Dark matter and dark force in the type-I inert 2HDM with local U(1) H gauge symmetry

We discuss dark matter (DM) physics in the Type-I inert two-Higgs-doublet model (2HDM) with local U(1) H Higgs gauge symmetry, which is assigned to the extra Higgs doublet in order to avoid the Higgs-mediated flavor problems. In this gauged inert DM setup, a U(1) H -charged scalar Φ is also introduced to break U(1) H spontaneously through its nonzero vacuum expectation value (VEV), 〈Φ〉, and then the remnant discrete subgroup appears according to the U(1) H charge assignment of Φ. The U(1) H -charged Higgs doublet does not have Yukawa couplings with the Standard-Model (SM) fermions, and its lightest neutral scalar component H is stable because of the remnant discrete symmetry. In order to suppress a too large Z -exchange diagram contribution in DM direct detection experiments, we have to introduce a non-renormalizable operator which can be generated by integrating out an extra heavy scalar. With these new particles contents, we first investigate the constraint on the U(1) H gauge interaction, especially through the kinetic and mass mixing between the SM gauge bosons and the extra gauge boson. Then we discuss dark matter physics in our 2HDM: thermal relic density, and direct/indirect detections of dark matter. The additional U(1) H gauge interaction plays a crucial role in reducing the DM thermal relic density. The most important result within the inert DM model with local U(1) H symmetry is that ∼ O (10) GeV dark matter scenario, which is strongly disfavored in the usual Inert Doublet Model (IDM) with Z 2 symmetry, is revived in our model because of newly open channels, HH → Z H Z H ,Z H Z . Exotic Higgs decays, h → Z H Z H ,ZZ H , would be distinctive signatures of the inert 2HDM with local U(1) H symmetry