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

Abstract

If heavy neutrinos with mass $m_{\hat{I}{½}_H}\ge$2$m_e$ are produced in the Sun via the decay ${}^8\mathrm{B}\to {}^8\mathrm{Be}+e^{+}+\hat{I}{½}_H$ in a side branch of pp-chain, they would undergo the observable decay into an electron, a positron and a light neutrino $\hat{I}{½}_H\to \hat{I}{½}_L+e^{+}+e^{-}$. 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 $\le m_{\hat{I}{½}_H}\le$ 14 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.

7 pages, 6 figures