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A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings |U₄₄|^2 and |U₄|^2 for a mainly sterile neutrino mass eigenstate ₄. 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 ₄. The agreement of the ratios of branching ratios R₄/^ () =BR (^+e^+₄) /BR (^+^+_), R₄/^ (K), R₄/^ ({Dₒ) }, R/^ ({Dₒ) }, and R₄/^ (D), and the branching ratios BR (B^+e^+₄) and BR (B^+^+_) decays with predictions of the Standard Model is utilized to derive new constraints on ₄ emission covering the ₄ mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a ₄ via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, decay, leptonic decay, and other experimental inputs.
Bryman et al. (Tue,) studied this question.