Search for massive neutrino branches in nuclear beta decays

A search for heavy neutrino branches in nuclear beta decays was performed by a comparative measurement of the β+ and β- spectral shape of the 64Cu decay, using the magnetic spectrometer BILL at the ILL reactor. A massive neutrino branch would have shown up in both spectra at the corresponding energies Esub(β)=Qsub(β) - msub(ν), where Qsub(β) is the beta end point energy for msub(ν)=0. No evidence was found for a massive neutrino branch in the range msub(ν)=30 keV to 500 keV, but stringent upper limits were established, ranging to 4x10-3 relative to the main msub(ν) approximately equal to the main msub(ν)=0 branch. Our results are compared with the various limits from other experiments