KeV scale new fermion from a hidden sector

We studied a simple model of hidden sector 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 righthanded SM singlet neutrinos, $N_R$'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 $ϕ^\dagger ϕH^\dagger H$ and this provides a third connection to the SM. Integrating out the $N_R$ at a high scale not only gives the active neutrinos, $ν$, masses but generates effective Dirac type of couplings between $ν$ and $χ$. This changes the usual Type-I seesaw results for active neutrino masses and makes $χ$ behave like a sterile neutrino even though its origin is in the hidden sector. $χ$ 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.

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