Indication of a composition anisotropy at energies above the ankle in the hybrid data of the Pierre Auger Observatory*

A study testing for a cosmic ray primary mass composition anisotropy correlated with the galactic plane of the Milky Way is presented. The depth of airshower maximum, X_max, from 14 years of hybrid data from the Pierre Auger Observatory is used as a mass sensitive parameter. The data are split to form two X_max distributions, the on- and off-plane datasets using the arriving galactic latitude of each event. These are then compared using the Anderson-Darling 2-samples test to generate a Test Statistic describing how much the distributions differ. 

A scan over half the data finds that a lower threshold energy of 10^18.7 eV and an opening angle of 30° in galactic latitude produce the largest difference between the two datasets. These thresholds are set as a prescription for the remaining data and result in the on-plane distribution having a shallower mean and narrower width than the off-plane distribution, indicating that, on average, primary cosmic rays arriving from near-to the galactic plane have a higher mass than those arriving far from it. Monte-Carlo studies penalized to include all systematic effects conservatively estimate a post-trial significance of 3.3σ. Discussions on the possible interpretations and impacts on astrophysical models, if confirmed, will be included.