We study a minimal extension of the Standard Model where a scalar field is coupled to the right-handed neutrino responsible for the seesaw mechanism for neutrino masses. In the absence of other couplings, below 8 TeV the scalar $A$ has a unique decay mode $A\to \nu \nu$, $\nu$ being the physical observed light neutrino state. Above 8 TeV (11 TeV), the 3-body (4-body) decay modes dominate. Imposing constraints on neutrino masses $m_{\nu }$ from atmospheric and solar experiments implies a long lifetime for $A$, much larger than the age of the Universe, making it a natural dark matter candidate. Its lifetime can be as large as ${10}^{29}\mathrm{seconds}$, and its signature below 8 TeV would be a clear monochromatic neutrino signal, which can be observed by ANTARES or IceCube. Under certain conditions, the scalar $A$ may be viewed as a Goldstone mode of a complex scalar field whose vacuum expectation value generates the Majorana mass for ${\nu }_R$. In this case, we expect the dark matter scalar to have a mass $\lesssim 10\mathrm{GeV}$.

• Received 18 January 2015

DOI:https://doi.org/10.1103/PhysRevD.91.075001