Two-Component Dark Matter

We study an extension of the Standard Model (SM) with two interacting cold Dark Matter (DM) candidates: a neutral Majorana fermion ($\nu$) and a neutral scalar singlet ($\varphi$). The scalar $\varphi$ interacts with the SM through the "Higgs portal" coupling while $\nu$ at the tree level interacts only with $\varphi$ through Yukawa interactions. The relic abundance of $\nu$ and $\varphi$ is found by solving the Boltzmann equations numerically; for the case $m_\nu > m_\varphi$ we also derive a reliable approximate analytical solution. Effects of the interaction between the two DM components are discussed. A scan over the parameter space is performed to determine the regions consistent with the WMAP data for DM relic abundance, and with the XENON100 direct detection limits for the DM-nucleus cross section. We find that although a large region of the parameter space is allowed by the WMAP constraints, the XENON100 data severely restricts the parameter space. Taking into account only amplitudes generated at the tree level one finds three allowed regions for the scalar mass: $m_\varphi \sim 62.5$ GeV (corresponding to the vicinity of the Higgs boson resonance responsible for $\varphi\varphi$ annihilation into SM particles), $m_\varphi \simeq 130-140$ GeV and $m_\varphi \gesim 3$TeV. 1-loop induced $\nu$-nucleon scattering has been also calculated and discussed. A possibility of DM direct detection by the CREST-II experiment was considered.