Revisiting Reactor Anti-Neutrino 5 MeV Bump with ¹³C Neutral-Current Interaction

For the first time, we systematically investigate the potential of neutrino-nucleus neutral current interactions with ¹³C to identify the origin of the 5 MeV bump observed in reactor anti-neutrino spectra in the inverse beta decay process. The distinctive signal is obtained from the de-excitation of ¹³C^* into the ground state emitting a 3.685 MeV photon in various liquid scintillator detectors. Such an interaction predominantly occurs for the reactor anti-neutrinos within the energy range coinciding with the 5 MeV bump. For a detector that has a capability of 95% level photon and electron separation and small thorium contamination below 5 × 10⁻¹⁷ gr/gr located in a site with an overburden of about a few hundred m.w.e, such as the location of near detectors of RENO and Daya Bay will have a great sensitivity to resolve the 5 MeV bump. In addition, we propose a novel approach to track the time evolution of reactor isotopes by analyzing our ¹³C signal shedding light on the contributions from ²³⁵U or ²³⁹Pu to the observed bump. This provides an extra powerful tool in both discriminating the flux models and testing any new physics possibilities for the 5 MeV bump at 3σ to 5σ level with much less systematic uncertainties and assuming 10 kt.year of data collection. Our detector requirements are realistic, aligning well with recent studies conducted for existing or forthcoming experiments.