RESUMEN
We describe the measurement of the depth of maximum, X{max}, of the longitudinal development of air showers induced by cosmic rays. Almost 4000 events above 10;{18} eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106{-21}{+35}) g/cm{2}/decade below 10{18.24+/-0.05} eV, and (24+/-3) g/cm{2}/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm{2}. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.
RESUMEN
The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional, variantE;{-gamma}, at energies between 4 x 10;{18} eV and 4 x 10;{19} eV is 2.69+/-0.02(stat)+/-0.06(syst), steepening to 4.2+/-0.4(stat)+/-0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.
RESUMEN
The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from nu(tau) charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of nu(tau) at EeV energies. Assuming an E(nu)(-2) differential energy spectrum the limit set at 90% C.L. is E(nu)(2)dN(nu)(tau)/dE(nu)<1.3 x 10(-7) GeV cm(-2) s(-1) sr(-1) in the energy range 2 x 10(17) eV< E(nu)< 2 x 10(19) eV.