Analysis of Moon shadow from 2021-2022 LHAASO WCDA Data

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) is a major ground-based array of cosmic-ray (CR) detectors in the Sichuan Province in China. The Water Cherenkov Detector Array (WCDA) is an essential component of LHAASO, which is designed to measure ~1-50 TeV CRs and gamma rays. As observed on Earth, the Moon blocks some CRs to create a shadow. Although CRs are mostly protons, WCDA can probabilistically identify some constituents (mostly electrons and photons) of CRs which produce electromagnetic (EM) showers in the atmosphere, using the “compactness” (C) of the showers as the criterion for which the higher C indicates the higher probability of an event being EM. The observed CR Moon shadow positions must be shifted from that of the actual Moon due to the deflection of charged particles by the Earth’s magnetic field. We analyze the LHAASO WCDA data from March 2021 to December 2022 to study the properties of the CR Moon shadows for events with C < 10 and C > 10, such as the shifted position and the size of the shadow at different energies. We found that i) CR shadows’ shifts decrease when energy increases, and ii) the higher the energy, the size of the CR shadows are more compact. This analysis is useful for detector calibration, event selection study, and understanding CR trajectories in the geomagnetic field.