2024 Theses Doctoral

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

Ge, Guanqun

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 𝜈_𝜇 → 𝜈_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 𝜈_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 Δ resonance production followed by Δ radiative decay: Δ → 𝛮_𝛾, and NC coherent single-photon production. Both searches take advantage of boosted decision trees to yield efficient background rejection, and a high-statistic NC ?0 measurement to constrain dominant background, and make use of MicroBooNE’s first three years of data.

The NC Δ → 𝛮_𝛾 measurement yielded a bound on the Δ 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 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 × 10−41cm² at 90% C.L., corresponding to 24.0 times the nominal prediction.