We study radiative plateaulike inflation and ${\mathrm{Z}}_{BL}$-portal freeze-in fermionic dark matter in a minimal $B-L$ extended model. The ${\mathrm{U}(1)}_{B-L}$ Higgs, responsible for heavy neutrino masses, also drives inflation in the early Universe, thanks to radiative corrections from the heavy neutrinos and the ${\mathrm{Z}}_{BL}$ gauge boson. In our benchmark choice for the ${\mathrm{U}(1)}_{B-L}$ gauge coupling $g_{B-L}\sim {10}^{-4}$, a light ${\mathrm{Z}}_{BL}$ boson can be explored by current and future lifetime frontier experiments, such as the Forward Search Experiment (FASER) and FASER 2 at the LHC, SHiP, Belle II, and LHCb. For the benchmark, the Hubble scale of inflation (${\mathcal{H}}_{\inf }$) is very low [${\mathcal{H}}_{\inf }=\mathcal{O}(100)\mathrm{eV}$] and the inflaton turns out to be very light with mass of $\mathcal{O}(1)\mathrm{eV}$, and consequently the decay width of the inflaton is extremely small. We investigate a two-field system with the $\mathrm{inflaton}/B-L$ Higgs and the Standard Model (SM) Higgs, and find that the reheating with a sufficiently high temperature occurs when the waterfall direction to the SM Higgs direction opens up in the trajectory of the scalar field evolution.

• Received 15 July 2022
• Accepted 18 October 2022

DOI:https://doi.org/10.1103/PhysRevD.106.095021

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Published by the American Physical Society