Speaker
Description
We test for a large-scale anisotropy in the mass of arriving cosmic-ray primaries as a function of galactic latitude. The sensitivity to primary mass is obtained through the depth of shower maximum, $X_{\text{max}}$, 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 $X_{\text{max}}$ 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 $|b| = 30^\circ$, which are set as a prescription for the remaining data. With these thresholds, the distribution of $X_{\text{max}}$ from the on-plane region is found to have a $9.1 \pm 1.6^{+2.1}_{-2.2}\,$g/cm$^2$ shallower mean and a $5.9\pm2.1^{+3.5}_{-2.5}\,$g/cm$^2$ narrower width than that of the off-plane region and is observed in all telescope sites independently. These differences 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 $5.9\times10^{-6}$ random chance probability for the result in the independent data, lowering to a $6.0\times10^{-7}$ 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 independent FD data recovered from the quality selection process. This test disfavors the null hypothesis of the on- and off-plane regions being uniform in composition at $2.2\,\sigma$ which is in good agreement with the expected sensitivity of the dataset used for this test. Possible interpretations, accompanying results and plans for further tests will be presented.
