Evidence for a mixed mass composition at the ‘ankle’ in the cosmic-ray spectrum
Résumé
We report a first measurement for ultrahighenergy cosmic rays of the correlation between the depth of
shower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneously
by the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlation
measurement is a unique feature of a hybrid air-shower observatory with sensitivity to both the electro-
magnetic and muonic components. It allows an accurate determination of the spread of primary masses
in the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominated
by systematic uncertainties. The present correlation measurement is not affected by systematics in the
measurement of the depth of shower maximum or the signal in the water Cherenkov stations. The
analysis relies on general characteristics of air showers and is thus robust also with respect to uncer-
tainties in hadronic event generators. The observed correlation in the energy range around the ‘ankle’ at
lg(E/eV) = 18.5–19.0 differs significantly from expectations for pure primary cosmic-ray compositions.
A light composition made up of proton and helium only is equally inconsistent with observations. The
data are explained well by a mixed composition including nuclei with mass A > 4. Scenarios such as
the proton dip model, with almost pure compositions, are thus disfavoredas the sole explanation of the
ultrahigh-energy cosmic-ray flux at Earth