Monochromatic neutrinos generated by dark matter and the seesaw mechanism

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 -> 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) 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 less than or similar to 10 GeV.