Update on the indication of a mass-dependent anisotropy above 1018.7 eV in the hybrid data of the Pierre Auger Observatory

Abstract

We test for an anisotropy in the mass of arriving cosmic-ray primaries associated with the galactic plane. The sensitivity to primary mass is obtained through the depth of shower maximum, X max, extracted from hybrid events measured over a 14-year period at the Pierre Auger Observatory. The sky is split into distinct on-and off-plane regions using the galactic latitude of each arriving cosmic ray to form two distributions of X max, which are compared using an Anderson-Darling 2-samples test. A scan over roughly half of the data is used to select a lower threshold energy of 1018.7 eV and a galactic latitude splitting at |b| = 30°, which are set as a prescription for the remaining data. With these thresholds, the distribution of X max from the on-plane region is found to have a 9.1±1.6−2.2+2.1g/cm2 shallower mean and a 5.9±2.1−2.5+3.5g/cm2 narrower width than that of the off-plane region and is observed in all telescope sites independently. These di↵erences indicate that the mean mass of primary particles arriving from the on-plane region is greater than that of those from the off-plane region. Monte Carlo studies yield a 5.9 ×10−6 random chance probability for the result in the independent data, lowering to a 6.0 × 10−7 post-penalization random chance probability when the scanned data is included. Accounting for systematic uncertainties leads to an indication for anisotropy in mass composition above 1018.7 eV with a 3.3 σ-significance. Furthermore, the result has been newly tested using additional FD data recovered from the selection process. This test independently disfavors the on-and off-plane regions being uniform in composition at the 2.2 σ-level, which is in good agreement with the expected sensitivity of the dataset used for this test.