Sterile Neutrino Search with MicroBooNE’s Electron Neutrino Disappearance Data

A sterile neutrino is a well motivated minimal new physics model that leaves an imprint in neutrino oscillations. Over the last two decades, a number of hints pointing to a sterile neutrino have emerged, many of which are pointing near $m_4\sim 1\mathrm{eV}$. Here, we show how MicroBooNE data can be used to search for electron neutrino disappearance using each of their four analysis channels. We find a hint for oscillations with the highest single channel significance of $2.4\sigma$ (using the Feldman-Cousins approach) coming from the Wire-Cell analysis and a simplified treatment of the experimental systematics. The preferred parameters are ${\sin }^2(2{\theta }_{14})=0.35_{-0.16}^{+0.19}$ and $\mathrm{\Delta }{m}_{41}^2=1.25_{-0.39}^{+0.74}{\mathrm{eV}}^2$. This region of parameter space is in good agreement with existing hints from source experiments, is at a similar frequency but higher mixing than indicated by reactor antineutrinos, and is at the edge of the region allowed by solar neutrino data. Existing unanalyzed data from MicroBooNE could increase the sensitivity to the $>3\sigma$ level.

Figure 1

Top: The disappearance probability in true energy for the best fit set of sterile oscillation parameters, $\mathrm{\Delta }{m}_{41}^2=1.25{\mathrm{eV}}^2$ and ${\sin }^2(2{\theta }_{14})=0.35$, for the Wire-Cell data. Bottom: The expected event rate at MicroBooNE in the Wire-Cell analysis in reconstructed neutrino energy [31], including contributions from backgrounds (red) and ${\nu }_e$ events (green) along with the systematic uncertainty (gray hatched). The actual data is shown in black and the expected data, assuming the best fit sterile hypothesis, is shown in orange.

Figure 2

The preferred regions in $\mathrm{\Delta }{m}_{41}^2\text{−}{\sin }^2(2{\theta }_{14})$ parameter space using data from MicroBooNE’s Wire-Cell analysis [31]. The blue (orange) contours are at $1\sigma$ ($2\sigma$) as determined by Wilks’ theorem; the Feldman-Cousins significance of the best fit compared to no oscillations is $2.4\sigma$ with a simplified treatment of systematics.

Figure 3

The preferred regions (Wilks’) from MicroBooNE’s Wire-Cell analysis [31] as calculated in this Letter (blue), from BEST [7] combined with other gallium data from SAGE [59] and GALLEX [60] (orange), from T2K [5] (red), and a global analysis of modern reactor antineutrino spectral data [4] (green). Additionally, solar (purple) [58] and reactor (light green) neutrinos disfavor large mixing angles.

Figure 4

The projected sensitivity in numbers of standard deviations as a function of POT when calculated with Feldman-Cousins in orange. The orange star shows the results from this analysis indicating that the data is experiencing mild fluctuations relative to the best fit sterile hypothesis.

Figure 5

The disappearance probability for the best fit oscillation parameters along with the primary kinematic range of each of the three detectors in the short baseline neutrino program at Fermilab.