The Mass Composition of UHECR above 10^17.7 eV Inferred from Pierre Auger Observatory Fluorescence Detector Observations

Since it began operating in 2004, the Pierre Auger Observatory has been leading the field in Ultra-High-Energy Cosmic Ray (UHECR) physics. Phase I of the Observatory has now finished and includes data through December 31^st, 2021. In this contribution, we present the final Phase I results on the depth of maximum of air-shower profiles, X_max, from measurements made with the fluorescence detector for energies above 10^17.7 eV.

First, we present the first two moments of the X_max distributions. We then combine this with predictions from air-showers simulated using different post-LHC hadronic interaction models, to infer the average logarithmic mass of cosmic rays arriving at Earth and its variance. We then present the energy-dependent fractional abundances of four representative primary-mass groups (H, He, CNO, Fe) obtained by fitting the observed X_max distributions with a weighted sum of elemental templates.

This analysis, which benefits from an improved event reconstruction and a 2.5 times larger data set, reinforces our previous findings: above 10^18.4 eV, the proton component gradually disappears giving way to a higher average mass and a decreasing diversity in elemental composition.

Finally, a comparison between southern and northern equatorial bands of the Pierre Auger Observatory fluorescence detector exposure is performed to evaluate the possibility of changes in mass composition as a function of declination.