摘要
We studied a simple model of the hidden sector that consists of a Dirac fermion χ and a spontaneously broken U(1)s symmetry. The dark sector is connected to the Standard Model (SM) via three right-handed SM singlet neutrinos, NR's, and the kinetic mixing between U(1)s and U(1)Y. A mixing between the scalar φ that breaks U(1)s and the SM Higgs boson H is implemented via the term φ†φH†H, and this provides a third connection to the SM. Integrating out the NR at a high scale not only gives the active neutrinos ν masses but generates effective Dirac-type couplings between ν and χ. This changes the usual type-I seesaw mechanism results for active neutrino masses and makes χ behave like a sterile neutrino even though its origin is in the hidden sector. Note that χ is also split into a pair of Majorana fermions. The amount of splitting depends on the parameters. If the lighter of the pair has a mass around keV, its lifetime is longer than the age of the Universe and it can be a warm dark matter candidate. Signatures of χ in high precision Kurie plots of nuclei β decays and low energy neutrino nuclei coherent scatterings are discussed. The model also induces new invisible Z decay modes that can be searched for in future Z factories.