Improved Constraints on Sterile Neutrinos in the MeV to GeV Mass Range

Improved upper bounds are presented on the coupling $|U_{e4}|^2$ of an electron to a sterile neutrino $ u_4$ from analyses of data on nuclear and particle decays, including superallowed nuclear beta decays, the ratios $R^{(pi)}_{e/mu}=BR(pi^+ to e^+ u_e)/BR(pi^+ to mu^+ u_mu)$, $R^{(K)}_{e/mu}$, $R^{(D_s)}_{e/tau}$, and $B^+_{e 2}$ decay, covering the mass range from MeV to GeV.