Constraints on sterile neutrinos in the MeV to GeV mass range

A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings |Uₑ₄|² and |U_μ4|² for a mainly sterile neutrino mass eigenstate ν₄. Using the excellent agreement among the Ft values (products of kinematic rate factors times half-lives with radiative corrections included) for superallowed nuclear beta decays, an improved upper limit is derived for emission of a ν₄. The agreement of the ratios of branching ratios ${R}_{e/{μ}}^{({π})}=BR({{π}}⁺{→}{e}⁺{{ν}}ₑ)/BR({{π}}⁺{→}{{μ}}⁺{{ν}}_{{μ}})$, ${R}_{e/{μ}}(K)$, ${R}_{e/{τ}}^{({D}ₛ)}$, ${R}_{{μ}/{τ}}^{({D}ₛ)}$, and ${R}_{e/{τ}}(D)$, and the branching ratios $BR({B}⁺{→}{e}⁺{{ν}}ₑ)$ and $BR({B}⁺{→}{{μ}}⁺{{ν}}_{{μ}})$ decays with predictions of the Standard Model is utilized to derive new constraints on ${{ν}}₄$ emission covering the ${{ν}}₄$ mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a ${{ν}}₄$ via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, ${μ}$ decay, leptonic ${τ}$ decay, and other experimental inputs.