Revisiting reactor antineutrino 5 MeV bump with ¹³C neutral-current interaction

For the first time, we comprehensively examine the potential of a neutral-current interaction of reactor neutrino with ¹³C emitting a 3.685 MeV photon to identify the origin of the 5 MeV bump in reactor antineutrino spectra observed through the inverse beta decay process. This anomaly may be due to new physics, reactor antineutrino flux inaccuracies, or inverse beta decay systematics. The 3.685 MeV photon released during the deexcitation of ¹³C* to its ground state is observable in liquid scintillator detectors. Remarkably, we confirm the powerfulness of our proposal by completely ruling out a new physics scenario explaining the bump from the existing NEOS data. We also explore the potential of current and forthcoming experiments, including solar neutrino studies at JUNO, pion and muon decay-at-rest experiments at OscSNS, and isotope decay-at-rest studies at Yemilab, to measure the cross section precisely enough to distinguish the expected bump and the theoretical flux models via our channel. Additionally, we propose a novel method to track the time evolution of reactor isotopes by analyzing the ¹³C signal, which yields critical insights into the contributions of ²³⁵U and ²³⁹Pu to the bump, acting as a robust tool.