On the influence of statistics on the determination of the mean value of Xmax

A. D. SUPANITSKY; G. MEDINA-TANCO

Malargüe

Otro; Reunion de la Colaboración Internacional Pierre Auger; 2011

Colaboración Internacional Pierre Auger

The chemical composition of the high energy cosmic rays is still uncertain, the latest results obtained by The Pierre Auger Observatory and The HiRes Collaboration, concerning the measurement of the mean value and the fluctuations of the atmospheric depth at which the showers reach the maximum development, Xmax, are inconsistent. From the comparison with air shower simulations it can be seen that, while the Auger data show a gradual transition to heavy nuclei for energies larger than 10^18 eV, the HiRes data are consistent with a composition dominated by protons. In Ref. [1] it is suggested that a possible explanation of the observeddeviation of the mean value of X max from the proton expectation, observed by Auger, could be originated by a statistical bias arisen by the approximated exponential shape of the X max distribution, combined with the decrease of the number of events as a function of primary energy. In this paper we show that, considering a better description of the X max distribution, the possible bias in the Auger data is at least one order of magnitude smaller than the one obtained for the exponential distribution. Therefore, we conclude that the deviation of the Auger datafrom the proton expectation can be unlikely explained by such statistical effect.