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

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 10(18.7) eV and a galactic latitude splitting at vertical bar b vertical bar = 30 degrees, 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)(+2.1) g/cm(2) shallower mean and a 5.9 +/- 2.1(-2.5)(+3.5) g/cm(2) 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 x 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 10(18.7) eV with a 3.3 sigma 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 sigma level, which is in good agreement with the expected sensitivity of the dataset used for this test.