New experimental limits on heavy neutrino mixing in 8B-decay obtained with the Borexino counting test facility

Abstract

If heavy neutrinos with mass $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} $$ are emitted in the decays of 8B in the Sun, then νH→νL+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 |U eH|2of a massive neutrino in the range of 1.1 MeV to 12 MeV have been derived (|U eH|2≤10−3−10−5). The obtained limits on the mixing parameter are stronger than obtained in previous experiments using nuclear reactors and accelerators.