WIMP-FIMP option and neutrino masses via a novel anomaly-free B-L symmetry

We propose a novel $U(1)_{B-L}$ model with singlet dark matter fermions composed of WIMP and FIMP, which is anomaly-free without a need for introducing right-handed neutrinos. Fermion dark matter masses are generated after the $U(1)_{B-L}$ is broken spontaneously, so the Yukawa couplings for WIMP and FIMP components can be distinguished by the hierarchical values of the vacuum expectation values of the single scalar fields. Moreover, the $U(1)_{B-L}$ gauge boson receives a TeV-scale mass for a tiny extra gauge coupling, so it goes out of equilibrium from the rest of the model content in the early Universe. Both the $U(1)_{B-L}$ gauge boson and FIMP component are produced from the decays of the bath particles, and the former can decay into FIMP DM and/or WIMP DM before BBN. The WIMP component can reside in the resonance region of the Higgs bosons or dominantly annihilate into a pair of singlet-like scalars. Thus, there is a flexibility to choose a small mixing between the visible and dark sectors, thereby evading all the current direct and indirect detection bounds. Furthermore, we show that WIMP and FIMP components can coexist in suitable fractions, depending on the choice of model parameters, allowing for additional protection for WIMP DM against various experimental bounds. Finally, we identify the dimension-6 and dimension-7 operators for Majorana neutrino masses in our model, being consistent with the $U(1)_{B-L}$ gauge symmetry, and provide a possibility of extending the model with additional singlet fermions for neutrino masses.