A muon-based observable for a photon search at 30-300PeV

Abstract

The observation of an ultra-high energy photon component of the cosmic radiation is one of the open problems in Astroparticle Physics. The stringent theoretical and experimental upper limits to the photon flux above 100 TeV make the search of a weak photon signal in the vast hadronic cosmic ray background a challenging task. At these energies, photon primaries entering the atmosphere develop an extensive air shower which is driven by electromagnetic processes with a poor muon component. The muon content of the air showers is one of the most promising observables that could lead to the best possible discrimination between photons and hadronic cosmic rays. In this article, we define a parameter capable of quantifying the muon component while reducing the fluctuations due to the unknown lateral distribution of muons. We explain the different features of this observable using simulated air showers between 30 and 300 PeV. We show that a merit factor of 5 in the separation between photon and proton primaries and a photon signal efficiency of at least ∼ 92% while rejecting 99.97% of the proton-initiated showers can be reached in the mentioned energy range of interest. This separation power can be achieved provided the shower features, specially the primary energy, are reconstructed sufficiently precise and without significant biases.