Speaker
Description
We test the hypothesis of an anisotropy in the mass of cosmic-ray primaries as a function of galactic latitude. The mass estimate is made using the depth of shower maximum, $X_{\text{max}}$, 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 $X_{\text{max}}$, 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 an angular split of the data into equally sized on- and off-plane subsamples. Applied to all events, the distribution from the on-plane region is found to have a mean $X_{\text{max}}$ which is $9.3 \pm 1.7^{+2.6}_{-2.2}\,\text{g}\,\text{cm}^{-2}$ shallower and a width which is $6.3\pm2.9^{+3.8}_{-2.8}\,\text{g}\,\text{cm}^{-2}$ narrower than that of the off-plane region. These differences are such as to indicate that the mean mass of the primary particles arriving from the on-plane region is higher than the mean mass of those coming from the off-plane region.
Monte-Carlo studies yield a preliminary $5.0^{+1.4}_{-1.5}\,\sigma$ post-trial statistical significance, where the uncertainties are of systematic origin. Penalizing for systematic uncertainties leads to an indication for anisotropy in mass composition above $10^{18.7}\,$eV at a preliminary confidence level of $3.5\,\sigma$. The anisotropy is observed independently at each of the four fluorescence telescope sites. Interpretations of possible causes of the observed effect will be discussed.
Keywords
UHECR, Mass Composition, Anisotropy, Galactic Plane, Xmax, Hybrid
| Subcategory | Experimental Results |
|---|---|
| Collaboration | Auger |
