Mass Composition of Cosmic Rays with Energies above 10$^{17.2}$ eV from the Hybrid Data of the Pierre Auger Observatory

We present updates on the measurements of the depth of the shower maximum $X_\rm{max}$ and the correlation between $X_\rm{max}$ and the signal in the water-Cherenkov stations of events registered simultaneously by the fluorescence and the surface detectors of the Pierre Auger Observatory. The measurements of $X_\rm{max}$ are performed for $E > 10^{17.2}$ eV using observations of the longitudinal development of air showers by the fluorescence telescopes. The evolution of the mean and the fluctuations of $X_\rm{max}$ with energy, as extracted from the data taken during $2004-2017$, is interpreted in terms of the evolution of the mean logarithmic mass and the spread of the masses in the primary beam using post-LHC hadronic interaction models. The measurements of the correlation between $X_\rm{max}$ and the signal in the surface stations allow one to obtain constraints on the spread of the masses in the primary beam. These constraints are weakly sensitive to the experimental systematic errors and to the uncertainties in the modelling of air showers. Previously, using data taken during $2004-2012$, we excluded with a significance of $5\sigma$ pure compositions and compositions consisting of only protons and helium for energies $10^{18.5}-10^{19.0}$ eV. In the update of the analysis presented here using the data from years $2004-2017$ and nearly doubled statistics, these conclusions are confirmed with a significance $>6.4\sigma$ in the energy range $10^{18.5}-10^{18.7}$ eV alone, while for higher energies, the correlation in data becomes consistent with less mixed compositions.

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