Speaker
Description
As a hybrid cosmic ray detector, the Pierre Auger Observatory can measure the longitudinal air shower
development with the fluorescence detector, and the lateral distribution of particles reaching the ground
with the surface detector. We report on the measurements of the first two moments of the Xmax
distributions measured as a function of energy with the fluorescence detector and convert them to \<\lnA> and sigma(lnA). This conversion depends on the adopted hadronic interaction model. To obtain
almost model-independent estimation of dispersion of primary masses sigma(lnA) near the ’ankle’ we
use the correlation between Xmax and the signal in the water-Cherenkov stations at 1000 m from the
shower core S(1000). The correlation analysis is robust with respect to uncertainties in hadronic models
and to experimental systematic uncertainties. The observed correlation between Xmax and S(1000)
differs significantly from expectations for pure primary compositions and is well described by a mixed
composition with sigma(lnA) >1.0.
| Presentation type | oral |
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