New experimental limits on heavy neutrino mixing in B-8-decay obtained with the Borexino Counting Test Facility

If heavy neutrinos with mass m(nu H)greater than or equal to2m(e) are emitted in the decays of B-8 in the Sun, then nu(H)-->nu(L)+e(+)+e(-) decays should be observed. In the present work, the results of background measurements with the Borexino Counting Test Facility have been used to obtain bounds on the number of these decays. As a result, new limits on the coupling textbackslash{}U-eHtextbackslash{}(2) of a massive neutrino in the range of 1.1 MeV to 12 MeV have been derived (textbackslash{}U-eHtextbackslash{}(2)less than or equal to10(-3)-10(-5)). The obtained limits on the mixing parameter are stronger than obtained in previous experiments using nuclear reactors and accelerators. (C) 2003 MAIK ``Nauka/Interperiodica{''}.