Abstract
We report a first measurement for ultra-high energy 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 electromagnetic 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 uncertainties in hadronic event generators. The observed correlation in the energy range around the `ankle' at $\lg(E/{\rm eV})=18.5-19.0$ 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 disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray flux at Earth.
Highlights
An important quantity to characterize the composition of cosmic rays is the spread in the range of masses in the primary beam
Observations cannot be reproduced by any pure composition of mass A ≥ 1, irrespective of the interaction model chosen
The observations are naturally explained by a mixed composition including nuclei heavier than helium A > 4, with a spread of masses σ(ln A) 1.35 ± 0.35
Summary
An important quantity to characterize the composition of cosmic rays is the spread in the range of masses in the primary beam. The dip model predicts an almost pure cosmic-ray composition with small spread in primary masses. The distributions of depths of shower maximum Xmax (the atmospheric depth where the number of particles in the air shower reaches a maximum value) observed at the Pierre Auger Observatory were interpreted in terms of primary masses [3] based on current hadronic interaction models. The spread of masses in the primary beam near the ankle, estimated from the moments of the Xmax distributions measured at the Pierre Auger Observatory [8, 9], depends as well on the details of the hadronic interactions and the results include the possibility of a pure mass composition. Observations of Xmax by the Telescope Array in the northern hemisphere were found compatible within uncertainties to both a pure proton composition [10] and to the data from the Auger Observatory [11]
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