Measurement of the composition of cosmic rays and proton-proton interaction cross section at ultrahigh energies with the Pierre Auger Observatory

We present a combined estimate of the cosmic-ray mass composition and particle interaction cross sections from the distributions of shower maximum (Xmax), measured with the fluorescence detector of the Pierre Auger Observatory. For this purpose, we adjust fractions of cosmic-ray mass groups to fit the data with Xmax-distributions from air shower simulations. In addition to the fractions, we fit for the proton-proton cross section at ultrahigh energies, from which simulations with modified nucleus-air cross sections are obtained via Glauber theory. Whereas previous analyses either fit the composition assuming the validity of hadronic interaction models or derived the cross sections assuming a particular composition, the combined fit presented in this contribution is the first self-consistent analysis of both quantities. We will present the resulting energy-dependent composition fractions and the ultrahigh-energy proton-proton interaction cross section and compare them to previous analyses and model extrapolations of low-energy accelerator data.