Reconstruction of Nearly-Horizontal Muons in the HAWC Observatory

Abstract

The volcanoes surrounding the HAWC observatory provide varying material depths from open sky to over 15 km of rock for nearly horizontal muon trajectories. The muon integral intensity as a function of material depth may be determined from the rate of muons as a function of arrival direction near the horizon. A Hough transform algorithm is used to identify nearly-horizontal muons traversing the HAWC observatory by finding a line in the 3d point cloud of PMT hits ($x_{i},y_{i},$ and $ct_{i}$). The arrival direction of the muon can be estimated from this line. Background Extensive Air Shower (EAS) fragments are identified by the presence of a lateral extension of PMT hits in a plane normal to the muon candidate trajectory. A geometry-based simulation has been developed to improve and estimate arrival direction reconstruction resolution and effective area. A description of the reconstruction techniques and estimates of detector resolution, backgrounds, and effective area as a function of arrival direction, will be provided.