Mass composition of cosmic rays with energies above 10^17:2 eV from the hybrid data of the Pierre Auger Observatory

S. J. SCIUTTO; THE PIERRE AUGER COLLABORATION.; A. YUSHKOV

Madison

Conferencia; 36th International Cosmic Ray Conference; 2019

We present updates on the measurements of the depth of the shower maximum Xmax and thecorrelation between Xmax and the signal in the water-Cherenkov stations of events registered simultaneouslyby the fluorescence and the surface detectors of the Pierre Auger Observatory. Themeasurements of Xmax are performed for E > 1017:2 eV using observations of the longitudinaldevelopment of air showers by the fluorescence telescopes. The evolution of the mean and thefluctuations of Xmax with energy, as extracted from the data taken during 2004􀀀2017, is interpretedin terms of the evolution of the mean logarithmic mass and the spread of the masses in theprimary beam using post-LHC hadronic interaction models.The measurements of the correlation between Xmax and the signal in the surface stations allowone to obtain constraints on the spread of the masses in the primary beam. These constraints areweakly sensitive to the experimental systematic errors and to the uncertainties in the modellingof air showers. Previously, using data taken during 2004􀀀2012, we excluded with a significanceof 5s pure compositions and compositions consisting of only protons and helium for energies1018:5 􀀀1019:0 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 > 6:4sin the energy range 1018:5 􀀀1018:7 eV alone, while for higher energies, the correlation in databecomes consistent with less mixed compositions.