Sterile neutrino decay as a common origin for LSND/MiniBooNe and T2K excess events

Abstract: We point out that the excess of electron-like neutrino events recently observed by the T2K collaboration may have a common origin with the similar excess events previously reported by the LSND and MiniBooNE experiments and interpreted as a signal from the radiative decays of a sterile neutrino ν_h with the mass around 50 MeV produced in muon neutrino neutral current (NC) interactions. In this work we assumed that the ν_h can also be produced in tau neutrino NC reactions.

• The paper demonstrates that heavy sterile neutrino decay (40–80 MeV) can account for the LSND, MiniBooNE, and T2K electron-like excesses through radiative decays producing e+e− pairs.
• It employs detailed simulations of neutrino production and decay within the T2K detector to replicate event rates and kinematic distributions observed experimentally.
• The findings challenge standard oscillation models, suggesting a unified decay mechanism that motivates targeted searches for sterile neutrino signatures in future experiments.

Sterile Neutrino Decay as a Common Origin for LSND/MiniBooNE and T2K Excess Events

Overview

This paper investigates the hypothesis that the observed electron-like excess events in LSND, MiniBooNE, and T2K can be explained through the production and subsequent radiative decay of heavy sterile neutrinos ($\nu_h$) with masses in the range 40–80 MeV. The sterile neutrino is hypothesized to be produced in neutral current interactions involving $\nu_\mu$ and $\nu_\tau$ and to decay dominantly into a photon and a lighter neutrino, with the photon converting into an $e^+e^-$ pair. This radiative decay model is proposed as a common mechanism for anomalous excesses across multiple experiments, challenging the standard oscillation-based interpretations.

Experimental Motivation

The LSND experiment reported a $3.8\,\sigma$ excess of $\overline{\nu}_e$-like events, originally attributed to $\overline{\nu}_\mu \rightarrow \overline{\nu}_e$ oscillations, but subsequent KARMEN and MiniBooNE experiments failed to confirm this in terms of oscillatory behavior. MiniBooNE data, however, exhibited unexplained low-energy electron-like events, both in neutrino and antineutrino modes, consistent with the LSND anomaly in the spectral region. Recently, T2K observed an excess of e-like events in its far detector, historically interpreted within the standard three-flavor oscillation paradigm, notably with a nonzero $\Theta_{13}$ mixing angle.

Heavy Sterile Neutrino Decay Hypothesis

The paper advances the paradigm that a heavy sterile neutrino ($\nu_h$), with properties:

• mass $40 \lesssim m_{\nu_h} \lesssim 80$ MeV,
• mixing parameter $10^{-3} \lesssim |U_{\mu h}|^2 \lesssim 10^{-2}$,
• lifetime $10^{-11} \lesssim \tau_h \lesssim 10^{-9}$ s,

can explain all three excesses observed in LSND, MiniBooNE, and T2K. The current work further extends the production channel to include neutral current interactions involving $\nu_\tau$, relevant in the T2K far detector, where the $\nu_\tau$ flux dominates due to underlying oscillations.

The sterile neutrino is assumed to decay predominantly via $\nu_h \rightarrow \nu \gamma$, facilitated by a possible transition magnetic moment. The radiative decay branching is taken to be unity for the targeted parameter space, evading strong constraints from previous heavy neutrino searches in two-body $\pi$ and $K$ decays and from high-energy beam-dump experiments designed to probe higher mass regions. The model is also consistent with PS191 and LEP bounds, given the rapid decay and exclusive radiative mode.

Model Implementation and Simulation

A quantitative simulation is undertaken for the T2K far detector (Super-Kamiokande), employing the following components:

• Calculation of $\nu_h$ production via neutral current quasi-elastic (NCQE) reactions with nucleons: $\nu_{\mu(\tau)} + N \rightarrow \nu_h + N$.
• Detailed modeling of sterile neutrino propagation and decay within and around the detector, including production in the detector’s fiducial volume, inner and outer detector regions, and surrounding rock.
• Assessment of the detection efficiency for photons converting into $e^+e^-$ pairs, considered indistinguishable from single electron tracks in the SK detector for opening angles below 1 degree.
• Calculation of the expected number and distribution of excess events as a function of $m_{\nu_h}$, $\tau_h$, mixing parameters $|U_{\mu h}|^2$, $|U_{\tau h}|^2$, and the asymmetry parameter $a$ (Dirac/Majorana nature).

Numerically, for T2K exposure of $1.43\times 10^{20}$ protons on target, the model can generate an excess of ~4–6 events, matching the observed $\Delta N=4.5$ electron-like events, with predicted distributions in reconstructed neutrino energy and angular variables consistent with data ($p$-values in simulations range from 0.79–0.92, depending on the $a$ parameter). The model captures both the energy range $200 < E^{QE}_\nu < 1000$ MeV and the wide angular distribution observed.

Constraints and Parameter Space

The required sterile neutrino parameter space:

• mass: $40 \lesssim m_h \lesssim 80$ MeV,
• mixing: $10^{-3} \lesssim |U_{\mu h}|^2 \lesssim 10^{-2}$ (LSND/MiniBooNE), $$10^{-2} \lesssim |U_{\tau h}|^2 \lesssim 3\times 10^{-2}$$ (T2K),
• lifetime: $10^{-10} \lesssim \tau_h \lesssim 10^{-9}$ s,

remains compatible with published laboratory, collider, cosmological, and astrophysical bounds. The short $\tau_h$ avoids BBN and SN1987A limits, and the observed branching ratios in $D_s^+$ decays can be interpreted as further, albeit indirect, support for sterile neutrino mixing with $\nu_\tau$.

Implications and Future Directions

This work demonstrates that sterile neutrino decay presents a viable, unified mechanism for the electron-like event excesses across LSND, MiniBooNE, and T2K. The model provides predictive power regarding both event rates and kinematic distributions, consistent with the observed data. The practical implication is that standard oscillation interpretations, especially those invoking $\Theta_{13}$ mixing, are not uniquely required to explain the observed excesses.

Theoretically, confirmation of such sterile neutrino properties would signify physics beyond the Standard Model, necessitating a revision of neutrino sector paradigms and possibly flavor mixing models involving sizable transition magnetic moments. From an experimental perspective, the results advocate for dedicated searches for signatures of sterile neutrino decay in future high-statistics neutrino experiments, $\mu$ decays, radiative $K$ decays, and with large-scale neutrino telescopes.

Potential future developments include:

• High-statistics runs in T2K and analogous long-baseline experiments to enhance $\nu_e$ event sample sizes and refine distributions.
• Targeted sterile neutrino searches using muon and kaon decays, exploiting the radiative channel and unique kinematics.
• Revisiting cosmological and astrophysical constraints with improved sterile neutrino models, including direct production and decay channels.

Conclusion

The paper elucidates that heavy sterile neutrino decay, particularly $\nu_h \rightarrow \nu\gamma$ with $m_{\nu_h} \sim 50$ MeV, short lifetimes, and moderate mixing with $\nu_\mu$ and $\nu_\tau$, constitutes a robust, consistent explanation for excess electron-like events observed in LSND, MiniBooNE, and T2K. Numerical simulations corroborate the plausibility of this hypothesis within the allowed parameter space. Future experimental work, including increased statistics and specialized decay searches, will be essential for further scrutiny of this mechanism and its impact on the neutrino sector and New Physics searches.