[Paper Review] Nuclear and Particle Conspiracy Solves Both Reactor Antineutrino Anomalies

Jeffrey M. Berryman, Vedran Brdar+1 more|arXiv (Cornell University)|Mar 22, 2018

Neutrino Physics Research3 citations

TL;DR

The paper proposes a sterile neutrino model with an Abelian gauge symmetry that induces nonstandard interactions with carbon-13 in scintillator detectors, producing a 4.4 MeV photon and neutron-induced proton recoils that mimic 5 MeV antineutrino events. This mechanism simultaneously explains the reactor antineutrino anomaly and the 5 MeV event excess.

ABSTRACT

We address the 5 MeV event excess in the measured antineutrino spectrum from nuclear reactors in a beyond the Standard model framework, a challenge which has not been met to date. We employ nonstandard neutrino interactions with baryons that can induce the reaction $^{13}$C$(\overline{ u}, \overline{ u}^\prime n)^{12}$C$^*$ in organic scintillator detectors. The de-excitation of $^{12}$C$^*$ 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 such that this process can mimic neutrinos at around 5 MeV energy. We find that the minimal viable model that could induce this reaction involves a sterile neutrino charged under an Abelian symmetry group. Such sterile neutrinos can simultaneously explain the discrepancy between the measured and predicted antineutrino fluxes at short-baseline reactor experiments.

Motivation & Objective

• To resolve the long-standing reactor antineutrino anomaly and the 5 MeV event excess observed in reactor experiments.
• To identify a beyond-Standard-Model mechanism that explains both anomalies within a single theoretical framework.
• To propose a minimal viable model involving sterile neutrinos charged under an Abelian symmetry to mediate nonstandard neutrino interactions with baryons.
• To demonstrate that the resulting signal from $^{13}$C$(\overline{u}, \overline{u}'n)^{12}$C$^*$ de-excitation can mimic 5 MeV antineutrino events in organic scintillators.
• To show that the same mechanism accounts for the discrepancy between predicted and measured antineutrino fluxes at short-baseline reactor experiments.

Proposed method

• Introduce a sterile neutrino charged under an Abelian gauge symmetry to mediate nonstandard neutrino interactions with baryons in scintillator detectors.
• Model the reaction $^{13}$C$(\overline{u}, \overline{u}'n)^{12}$C$^*$, where the excited $^{12}$C$^*$ state decays via a 4.4 MeV gamma emission.
• Include neutron thermalization effects that produce proton recoils, contributing a prompt energy deposit with finite width.
• Calculate the effective cross-section and energy spectrum of the resulting signal to compare with experimental data.
• Assess the compatibility of the model with existing short-baseline reactor antineutrino flux measurements.
• Determine the minimal parameter space of the sterile neutrino model that can simultaneously explain both anomalies.

Experimental results

Research questions

• RQ1 Can a single beyond-Standard-Model mechanism explain both the 5 MeV event excess and the reactor antineutrino flux anomaly?
• RQ2 What type of sterile neutrino interaction can produce a detectable signal mimicking 5 MeV antineutrinos in organic scintillators?
• RQ3 How do nonstandard neutrino interactions with $^{13}$C contribute to a prompt energy deposit with finite width resembling antineutrino events?
• RQ4 What is the minimal viable model involving an Abelian symmetry that can account for both anomalies?
• RQ5 To what extent can this mechanism reconcile predicted versus measured antineutrino fluxes at short-baseline reactor experiments?

Key findings

• The reaction $^{13}$C$(\overline{u}, \overline{u}'n)^{12}$C$^*$ produces a 4.4 MeV gamma decay and neutron-induced proton recoils, generating a prompt signal with finite width.
• The combined energy deposit from the gamma and proton recoil can mimic antineutrino events peaking near 5 MeV.
• A sterile neutrino charged under an Abelian symmetry group provides the minimal viable model to mediate this process.
• The model simultaneously explains the 5 MeV event excess and the discrepancy in measured versus predicted antineutrino fluxes at short-baseline reactors.
• The nonstandard interaction mechanism is consistent with current experimental constraints on antineutrino spectra and fluxes.
• The proposed mechanism offers a unified explanation for two long-standing anomalies in reactor neutrino physics.