Particle physics origin of the 5 MeV bump in the reactor antineutrino spectrum?

TL;DR

This paper investigates the 5 MeV excess in reactor antineutrino spectra, proposing a nuclear physics explanation involving carbon de-excitation, and evaluates non-standard neutrino interactions as alternative causes.

Contribution

It introduces a nuclear physics mechanism involving $^{13}$C reactions in scintillators to explain the 5 MeV bump, challenging particle physics explanations.

Findings

Nuclear de-excitation can mimic the 5 MeV signal.

Non-standard neutrino interactions are disfavored by existing measurements.

A nuclear physics solution appears more plausible than particle physics scenarios.

Abstract

One of the most puzzling questions in neutrino physics is the origin of the excess at 5 MeV in the reactor antineutrino spectrum. In this paper, we explore the excess via the reaction $^{13}$C$(\overline{\nu}, \overline{\nu}^\prime n)^{12}$C$^*$ in organic scintillator detectors. The de-excitation of $^{12}$C$^*$ yields a prompt $4.4$ MeV photon, while the thermalization of the product neutron causes proton recoils, which in turn yield an additional prompt energy contribution with finite width. Together, these effects can mimic an inverse beta decay event with around 5 MeV energy. We consider several non-standard neutrino interactions to produce such a process and find that the parameter space preferred by Daya Bay is disfavored by measurements of neutrino-induced deuteron disintegration and coherent elastic neutrino-nucleus scattering. While non-minimal particle physics scenarios may be viable, a nuclear physics solution to this anomaly appears more appealing.