Direct detection of supersymmetric dark matter and the role of the target nucleus spin

We investigate the role of nuclear spin in elastic scattering of dark matter (DM) neutralinos from nuclei in the framework of the minimal supersymmetric standard model (MSSM). The relative contribution of spin-dependent axial-vector and spin-independent scalar interactions to the event rate in a DM detector has been analyzed for various nuclei. Within general assumptions about the nuclear and nucleon structure we find that for nuclei with atomic weights A>50 the spin-independent part of the event rate ${\mathit{R}}_{\mathrm{SI}}$ is larger than the spin-dependent one ${\mathit{R}}_{\mathrm{SD}}$ in the domain of the MSSM parameter space allowed by the known experimental data and where the total event rate is R=${\mathit{R}}_{\mathrm{SD}}$+${\mathit{R}}_{\mathrm{SI}}$>0.01 events/(kg day). The latter condition reflects realistic sensitivities of present and near future DM detectors. Therefore we expect equal chances for discovering a DM event either with spin-zero or with spin-nonzero isotopes if their atomic weights are ${\mathit{A}}_1$∼${\mathit{A}}_2$>50. We discuss several examples of spin-nonzero nuclei ${(}^{19}$F, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$, ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$) as a target material for DM detectors and compare their axial-vector couplings to the neutralino.