http://arxiv.org/abs/2303.16336
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, $X_{\rm max}$, 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 $X_{\rm max}$, 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 $X_{\rm max}$ 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 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.
Eric Mayotte
Thu, 30 Mar 23
52/66
Comments: Proceedings of the 6th International Symposium on Ultra High Energy Cosmic Rays (UHECR 2022), Oct 3-7, 2022, Gran Sasso Science Institute, L’Aquila, Italy. Accepted for publications in EPJ Web of Conferences series
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