Lepton flavor violation and scotogenic Majorana neutrino mass in a Stueckelberg U(1)_X model

We construct a scotogenic Majorana neutrino mass model in a gauged U(1)_X extension of the standard model, where the mass of the gauge boson and the unbroken gauge symmetry, which leads to a stable dark matter (DM), can be achieved through the Stueckelberg mechanism. It is found that the simplest version of the extended model consists of the two inert-Higgs doublets and one vector-like singlet fermion. In addition to the Majorana neutrino mass, we study the lepton flavor violation (LFV) processes, such as ℓ_i → ℓ_jγ, ℓ_i → 3ℓ_j, μ − e conversion rate in nucleus, and muonium-antimuonium oscillation. We show that the sensitivities of μ → 3e and μ − e conversion rate designed in Mu3e and COMET/Mu2e experiments make both decays the most severe constraints on the μ → e LFV processes. It is found that τ → μγ and τ → 3μ can reach the designed significance level of Belle II. In addition to explaining the DM relic density, we also show that the DM-nucleon scattering cross section can satisfy the currently experimental limit of DM direct detection.