Search for a Heavy Neutrino in the Mass Range under 750 keV Using Electron Capture in 7 Be

We present reanalysis of the experimental data of electron capture in 7 Be embedded in Ta, which have been published by other authors. Our goal is to set upper limits on a mixture of electron neutrino with a possible right-handed heavy neutrino in the 300–750 keV mass range. In the published experiment, a 7 Li recoil energy spectrum in the 20–200 eV range was measured. In case of electron capture with emission of a heavy neutrino, the recoil spectrum should be shifted to the lower energies. We search for an additional Gauss-shaped structure with the same energy width as the main K -shell transition peak. We present the upper limits at a 95