Direct dark matter detection: A spin3/2WIMP candidate

Abstract

We consider the prospects for direct dark matter detection in the case when the candidate particle couples to matter primarily through ordinary weak interactions. Existing experimental limits exclude the Dirac case by the very large coherent cross section and narrow down the possibilities to a Majorana particle with a purely axial coupling to the $Z$, while considerations based on relic abundance disfavor the case of spin $1/2$. We then focus on the new spin $3/2$ candidate with weak interactions. In the Majorana case, the particle can account for all of the dark matter for a range of masses between 70 and 160 GeV, depending on the strength of the Higgs couplings. The elementary spin-dependent cross section on the nucleon is calculated to be 17 fb and does not depend on the mass or any additional parameters. The amplitude at the nuclear level is of the purely isovector form. We make detailed predictions for differential and total rates of scattering on a variety of nuclear targets of interest to the current direct detection experiments. For heavy targets the annual modulation amplitude is predicted to be negative, which may help to determine the mass of the particle if and when data become available.