Auger-TA energy spectrum working group report

Abstract

The energy spectrum of ultra-high energy cosmic rays is the most emblematic observable for describing these particles. Beyond a few tens of EeV, the Pierre Auger Observatory and the Telescope Array, currently being exploited, provide the largest exposures ever accumulated in the Southern and Northern hemispheres to measure independently a suppression of the intensity, in a complementary way in terms of the coverage of the sky. However, the comparison of the spectra shows differences that are not reducible to an overall uncertainty on the calibration of the energy scale used to reconstruct the extensive air showers. In line with the previous editions of the UHECR workshops, a working group common to both experiments examined these differences by focusing this time on quantification of these differences in the region of the sky commonly observed, where the spectra should be in agreement within uncertainties when directional-exposure effects are taken into account. These differences are compared with the systematic uncertainties of each experiment. We have also revisited the methods of determining cosmic-ray energies and deriving the energy spectrum. We describe the surface detector (SD) spectrum obtained adopting an energy calibration based on the constant intensity cut method (CIC), a Monte Carlo-based attenuation correction, and an energy-dependent CIC attenuation correction.