How to measure the reactor neutrino flux below the inverse beta decay threshold with CE$\nu$NS

TL;DR

This paper proposes using coherent elastic neutrino-nucleus scattering with ultra-low energy detectors to measure reactor neutrino flux below the inverse beta decay threshold, providing new insights into low-energy antineutrinos.

Contribution

It introduces a method to detect sub-1.8 MeV reactor neutrinos using CE$ u$NS with ultra-low thresholds, which was previously unmeasured.

Findings

A regularized unfolding can set an upper bound on low-energy flux.

Detection of neutron capture component remains inconclusive.

Method demonstrates potential for measuring sub-threshold reactor neutrinos.

Abstract

Most antineutrinos produced in a nuclear reactor have energies below the inverse beta decay threshold, and have not yet been detected. We show that a coherent elastic neutrino-nucleus scattering experiment with an ultra-low energy threshold like NUCLEUS can measure the flux of reactor neutrinos below 1.8 MeV. Using a regularized unfolding procedure, we find that a meaningful upper bound can be placed on the low energy flux, but the existence of the neutron capture component cannot be established.