Neutrino Portals, Terrestrial Upscattering, and Atmospheric Neutrinos

TL;DR

This paper investigates how atmospheric neutrinos can produce heavy neutral leptons inside the Earth, leading to detectable signals in large volume detectors, and sets new constraints on their properties.

Contribution

It introduces a novel method to constrain heavy neutral leptons via terrestrial upscattering of atmospheric neutrinos using existing detector data.

Findings

New constraints on dipole couplings for HNLs between 10-100 MeV.

Probing parameter space near 20 MeV HNL mass with current data.

Future experiments can significantly improve these constraints.

Abstract

We consider the upscattering of atmospheric neutrinos in the interior of the Earth producing heavy neutral leptons (HNLs) which subsequently decay inside large volume detectors (e.g. Super-Kamiokande or DUNE). We compute the flux of upscattered HNLs arriving at a detector, and the resultant event rate of visible decay products. Using Super-Kamiokande's atmospheric neutrino dataset we find new leading constraints for dipole couplings to any flavor with HNL masses between roughly 10 MeV and 100 MeV. For mass mixing with tau neutrinos, we probe new parameter space near HNL masses of $\sim 20$ MeV with prospects for substantial future improvements. We also discuss prospects at future experiments such as DUNE, JUNO, and Hyper-Kamiokande.