New Experimental Limits on Heavy Neutrino Mixing in [sup 8]B-Decay Obtained with the Borexino Counting Test Facility.

If heavy neutrinos with mass m[sub ν[sub H]] ≥ 2m[sub e] are emitted in the decays of [sup 8]B in the Sun, then ν[sub H] → ν[sub L] + e[sup +] + e[sup –] 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 |U[sub eH]|[sup 2] of a massive neutrino in the range of 1.1 MeV to 12 MeV have been derived (|U[sub eH]|[sup 2] ≤ 10[sup –3]–10[sup –5]). The obtained limits on the mixing parameter are stronger than obtained in previous experiments using nuclear reactors and accelerators. © 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]