Right-Handed Sneutrino as Cold Dark Matter of the Universe

We consider the minimal supersymmetric standard model (MSSM) extended by introducing three right-handed (s)neutrinos to account for neutrino masses in the oscillation experiments. Assuming that the neutrino masses are purely Dirac-type, the lightest right-handed sneutrino $\tilde \nu_R$ can be the lightest superparticle (LSP), which is a good candidate of cold dark matter (CDM) of the universe. We study the possibility of realizing $\tilde \nu_R$-CDM, paying a special attention to the production of $\tilde \nu_R$ via decay of the next-to-lightest superparticle (NLSP) after its freeze-out time. It is shown that the late decay of the MSSM-LSP (the LSP among superparticles in the MSSM) can produce a sufficient amount of $\tilde \nu_R$ to explain the observed dark-matter density, and that the $\tilde \nu_R$-CDM scenario can be realized in a wide range of parameter space. We also consider the constraint on the decay of MSSM-LSP from the big-bang nucleosynthesis (BBN); we found that the case with stau being the MSSM-LSP is severely constrained.