Investigation of very high energy cosmic rays by means of inclined muon bundles

In a typical approach to extensive air shower (EAS) investigations, horizontal arrays are used and near-vertical EAS are detected. In contrast, in this work vertically arranged muon detectors are used to study inclined EAS. At large zenith angles, EAS consisting solely of muon component are employed. The transverse dimensions of EAS rapidly increase when the zenith angle increases. Hence, EAS in a wide energy interval can be explored by means of a relatively small detector. Here we present results of the analysis of the data on inclined muon bundles accumulated from 2002 to 2016 in the DECOR experiment. For the first time, these results demonstrate with more than 3σ significance the existence of the second knee in the EAS muon component spectrum near 1017 eV primary energy. An excess of muon bundles at energies about 1 EeV found earlier in DECOR data has been confirmed and analyzed in detail. It is highly likely that the obtained outcomes indicate the appearance of new processes of muon generation.