New experimental limits on heavy neutrino mixing in $^{8}$B-decay obtained with the Borexino Counting Test Facility

If heavy neutrinos with mass $m_{\nu_{H}}\geq2 m_e $ are emitted in the decays of $^8$B in the Sun, then decays $\nu_{H}\rightarrow\nu_{L}+e^++e^-$ 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 $|U_{eH}|^2$ of a massive neutrino in the range of 1.1  MeV to 12 MeV have been derived ($|U_{eH}|^2\leq 10^{-3}$–$10^{-5}$). The obtained limits on the mixing parameter are stronger than obtained in previous experiments using nuclear reactors and accelerators.