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

THE PIERRE AUGER COLLABORATION, INCLUYE S. MOLLERACH

PHYSICS LETTERS B

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 vol. 762

0370-2693

We report a first measurement for ultrahigh energy cosmic rays of thecorrelation between the depth of shower maximum and the signal in thewater Cherenkov stations of air-showers registered simultaneously by thefluorescence and the surface detectors of the Pierre Auger Observatory.Such a correlation measurement is a unique feature of a hybridair-shower observatory with sensitivity to both the electromagnetic andmuonic components. It allows an accurate determination of the spread ofprimary masses in the cosmic-ray flux. Up till now, constraints on thespread of primary masses have been dominated by systematicuncertainties. The present correlation measurement is not affected bysystematics in the measurement of the depth of shower maximum or thesignal in the water Cherenkov stations. The analysis relies on generalcharacteristics of air showers and is thus robust also with respect touncertainties in hadronic event generators. The observed correlation inthe energy range around the 'ankle' at lg ⁡ (E /eV) = 18.5- 19.0differs significantly from expectations for pure primary cosmic-raycompositions. A light composition made up of proton and helium only isequally inconsistent with observations. The data are explained well by amixed composition including nuclei with mass A > 4. Scenarios such asthe proton dip model, with almost pure compositions, are thus disfavoredas the sole explanation of the ultrahigh-energy cosmic-ray flux atEarth.