New limits on heavy sterile neutrino mixing in B8 decay obtained with the Borexino detector

If heavy neutrinos with mass m_νH≥2m_e are produced in the Sun via the decay B8→Be8+e⁺+ν_H in a side branch of the pp chain, they would undergo the observable decay into an electron, a positron and a light neutrino ν_H→ν_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 1.5MeV≤m_νH≤14MeV to be |U_eH|²≤(10^-3-4×10^-6), respectively. These are tighter limits on the mixing parameters than obtained in previous experiments at nuclear reactors and accelerators.