If heavy neutrinos with mass $m_{{\nu }_H}\ge 2m_e$ are produced in the Sun via the decay ${}^8\mathrm{B}\to {}^8\mathrm{Be}+e^{+}+{\nu }_H$ in a side branch of the $pp$ 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 $1.5\mathrm{MeV}\le m_{{\nu }_H}\le 14\mathrm{MeV}$ to be $|U_{eH}{|}^2\le ({10}^{-3}-4\times {10}^{-6})$, respectively. These are tighter limits on the mixing parameters than obtained in previous experiments at nuclear reactors and accelerators.

• Received 27 May 2013

DOI:https://doi.org/10.1103/PhysRevD.88.072010