Detection of Near Horizontal Muons with the HAWC Observatory

TL;DR

The paper reports the first high-altitude measurement of near horizontal muons using the HAWC observatory, demonstrating its capability to observe multi-TeV muons at large zenith angles and distinguishing them from air showers.

Contribution

It introduces HAWC's novel ability to detect near horizontal muons at high altitude with large separations, expanding muon observation capabilities.

Findings

First high-altitude measurement of near horizontal muons.

Observation of multi-TeV muons at zenith angles >75°.

Measurement of muon angular distribution and rate.

Abstract

The HAWC (High Altitude Water Cherenkov) gamma ray observatory is able to observe muons with nearly horizontal trajectories. HAWC is located at an altitude of 4100 meters a.s.l. on the Sierra Negra volcano in Mexico. The HAWC detector is composed of 300 water tanks, each 7.3 m in diameter and 4.5 m tall, densely packed over a physical area of 22,000 m$^{2}$. Previous and current experiments have observed high zenith angle (near horizontal) muons at or near sea level. HAWC operates as a hodoscope able to observe multi-TeV muons at zenith angles greater than 75 degrees. This is the first experiment to measure near horizontal muons at high altitude and with large ($\geq$ 10 m) separations for multiple muons. These muons are distinguishable from extensive air showers by observing near horizontal particles propagating with the speed of light. The proximity of Sierra Negra and Pico de Orizaba volcanoes provides an additional measurement of muons with rock overburdens of several km water equivalent. We will present the angular distribution and rate at which HAWC observes these muon events