Freeze-in sterile neutrino dark matter in a feebly gauged B − L model

Abstract

We consider the gauged U(1)_B−L model and examine the situation where the sterile neutrino is a dark matter candidate produced by the freeze-in mechanism. In our model, the dark matter N is mainly produced by the decay of a U(1)_B−L breaking scalar boson ϕ. We point out that the on-shell production of ϕ through annihilation of the U(1)_B−L gauge boson Z^′ plays an important role. We find that the single production of Z^′ from the gluon bath in the early Universe can become the main production modes for Z^′ in some parameter regions. To prevent N from being overproduced, we show that the U(1)_B−L gauge coupling constant g_B−L must be as small as 10⁻¹⁶–10⁻¹⁰. We also consider the case where the decay of ϕ into N is kinematically forbidden. In this case, N is generated by the scattering of Z^′ and the g_B−L takes values of 10⁻¹⁰–10⁻⁶, which can be explored in collider experiments like FASER and SHiP.