Coannihilation Effects and Relic Abundance of Higgsino-Dominant LSPs

We calculate the relic abundance of Higgsino-dominant lightest superparticles, taking account of coannihilations with the superparticles which are almost degenerate with the lightest one. We show that their relic abundance is reduced drastically by the coannihilation processes and hence they are cosmologically of no interest. The lightest superparticle (LSP) in supersymmetric (SUSY) standard models, mostly taken to be a neutralino, is an intriguing candidate for dark matter [1, 2, 3]. An important task is therefore to compute the relic abundance of the LSPs, which is conveniently represented by Ωχh 2 where Ωχ is the ratio of the mass density of the LSPs to the critical one to close the Universe and h (0.4 < ∼ h < ∼ 1) stands for the Hubble constant in units of 100 km secMpc. This issue has been discussed extensively in the literature [4, 5, 6, 7], assuming (1) the minimal particle content of the SUSY standard model, (2) a GUT relation on gaugino mass parameters (see below), (3) the neutralino LSP and (4) R-parity conservation which guarantees the stability of the LSP. The abundance of the neutralino LSPs crucially depends not only on their mass but also on the composition of them, since the LSP is a linear combination of neutral gauginos, B̃, W̃3, and neutral Higgsinos H̃1, H̃2, and each of them has different pair annihilation processes which determine its abundance. The results obtained previously are summarized as follows: let us first consider the case where the LSP is lighter than the W-boson. If it is almost a pure gaugino or a pure Higgsino, its density can reach (or even exceed) the critical one and hence it is a good candidate for the dark matter of the Universe. If the LSP is an admixture of the gauginos and the Higgsinos, the relic abundance is generally small. On the contrary, in the case where the LSP is heavier than the W-boson, the annihilation mode to a W-pair opens if it is a pure Higgsino or a mixed LSP and the relic abundance is very small until the mass increases to a TeV region where Ωχh 2 of the LSPs becomes again of order unity. If the LSP is a gaugino, there is no annihilation process