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Volume 358 - 36th International Cosmic Ray Conference (ICRC2019) - CRI - Cosmic Ray Indirect
Mass Composition of Cosmic Rays with Energies above 10$^{17.2}$ eV from the Hybrid Data of the Pierre Auger Observatory
A. Yushkov
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Pre-published on: 2019 July 22
Published on:
Abstract
We present updates on the measurements of the depth of the
shower maximum $X_\mathrm{max}$ and the correlation between $X_\mathrm{max}$ 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 $X_\mathrm{max}$
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 $X_\mathrm{max}$
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 $X_\mathrm{max}$ 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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