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

H. WAHLBERG Y PIERRE AUGER COLLABORATION

PHYSICS LETTERS B

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016

0370-2693

We report a first measurement for ultra-high energy cosmic rays of the cor-relation between the depth of shower maximum and the signal in the waterCherenkov stations of air-showers registered simultaneously by the fluorescenceand the surface detectors of the Pierre Auger Observatory. Such a correlationmeasurement is a unique feature of a hybrid air-shower observatory with sensi-tivity to both the electromagnetic and muonic components. It allows an accuratedetermination of the spread of primary masses in the cosmic-ray flux. Up tillnow, constraints on the spread of primary masses have been dominated by sys-tematic uncertainties. The present correlation measurement is not affected bysystematics in the measurement of the depth of shower maximum or the signalin the water Cherenkov stations. The analysis relies on general characteristicsof air showers and is thus robust also with respect to uncertainties in hadronicevent generators. The observed correlation in the energy range around the ?an-kle? at lg(E/eV) = 18.5 − 19.0 differs significantly from expectations for pureprimary cosmic-ray compositions. A light composition made up of proton andhelium only is equally inconsistent with observations. The data are explainedwell by a mixed composition including nuclei with mass A > 4. Scenarios suchas the proton dip model, with almost pure compositions, are thus disfavouredas the sole explanation of the ultrahigh-energy cosmic-ray flux at Earth.