Experimental and Phenomenological Investigations of the MiniBooNE Anomaly

TL;DR

This thesis investigates the MiniBooNE anomaly through experimental data from MicroBooNE, exploring neutrino interactions, testing models, and proposing new explanations involving heavy neutral leptons, with implications for beyond-Standard Model physics.

Contribution

It presents a novel deep-learning-based analysis of MicroBooNE data, rules out a pure $ u_e$ explanation, and explores phenomenological models involving heavy neutral leptons for the MiniBooNE excess.

Findings

MicroBooNE's $ u_e$ CCQE analysis rules out a $ u_e$-only explanation at 2.4$\sigma$.

Allowed regions in the $3+1$ model persist at 3$\sigma$ after MicroBooNE data.

A 500 MeV heavy neutral lepton can fit the excess at 2$\sigma$ confidence level.

Abstract

This thesis covers a range of experimental and theoretical efforts to elucidate the origin of the $4.8\sigma$ MiniBooNE low energy excess (LEE). We begin with the follow-up MicroBooNE experiment, which took data along the BNB from 2016 to 2021. This thesis specifically presents MicroBooNE's search for $\nu_e$ 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 $\nu_e$ CCQE interaction candidates with $75\%$ purity. The analysis rules out an entirely $\nu_e$-based explanation of the MiniBooNE excess at the $2.4\sigma$ 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 $\Delta m^2$-$\sin^2 2_{\theta_{\mu e}}$ parameter space exist at the $3\sigma$ confidence level. This thesis also demonstrates that the MicroBooNE data are consistent with a $\overline{\nu}_e$-based explanation of the MiniBooNE LEE at the $<2\sigma$ confidence level. Next, we investigate a phenomenological explanation of the MiniBooNE excess combining the $3+1$ model with a dipole-coupled heavy neutral lepton (HNL). It is shown that a 500 MeV HNL can accommodate the energy and angular distributions of the LEE at the $2\sigma$ confidence level while avoiding stringent constraints derived from MINER$\nu$A elastic scattering data. Finally, we discuss the Coherent CAPTAIN-Mills 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 enable world-leading sensitivity to a number of beyond-the-Standard Model physics scenarios, including dipole-coupled HNLs.