KeV scale new fermion from a hidden sector

Abstract

We studied a simple model of hidden sector consists of a Dirac fermion $\chi$ 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 $\varphi$ that breaks $U(1{)}_s$ and the SM Higgs boson, $H$, is implemented via the term ${\varphi }^{†}\varphi H^{†}H$ and this provides a third connection to the SM. Integrating out the $N_R$ at a high scale not only gives the active neutrinos, $\nu$, masses but generates effective Dirac type of couplings between $\nu$ and $\chi$. This changes the usual Type-I seesaw results for active neutrino masses and makes $\chi$ behave like a sterile neutrino even though its origin is in the hidden sector. $\chi$ 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 $\chi$ in high precision Kurie plots of nuclei $\beta$ 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.

27 pages, 4 figures; References added; Typos and minor mistakes corrected

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• Observation of Coherent Elastic Neutrino-Nucleus Scattering
• Sterile Neutrino Dark Matter
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Cited by in corpus (2)

• Constraints on general neutrino interactions with exotic fermion from neutrino-electron scattering experiments
• Constraints on light singlet fermion interactions from coherent elastic neutrino-nucleus scattering