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In this paper, we present a novel methodology to search for intranuclearneutron-antineutron transition ($n\rightarrow\bar{n}$) followed by annihilationwithin an $^{40}$Ar nucleus, using the MicroBooNE liquid argon time projectionchamber (LArTPC) detector. A discovery of $n\rightarrow\bar{n}$ transition orincreased lower limit on the lifetime of this process would either constitutephysics beyond the Standard Model or greatly constrain theories ofbaryogenesis, respectively. The approach presented in this paper makes use ofdeep learning methods to select $n\rightarrow\bar{n}$ events based on theirunique features and differentiate them from cosmogenic backgrounds. Theachieved signal and background efficiencies are (70$\pm$6)\% and(0.0020$\pm$0.0003)\%, respectively. A demonstration of a search is performedwith a data set corresponding to an exposure of $3.32\times10^{26}\,$neutron-years, and where the background rate is constrainedthrough direct measurement, assuming the presence of a negligible signal. Withthis 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 $\tau_{\textrm m} > 1.1\times10^{26}\,$years, and on the free$n\rightarrow\bar{n}$ transition time of $\tau_{\textrm n-\bar{n}} >2.6\times10^{5}\,$s, each at the $90\%$ confidence level. This analysisrepresents a first-ever proof-of-principle demonstration of the ability tosearch for this rare process in LArTPCs with high efficiency and lowbackground.

First demonstration for a LArTPC-based search for intranuclear  neutron-antineutron transitions and annihilation in $^{40}$Ar using the  MicroBooNE detector