermultiplet includes axion, saxion (the scalar partner) and axino. Both saxion and axino masses are split from the almost vanishing axion mass if SUSY is broken. The precise value of the axino mass depends on the model, specified by the SUSY breaking sector and the mediation sector to the axion supermultiplet [4]. In principle, the axion supermultiplet is independent from the observable sector in which case we may take the axino mass as a free parameter of order from keV to a value much larger than the gravitino mass [5,6]. Light axinos can be a dark matter (DM) candidate, which has been studied extensively [7,8,9]. Heavy axinos, however, cannot be the LSP and can decay to the LSP plus light particles. This heavy axino decay to neutralino was considered in the literature [6] where the neutralino relic density was not considered seriously. Some considered the axino as the next LSP decaying to the gravitino LSP in the gauge mediated SUSY breaking scenario [10]. Recently, supersymmetric axion models were studied with an emphasis on saxion [11], where the heavy axino possibility was also considered briefly [12].In this paper, we present a more or less complete cosmological analysis of a heavy axino with mass in the TeV region so that it is heavier than the LSP neutralino. Compared with the saxi...

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