# KeV scale new fermion from a hidden sector

## TL;DR

This paper proposes a hidden sector model with a keV-scale fermion that can serve as warm dark matter, connecting to the Standard Model through neutrinos, scalar mixing, and kinetic mixing, with potential experimental signatures.

## Contribution

It introduces a novel hidden sector model where a keV-scale fermion acts as warm dark matter and modifies neutrino mass generation, with multiple experimental signatures.

## Key findings

- The keV fermion can be a long-lived dark matter candidate.
- The model alters active neutrino mass generation mechanisms.
- Potential signals in Z decays and neutrino scattering experiments.

## 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 $\phi$ that breaks $U(1)_s$ and the SM Higgs boson, $H$, is implemented via the term $\phi^\dagger \phi H^\dagger H$ and this provides a third connection to the SM. Integrating out the $N_R$ at a high scale not only gives the active neutrinos, $\nu$, 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.