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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 C-13((nu) over bar,(nu) over bar 'n) C-12((*)) in organic scintillator detectors. The deexcitation of C-12((*)) 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 nonstandard 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 nonminimal particle physics scenarios may be viable, a nuclear physics solution to this anomaly appears more appealing.