First application of a liquid argon time projection chamber for the search for intranuclear neutron-antineutron transitions and annihilation in $^{40}$Ar using the MicroBooNE detector

TL;DR

This paper demonstrates a novel deep learning-based method using the MicroBooNE liquid argon TPC to search for neutron-antineutron transitions within argon nuclei, setting new lower bounds on their lifetimes with high efficiency and minimal background.

Contribution

It introduces the first application of a liquid argon TPC for intranuclear neutron-antineutron transition searches utilizing deep learning for event selection.

Findings

Achieved 70.22% signal efficiency and 0.002% background rate.

Set a lower bound on $n ightarrow\bar{n}$ lifetime in $^{40}$Ar of 1.1×10^{26} years.

Demonstrated the feasibility of high-efficiency, low-background searches for rare processes in LArTPCs.

Abstract

We present a novel methodology to search for intranuclear neutron-antineutron transition ($n\rightarrow\bar{n}$) followed by $\bar{n}$-nucleon annihilation within an $^{40}$Ar nucleus, using the MicroBooNE liquid argon time projection chamber (LArTPC) detector. A discovery of $n\rightarrow\bar{n}$ transition or a new best limit on the lifetime of this process would either constitute physics beyond the Standard Model or greatly constrain theories of baryogenesis, respectively. The approach presented in this paper makes use of deep learning methods to select $n\rightarrow\bar{n}$ events based on their unique features and differentiate them from cosmogenic backgrounds. The achieved signal and background efficiencies are (70.22$\pm$6.04)\% and (0.0020$\pm$0.0003)\%, respectively. A demonstration of a search is performed with a data set corresponding to an exposure of $3.32 \times10^{26}\,$neutron-years, and where the background rate is constrained through direct measurement, assuming the presence of a negligible signal. With this approach, no excess of events over the background prediction is observed, setting a demonstrative lower bound on the $n\rightarrow\bar{n}$ lifetime in $^{40}$Ar of $\tau_{\textrm{m}} \gtrsim 1.1\times10^{26}\,$years, and on the free $n\rightarrow\bar{n}$ transition time of $\tau_{\textrm{\nnbar}} \gtrsim 2.6\times10^{5}\,$s, each at the $90\%$ confidence level. This analysis represents a first-ever proof-of-principle demonstration of the ability to search for this rare process in LArTPCs with high efficiency and low background.