Depths of Maximum of Air-Shower Profiles at the Pierre Auger Observatory: Composition Implications

THE PIERRE AUGER COLLABORATION; GARCIA, B (500 AUTHORS IN ALPHABETICAL ORDER)

PHYSICAL REVIEW D

AMER PHYSICAL SOC

Lugar: New York; Año: 2014 vol. 90 p. 1 - 12

1550-7998

Using the data taken at the Pierre Auger Observatory between December 2004 and December 2012, wehave examined the implications of the distributions of depths of atmospheric shower maximum (Xmax),using a hybrid technique, for composition and hadronic interaction models. We do this by fitting thedistributions with predictions from a variety of hadronic interaction models for variations in thecomposition of the primary cosmic rays and examining the quality of the fit. Regardless of whatinteraction model is assumed, we find that our data are not well described by a mix of protons and ironnuclei over most of the energy range. Acceptable fits can be obtained when intermediate masses areincluded, and when this is done consistent results for the proton and iron-nuclei contributions can be foundusing the available models. We observe a strong energy dependence of the resulting proton fractions, andfind no support from any of the models for a significant contribution from iron nuclei. However, we alsoobserve a significant disagreement between the models with respect to the relative contributions of theintermediate components.