Search for UHE gamma photons in cosmic rays by studying the geomagnetic influence on air-shower muons
A major challenge in ground-based ultra-high-energy gamma-ray observations remains in discriminating sporadic gamma-ray signals from a huge background of cosmic-ray events. To achieve good discrimination power of gamma rays against protons, an unconventional approach is presented that exploits the geomagnetic influence on air shower muons in Monte Carlo shower events. A recognizable polar asymmetry in the positive and negative muon distributions is noticed after transforming the impact coordinates of each muon from the observer plane onto the shower front plane, plus a judicious ignoring of the minimal attenuation of muons in the space between the two planes. A couple of observables, $d_{\text{max}}$ and $N_{μ; \text{IQS}}^{\text{tr.}}$, can be extracted from the formation of two densely populated positive and negative muon zones in each shower. Both observables demonstrate excellent gamma/proton discrimination power, with efficiencies of $93.2\%$ and $98.6\%$. This analysis permitted the usage of muon detectors just covering a small area of $\simeq{1.4}\%$ of a square-km array for ultra-high-energy gamma-ray observations. It is expected that the present and future gamma-ray observatories will implement the current approach.