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

We present a novel methodology to search for intranuclear neutron-antineutron transition (n → n̄) followed by n̄-nucleon annihilation within an ⁴⁰Ar nucleus, using the MicroBooNE liquid argon time projection chamber (LArTPC) detector. A discovery of n → n̄ 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 → n̄ events based on their unique features and differentiate them from cosmogenic backgrounds. The achieved signal and background efficiencies are (70.22 ± 6.04)% and (0.0020 ± 0.0003)%, respectively. A demonstration of a search is performed with a data set corresponding to an exposure of 3.32 × 10²⁶ 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 → n̄ lifetime in ⁴⁰Ar of τ_m ≳ 1.1 × 10²⁶ years, and on the free n → n̄ transition time of τ_n→_n̄ ≳ 2.6 × 10⁵ 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.