The 5 MeV bump - a nuclear whodunit mystery

TL;DR

This study analyzes a large dataset of reactor antineutrino events to investigate the origin of the 5 MeV bump, finding that plutonium isotopes are unlikely the sole source, and introduces a method applicable to future high-statistics experiments.

Contribution

It presents a novel combined analysis method that reduces model and detector uncertainties, providing new insights into the 5 MeV bump's origin in reactor neutrino spectra.

Findings

Plutonium isotopes are disfavored as the sole source of the 5 MeV bump.

The analysis method effectively cancels flux and detector response uncertainties.

Higher statistics data can further clarify the bump's origin and improve sterile neutrino search sensitivity.

Abstract

We perform a combined analysis of recent NEOS and Daya Bay data on the reactor antineutrino spectrum. This analysis includes approximately 1.5 million antineutrino events, which is the largest neutrino event sample analyzed to date. We use a double ratio which cancels flux model dependence and related uncertainties as well as the effects of the detector response model. We find at 3-4 standard deviation significance level, that plutonium-239 and plutonium-241 are disfavored as the single source for the the so-called 5 MeV bump. This analysis method has general applicability and in particular with higher statistics data sets will be able to shed significant light on the issue of the bump. With some caveat this also should allow to improve the sensitivity for sterile neutrino searches in NEOS.