First Observation of Antiproton Annihilation At Rest on Argon in the LArIAT Experiment

We report the first observation and measurement of antiproton annihilation at rest on argon using the LArIAT experiment. Antiprotons from a charged particle test beam that come to rest inside LArIAT's liquid argon time projection chamber (LArTPC) are identified through beamline instrumentation and LArTPC track reconstruction algorithms. The multiplicity of charged particle tracks originating from the annihilation vertex is manually assessed through hand-scanning, resulting in a distribution with a mean of 3.2 $\pm$ 0.4 tracks and a standard deviation of 1.3 tracks. This is consistent with an automated track reconstruction, which produces a mean of 2.8 $\pm$ 0.4 tracks and a standard deviation of 1.2 tracks. Good agreement is found between data and Monte Carlo simulations for both methods. Additionally, we report the shower multiplicity and particle identification of outgoing tracks, both of which align closely with theoretical predictions. These findings will contribute to modeling of intranuclear annihilation interactions on argon, including scenarios such as neutron-antineutron oscillations.