Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum

Understand

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.