Antineutrino spectral anomaly within the neutron spectrum

TL;DR

This study investigates the antineutrino spectral anomaly observed in reactor experiments by analyzing neutron spectra and fission yields, revealing a spectral bump around 5-7 MeV and flux effects at 7-8 MeV.

Contribution

It provides a detailed analysis of neutron spectra and their impact on antineutrino spectral discrepancies, highlighting the role of neutron energy distribution in the anomaly.

Findings

A spectral bump appears in the 5-7 MeV range.

Differences between average and thermal antineutrino spectra are minimal.

Neutron flux influences the high-energy antineutrino spectrum at 7-8 MeV.

Abstract

Recently, three antineutrino experiments at Daya Bay, Double Chooz, and RENO measured the neutrino mixing angle $\theta_{13}$, each using a nuclear reactor power plant. However, significant discrepancies were found, both in the absolute flux and spectral shape. In the reactor, the neutrons have a range of energies, with different neutron energies generating different fission yields. The different fission yields may be the reason for the antineutrino spectrum discrepancies. In our study, the neutron spectrum has been analyzed to understand the possible reasons for the discrepancies. In comparing results from the Huber--Muller model, we found that a spectral bump appears in the energy region from 5--7~MeV. Nevertheless, the differences between the average antineutrino spectrum and the only-thermal antineutrino spectrum are small. That is, they are unable to account for this bump. However, in the energy region of 7--8~MeV, the neutron flux induces a decrease in the antineutron flux, and therefore, the distribution of the antineutrino spectrum is affected, especially in the high-energy region.