Measurement of cosmic-ray reconstruction efficiencies in the MicroBooNE LArTPC using a small external cosmic-ray counter

The MicroBooNE detector is a liquid argon time projection chamberat Fermilab designed to study short-baseline neutrino oscillationsand neutrino-argon interaction cross-section. Due to its location near the surface, a good understanding of cosmic muonsas a source of backgrounds is of fundamental importance for the experiment. We present a method of using an external 0.5 m (L) x 0.5 m (W) muoncounter stack, installed above the main detector, to determine the cosmic-rayreconstruction efficiency in MicroBooNE. Data are acquired with this external muoncounter stack placed in three different positions, corresponding to cosmic raysintersecting different parts of the detector. The data reconstruction efficiency of tracks in the detector is found to be $\epsilon_{\mathrm{data}}=(97.1\pm0.1~(\mathrm{stat}) \pm 1.4~(\mathrm{sys}))\%$, in good agreement with the Monte Carlo reconstruction efficiency $\epsilon_{\mathrm{MC}} = (97.4\pm0.1)\%$. This analysis represents a small-scale demonstration of the method that can be used with future data coming from a recently installed cosmic-raytagger system, which will be able to tag $\approx80\%$ of the cosmic rayspassing through the MicroBooNE detector.