[Paper Review] First constraints on light sterile neutrino oscillations from combined appearance and disappearance searches with the MicroBooNE detector

MicroBooNE collaboration, P. Abratenko+185 more|arXiv (Cornell University)|Oct 18, 2022

Neutrino Physics Research4 citations

TL;DR

This paper presents the first combined constraints on light sterile neutrino oscillations using both appearance and disappearance channels in the MicroBooNE experiment. By analyzing neutrino events in multiple final states and applying a Feldman-Cousins statistical approach, it rules out significant regions of the $\Delta m^2_{41}$--$\sin^2 2\theta_{\mu e}$ parameter space, showing a $2-3\sigma$ tension with the MiniBooNE anomaly and disfavoring a pure $\nu_e$ excess explanation.

ABSTRACT

We present a search for eV-scale sterile neutrino oscillations in the MicroBooNE liquid argon detector, simultaneously considering all possible appearance and disappearance effects within the $3+1$ active-to-sterile neutrino oscillation framework. We analyze the neutrino candidate events for the recent measurements of charged-current $ν_e$ and $ν_μ$ interactions in the MicroBooNE detector, using data corresponding to an exposure of 6.37$ imes$10$^{20}$ protons on target from the Fermilab booster neutrino beam. We observe no evidence of light sterile neutrino oscillations and derive exclusion contours at the $95\%$ confidence level in the plane of the mass-squared splitting $Δm^2_{41}$ and the sterile neutrino mixing angles $θ_{μe}$ and $θ_{ee}$, excluding part of the parameter space allowed by experimental anomalies. Cancellation of $ν_e$ appearance and $ν_e$ disappearance effects due to the full $3+1$ treatment of the analysis leads to a degeneracy when determining the oscillation parameters, which is discussed in this paper and will be addressed by future analyses.

Motivation & Objective

• To test the $3+1$ sterile neutrino oscillation model using both $\nu_e$ appearance and $\nu_\mu$/$\nu_e$ disappearance signals in the MicroBooNE detector.
• To resolve the tension between the MiniBooNE anomaly and subsequent null results by including full oscillation framework effects.
• To improve sensitivity by combining multiple neutrino interaction channels (CC, NC, $\pi^0$) in a joint fit.
• To assess the compatibility of the MiniBooNE $\nu_e$ excess with MicroBooNE data under the $3+1$ model.
• To provide updated exclusion limits at 95% CL in the $\Delta m^2_{41}$--$\sin^2 2\theta_{\mu e}$ and $\Delta m^2_{41}$--$\sin^2 2\theta_{ee}$ planes.

Proposed method

• A joint likelihood fit is performed on seven neutrino selection channels: fully and partially contained $\nu_e$ CC, $\nu_\mu$ CC, $\nu_\mu$ CC with $\pi^0$, and NC $\pi^0$ events.
• The Feldman-Cousins method is used to compute $\Delta\chi^2$ distributions, ensuring proper statistical coverage for hypothesis testing.
• The analysis includes both $\nu_e$ appearance and $\nu_\mu$/$\nu_e$ disappearance effects within the $3+1$ neutrino oscillation framework.
• Event-weighted neutrino fluxes from BNB and NuMI beams are used to model signal expectations, with ratios of $\sim185$ and $\sim25$ for $\nu_\mu/\nu_e$ respectively.
• Data exclusion contours are computed at 95% CL using the $CL_s$ method, with sensitivity curves derived from Asimov datasets.
• Results are compared with prior experiments (LSND, KARMEN, MiniBooNE, Neutrino-4, gallium anomalies) and reactor-based experiments (STEREO, PROSPECT, etc.).

Experimental results

Research questions

• RQ1 Is the MiniBooNE $\nu_e$ excess consistent with a $3+1$ sterile neutrino oscillation model when both appearance and disappearance effects are included?
• RQ2 What regions of the $\Delta m^2_{41}$--$\sin^2 2\theta_{\mu e}$ parameter space are excluded by MicroBooNE data at 95% CL?
• RQ3 How do the MicroBooNE results compare with the LSND, gallium anomaly, and Neutrino-4 results in the context of sterile neutrino oscillations?
• RQ4 To what extent does the observed deficit in $\nu_e$ CC events disfavor a pure $\nu_e$ appearance explanation of the MiniBooNE anomaly?
• RQ5 What is the significance of the tension between MicroBooNE and MiniBooNE in the $\nu_e$ low-energy excess and sterile neutrino oscillation searches?

Key findings

• MicroBooNE excludes a large portion of the parameter space previously suggested by the MiniBooNE anomaly, particularly in the $\Delta m^2_{41} \sim 0.1-10$ eV$^2$ range.
• The $\nu_e$ appearance-only scenario is disfavored at $2-3\sigma$ when compared to the MiniBooNE $2\sigma$ allowed region, indicating inconsistency in the pure $\nu_e$ excess model.
• The data exclusion contour at 95% CL in the $\Delta m^2_{41}$--$\sin^2 2\theta_{\mu e}$ plane excludes values of $\sin^2 2\theta_{\mu e} \gtrsim 0.02$ for $\Delta m^2_{41} \sim 1$ eV$^2$.
• The $\nu_e$ disappearance-only scenario excludes $\sin^2 2\theta_{ee} \gtrsim 0.01$ for $\Delta m^2_{41} \sim 1$ eV$^2$, consistent with reactor anomaly constraints.
• A $2-3\sigma$ tension is observed between MicroBooNE and MiniBooNE results in both the $\nu_e$ low-energy excess and the sterile neutrino oscillation parameter space.
• The MicroBooNE results are incompatible with the LSND $90\%$ CL allowed region in the $\nu_e$ appearance-only approximation, further challenging the sterile neutrino explanation of the MiniBooNE anomaly.