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

Discussion timeslot (ZOOM-Meeting): 13. July 2021 - 18:00
ZOOM-Meeting URL: https://desy.zoom.us/j/92210078166
ZOOM-Meeting ID: 92210078166
ZOOM-Meeting Passcode: ICRC2021
Corresponding Session: https://icrc2021-venue.desy.de/channel/05-CR-Mass-composition-CRI/73
Live-Stream URL: https://icrc2021-venue.desy.de/livestream/Discussion-03/4

Abstract:
'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, xmax{}, 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 xmax{}, 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 $|b| = 30^circ$, which are then set as a prescription for the remaining data. With these thresholds, the distribution of xmax{} from the on-plane region is found to have a $9.1 pm 1.6^{+2.1}_{-2.2}$,gcm{} shallower mean and a $5.9pm2.1^{+3.5}_{-2.5}$,gcm{} 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 $4.4,sigma$ post-penalization statistical significance for the independent data. Including the scanned data results in a $4.9^{+1.4}_{-1.5},sigma$ 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 $3.3,sigma$. The anisotropy is observed independently at each of the four fluorescence telescope sites. Interpretations of possible causes of the observed effect are discussed.'

Authors: Eric Mayotte | For the Pierre Auger Collaboration
Collaboration: Auger

Indico-ID: 630
Proceeding URL: https://pos.sissa.it/395/321