Indication of a mass-dependent anisotropy above 10^18.7 eV in the hybrid data of the Pierre Auger Observatory

We test the hypothesis of an anisotropy laying along the galactic plane which depends on the mass of primary cosmic-rays. The sensitivity to primary mass is provided by the depth of shower maximum, X_max, from hybrid events measured at the Pierre Auger Observatory. The 14 years of available data are split into on- and off-plane regions using the galactic latitude of each event to form two distributions in X_max, which are compared using the Anderson-Darling 2-samples test. A scan over a subset of the data is used to select an optimal threshold energy of 10^18.7 eV and a galactic latitude splitting at |b| = 30^◦, which are then 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 ¹ ₂ g/cm² shallower mean and a 5.9 ± 2.1⁺ ₋ ³ ₂ ⁵ ₅ g/cm² narrower width than that of the off-plane region. These differences are as such to indicate that the mean mass of primary particles arriving from the on-plane region is greater than that of those coming from the off-plane region. Monte-Carlo studies yield a 4.4 σ post-penalization statistical significance for the independent data. Including the scanned data results in a 4.9⁺ ₋ ¹ ₁ ⁴ ₅ σ post-penalization statistical significance, where the uncertainties are of systematic origin. Accounting for systematic uncertainties leads to an indication for anisotropy in mass composition above 10^18.7 eV at a confidence level of 3.3 σ. The anisotropy is observed independently at each of the four fluorescence telescope sites. Interpretations of possible causes of the observed effect are discussed.