Experimental and Phenomenological Investigations of the MiniBooNE Anomaly

Abstract

The 4.8σ excess of electron neutrino-like events reported by the MiniBooNE experiment at Fermilab's Booster Neutrino Beam (BNB) is one of the most significant and longest standing anomalies in particle physics. This thesis covers a range of experimental and theoretical efforts to elucidate the origin of the MiniBooNE low energy excess (LEE). We begin with the follow-up MicroBooNE experiment, which took data along the BNB from 2016 to 2021. The detailed images produced by the MicroBooNE liquid argon time projection chamber enable a suite of measurements that each test a different potential source of the MiniBooNE anomaly. This thesis specifically presents MicroBooNE's search for vₑ charged-current quasi-elastic (CCQE) interactions consistent with two-body scattering. The two-body CCQE analysis uses a novel reconstruction process, including a number of deep-learning based algorithms, to isolate a sample of vₑ CCQE interaction candidates with 75% purity. The analysis rules out an entirely vₑ-based explanation of the MiniBooNE excess at the 2.4σ confidence level. We next perform a combined fit of MicroBooNE and MiniBooNE data to the popular 3+1 model; even after the MicroBooNE results, allowed regions in [formula] parameter space exist at the 3σ confidence level. This thesis also demonstrates that, due to nuclear effects in the low-energy cross section behavior, the MicroBooNE data are consistent with a [notation]-based explanation of the MiniBooNE LEE at the <2σ confidence level. Next, we investigate a phenomenological explanation of the MiniBooNE excess involving both an eV-scale sterile neutrino and a dipole-coupled MeV-scale heavy neutral lepton (HNL). It is shown that a 500~MeV HNL can accommodate the energy and angular distributions of the LEE at the 2σ confidence level while avoiding stringent constraints derived from MINERvA elastic scattering data. Finally, we discuss the Coherent CAPTAIN-Mills (CCM) experiment--a 10-ton light-based liquid argon detector at Los Alamos National Laboratory. The background rejection achieved from a novel Cherenkov-based reconstruction algorithm will give CCM world-leading sensitivity to a number of beyond-the-Standard Model physics scenarios, including dipole-coupled HNLs.