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

If heavy neutrinos with mass $m_{{\nu }_H}\ge$2$m_e$ are produced in the Sun via the decay ${}^8\mathrm{B}\to {}^8\mathrm{B}\mathrm{e}+e^{+}+{\nu }_H$ in a side branch of pp-chain, they would undergo the observable decay into an electron, a positron and a light neutrino ${\nu }_H\to {\nu }_L+e^{+}+e^{-}$. 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$14MeVtobe$|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.