Cosmic-ray events as background in imaging atmospheric Cherenkov telescopes

The dominant background for observations of γ-rays in the energy region above 50 GeV with Imaging Atmospheric Cherenkov telescopes are cosmic-ray events. The images of most of the cosmic-ray showers look significantly different from those of γ-rays and are therefore easily discriminated. However, a small fraction of events seems to be indistinguishable from γ-rays. This constitutes an irreducible background to the observation of high-energy γ-ray sources, and limits the sensitivity achievable with a given instrument. Here, a Monte Carlo study of γ-like cosmic-ray events is presented. The nature of γ-like cosmic-ray events, the shower particles that are responsible for the γ-like appearance, and the dependence of these results on the choice of the hadronic interaction model are investigated. Most of the γ-like cosmic-ray events are characterised by the production of high-energy π0’s early in the shower development which dump most of the shower energy into electromagnetic sub-showers. Also Cherenkov light from single muons can mimic γ-rays in close-by pairs of telescopes. Differences of up to 25% in the collection area for γ-like proton showers between QGSJet/FLUKA and Sibyll/FLUKA simulations have been found.