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

If heavy neutrinos with mass $m_{{\nu }_H}\ge 2m_e$ 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.

Figure 1

Feynman graphs describing the appearance ${}^8\mathrm{B}\to {}^8\mathrm{Be}+e^{+}+{\nu }_H$ (a) and decay ${\nu }_H\to {\nu }_e+e^{+}+e^{-}$ (b),(c) of a heavy neutrino. The invisible decay mode ${\nu }_H\to {\nu }_e{\nu }_i{\tilde{\nu }}_i$ is shown also (d).

Figure 2

Energy spectra of the events surviving incremental selection cuts. From top to bottom: (1) raw spectrum; (2) 2 ms postmuon veto cut; (3) 20 s after muons crossing the SSS cut; (4) FV cut. See text for details.

Figure 3

The expected spectra of signals due to ${\nu }_H\to {\nu }_L+e^{+}+e^{-}$ decay for different neutrino masses $m_{{\nu }_H}=4$, 8, 10, 12 MeV and mixing parameter $|U_{eH}{|}^2=1.0\times {10}^{-5}$.

Figure 4

The Borexino spectrum in the (4.8–12.8) MeV range. 1, optimal fit; 2, $(\nu ,e)$ scattering of ${}^8\mathrm{B}$ neutrino [LMA MSW BPS06(GS98)]; 3, 15 ${}^{11}\mathrm{Be}$ decays; 4, the detector response function for the decay of neutrino with $m_{\nu H}=8\mathrm{MeV}$ and $|U_{eH}{|}^2=8\times {10}^{-6}$. The inset shows the dependence of $L$ on $|U_{eH}{|}^4$ for $m_{\nu H}=8\mathrm{MeV}$.

Figure 5

Limits on the mixing parameter $|U_{eH}{|}^2$ as a function of neutrino mass $m_{\nu }$ (90% C.L.). 1: Present work excludes values of $|U_{eH}{|}^2$ and $m_{\nu }$ inside region 1. 2: Upper limits from reactor experiments on the search for ${\nu }_H\to {\nu }_L+e^{+}+e^{-}$ decay due to $W$-exchange mode [26]. 3: Upper limits from $\pi \to e+\nu$ decay [28].

Figure 6

The Borexino constraints (region 1) and limits on $|U_{eH}{|}^2$ versus $m_{\nu H}$ in the mass range (0.01–100) MeV from different experiments (see text).