MicroBooNE investigations on the photon interpretation of the MiniBooNE low energy excess

TL;DR

MicroBooNE investigated whether photon-related backgrounds could explain the MiniBooNE low-energy excess, setting new experimental limits on related neutrino interactions and disfavoring photon interpretation as the primary cause.

Contribution

This study provides the first experimental limits on NC coherent single-photon production below 1 GeV and constrains the photon background hypothesis for the MiniBooNE excess.

Findings

Disfavored photon interpretation of MiniBooNE excess

Set world’s first limit on NC coherent single-photon cross-section below 1 GeV

Bound on NC Δ radiative decay rate at 2.3 times the predicted value

Abstract

The MicroBooNE experiment is a liquid argon time projection chamber with 85-ton active volume at Fermilab, operated from 2015 to 2020 to collect neutrino data from Fermilab's Booster Neutrino Beam. One of MicroBooNE's physics goals is to investigate possible explanations of the low-energy excess observed by the MiniBooNE experiment in $\nu_{\mu}\rightarrow \nu_{e}$ neutrino oscillation measurements. MicroBooNE has performed searches to test hypothetical interpretations of the MiniBooNE low-energy excess, including the underestimation of the photon background or instrinic $\nu_{e}$ background. This thesis presents MicroBooNE's searches for two neutral current (NC) single-photon production processes that contribute to the photon background of the MiniBooNE measurement: NC $\Delta$ resonance production followed by $\Delta$ radiative decay: $\Delta \rightarrow N\gamma$, and NC coherent single-photon production. Both searches take advantage of boosted decision trees to yield efficient background rejection, and a high-statistic NC $\pi^0$ measurement to constrain dominant background, and make use of MicroBooNE's first three years of data. The NC $\Delta \rightarrow N\gamma$ measurement yielded a bound on the $\Delta$ radiative decay process at 2.3 times the predicted nominal rate at 90% confidence level (C.L.), disfavoring a candidate photon interpretation of the MiniBooNE low-energy excess as a factor of 3.18 times the nominal NC $\Delta$ radiative decay rate at the 94.8% C.L. The NC coherent single photon measurement leads to the world's first experimental limit on the cross-section of this process below 1 GeV, of $1.49 \times 10^{-41} \text{cm}^2$ at 90% C.L., corresponding to 24.0 times the nominal prediction.