We study an extension of the Standard Model (SM) with two interacting cold
Dark Matter (DM) candidates: a neutral Majorana fermion (ν) and a neutral
scalar singlet (φ). The scalar φ interacts with the SM through
the "Higgs portal" coupling while ν at the tree level interacts only with
φ through Yukawa interactions. The relic abundance of ν and
φ is found by solving the Boltzmann equations numerically; for the case
mν>mφ 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: mφ∼62.5 GeV (corresponding to the vicinity of the Higgs
boson resonance responsible for φφ annihilation into SM
particles), mφ≃130−140 GeV and m_\varphi \gesim 3 TeV. 1-loop
induced ν-nucleon scattering has been also calculated and discussed. A
possibility of DM direct detection by the CREST-II experiment was considered.Comment: 22 pages, 17 figures; v2: references added, published in JHEP, v3:
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