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New limits on heavy sterile neutrino mixing in ${^{8}\rm{B}}$-decay obtained with the Borexino detector

If heavy neutrinos with mass $m_{ν_{H}}\geq$2$ m_e $ are produced in the Sun via the decay ${^8\rm{B}} \rightarrow {^8\rm{Be}} + e^+ + ν_H$ in a side branch of pp-chain, they would undergo the observable decay into an electron, a positron and a light neutrino $ν_{H}\rightarrowν_{L}+e^++e^-$. 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 $\leq m_{ν_{H}} \le$ 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.