Muons as a tool for background rejection in imaging atmospheric Cherenkov telescope arrays

L Olivera-Nieto,K Bernlöhr,J A Hinton,A M W Mitchell

doi:10.1140/epjc/s10052-021-09869-0

Abstract

The presence of muons in air-showers initiated by cosmic ray protons and nuclei is well established as a powerful tool to separate such showers from those initiated by gamma rays. However, so far this approach has been fully exploited only for ground level particle detecting arrays. We explore the feasibility of using Cherenkov light from muons as a background rejection tool for imaging atmospheric Cherenkov telescope arrays at the highest energies. We adopt an analytical model of the Cherenkov light from individual muons to allow rapid simulation of a large number of showers in a hybrid mode. This allows us to explore the very high background rejection power regime at acceptable cost in terms of computing time. We show that for very large (gtrsim 20 m mirror diameter) telescopes, efficient identification of muon light can potentially lead to background rejection levels up to 10^{-5} whilst retaining high efficiency for gamma rays. While many challenges remain in the effective exploitation of the muon Cherenkov light in the data analysis for imaging Cherenkov telescope arrays, our study indicates that for arrays containing at least one large telescope, this is a very worthwhile endeavor.

Highlights

The hadronic cascades associated to charged cosmic ray primary particles typically produce large numbers of muons, primarily from the decay of charged pions
The presence of muons in air-showers initiated by cosmic ray protons and nuclei is well established as a powerful tool to separate such showers from those initiated by gamma rays
We explore the feasibility of using Cherenkov light from muons as a background rejection tool for imaging atmospheric Cherenkov telescope arrays at the highest energies

Summary

Introduction

The hadronic cascades associated to charged cosmic ray primary particles typically produce large numbers of muons, primarily from the decay of charged pions. The current background rejection power attained by the traditional separation methods at energies above a few tens of TeVs reaches levels between 10−2 and 10−3 [6,7,9]. [13] and the Large-Sized Telescopes (LSTs) of CTA [14] enable the detection of individual muons out to large impact distance This has traditionally been seen as a problem due to their apparent similarity to gamma-like events [15], but can be seen as an opportunity for improvement of the background rejection power at the highest energies if characteristic differences of muons to gamma rays are identifiable. In this paper we explore the potential for muon measurement with IACTs as a tool for background rejection by characterizing the number of muons that are detectable by a large Cherenkov telescope in proton- and gamma-initiated showers of different energies.

Cherenkov light from muons

Detectable muons in showers

Results

Smaller telescopes

Discussion