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We present a novel methodology to search for intranuclear neutron-antineutrontransition ($n\rightarrow\bar{n}$) followed by $\bar{n}$-nucleon annihilationwithin an $^{40}$Ar nucleus, using the MicroBooNE liquid argon time projectionchamber (LArTPC) detector. A discovery of $n\rightarrow\bar{n}$ transition or anew best limit on the lifetime of this process would either constitute physicsbeyond the Standard Model or greatly constrain theories of baryogenesis,respectively. The approach presented in this paper makes use of deep learningmethods to select $n\rightarrow\bar{n}$ events based on their unique featuresand differentiate them from cosmogenic backgrounds. The achieved signal andbackground efficiencies are (70.22$\pm$6.04)\% and (0.0020$\pm$0.0003)\%,respectively. A demonstration of a search is performed with a data setcorresponding to an exposure of $3.32 \times10^{26}\,$neutron-years, and wherethe background rate is constrained through direct measurement, assuming thepresence of a negligible signal. With this approach, no excess of events overthe background prediction is observed, setting a demonstrative lower bound onthe $n\rightarrow\bar{n}$ lifetime in $^{40}$Ar of $\tau_{\textrm{m}} \gtrsim1.1\times10^{26}\,$years, and on the free $n\rightarrow\bar{n}$ transition timeof $\tau_{\textrm{\nnbar}} \gtrsim 2.6\times10^{5}\,$s, each at the $90\%$confidence level. This analysis represents a first-ever proof-of-principledemonstration of the ability to search for this rare process in LArTPCs withhigh efficiency and low background.

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