Enhanced photon triggers using the underground muon detector of AugerPrime

We present a method for increasing the aperture of the Pierre Auger Observatory for ultra-high-energy photons with energies exceeding a couple dozen PeV. The method involves lowering the trigger thresholds of individual water-Cherenkov detector stations when their partner underground muon detectors measure below a certain number of muons. This increases the detection efficiency for muon-poor photon primaries while mitigating otherwise significant increases in the total rate of event-level triggers due to the steep fall off of the cosmic ray energy spectrum. A detailed evaluation of the method is presented for the 23.5 km2 dense sector of the observatory’s surface detector array, where the spacing between individual 12-ton water-Cherenkov detectors is 750 m, and where each detector position is currently also being equipped with a 30 m2 buried muon counter.