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

THE PIERRE AUGER COLLABORATION (P HANSEN PERTENECE A LA COLABORACIÓN AUGER)

PHYSICS LETTERS B

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 vol. 762 p. 288 - 295

0370-2693

We report a first measurement for ultrahighenergy cosmic rays of the correlation between the depth ofshower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneouslyby the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlationmeasurement 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 massesin the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominatedby systematic uncertainties. The present correlation measurement is not affected by systematics in themeasurement of the depth of shower maximum or the signal in the water Cherenkov stations. Theanalysis 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? atlg(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. Thedata are explained well by a mixed composition including nuclei with mass A > 4. Scenarios such asthe proton dip model, with almost pure compositions, are thus disfavoredas the sole explanation of theultrahigh-energy cosmic-ray flux at Earth.