If heavy neutrinos with mass $m_{ \nu_{ H}}\geq$2$ m_e $ are produced in the Sun via the decay ${ ^8\rm{ B}} \rightarrow { ^8\rm{ Be}} + e^+ + \nu_H$ in a side branch of pp-chain, they would undergo the observable decay into an electron, a positron and a light neutrino $\nu_{ H}\rightarrow\nu_{ L}+e^++e^-$. In the present work Borexino data are used to set a bound on the existence of such decays. We constrain the mixing of a heavy neutrino with mass 1.5 MeV $\leq m_{ \nu_{ H}} \le$ 14 MeV to be $|U_{ eH}|^2\leq (10^{ -3}-4\times10^{ -6})$ respectively. These are tighter limits on the mixing parameters than obtained in previous experiments at nuclear reactors and accelerators.
Categories: hep-ex hep-ph nucl-ex