Luminous solar neutrinos II: Mass-mixing portals

TL;DR

This paper explores how solar neutrinos can produce heavy neutral leptons in Earth's mantle, leading to detectable signals in large detectors and setting new limits on certain neutrino mixing parameters.

Contribution

It introduces a novel mechanism for solar neutrino upscattering to heavy neutral leptons in Earth's mantle and assesses its detectability and constraints on neutrino mixing.

Findings

Upscattering of solar neutrinos can produce detectable heavy neutral leptons.

New limits on third-generation lepton mixing are established.

Sensitivity to first- and second-generation mixing is limited.

Abstract

Solar neutrinos can be efficiently upscattered to MeV scale heavy neutral leptons (HNLs) within the Earth's mantle. HNLs can then decay to electron-positron pairs leading to energy deposition inside large-volume detectors. In this paper we consider mass-portal upscattering of solar neutrinos to HNLs of mass 20 MeV $\geq m_N \geq 2 m_e$. The large volume of the Earth compensates for the long decay-length of the HNLs leading to observable rates of $N\rightarrow \nu_\alpha e^+e^-$ in large volume detectors. We find that searches for mantle-upscattered HNLs can set the novel limits on mixing with third generation leptons, $|U_{\tau N}|$ for masses in the MeV regime; sensitivity to mixing with first- and second-generation leptons is not competitive with existing search strategies.