Speaker
Description
We consider the gauged U(1)$_\mathrm{B−L}$ 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)$_\mathrm{B−L}$ breaking scalar boson $\phi$. We point out that the on-shell production of $\phi$ through annihilation of the U(1)$_\mathrm{B−L}$ 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 mode for $Z'$ in some parameter regions. To prevent $N$ from being overproduced, we show that the U(1)$_\mathrm{B−L}$ gauge coupling constant $g_{B-L}^{}$ must be as small as $10^{−16}−10^{−10}$. We also consider the case where the decay of $\phi$ 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 FASER, FASER2 and SHiP. We will show the sensitivity of FASER, FASER2, and SHiP. This talk is based on JHEP 05 (2025) 147 and arXiv:2603.28882.
