OBSERVABLES SENSITIVE TO PRIMARY MASS COMPOSITION AT THE PIERRE AUGER OBSERVATORY

A. G. MARIAZZI FOR THE PIERRE AUGER COLLABORATION

La Thuile, Aosta Valley, Italia

Conferencia; 44th Rencontres de Moriond on Very High Energy Phenomena in the Universe; 2009

The knowledge of the mass composition of the ultra high energy cosmic rays (UHECR) is a crucial point towards the determination of their origin, acceleration and propagation mechanism.A unique feature of the Pierre Auger Observatory is its hybrid nature, where showers are observed simultaneously by fluorescence telescopes and water Cherenkov tanks. The combined use of these two detection techniques provides cross-calibration and a better event reconstruction accuracy. Ideally all mass-sensitive observables should be combined to maximise the discrimination,but the 10 % duty cycle of the fluorescence detector limits the use of direct measurements of shower maximum at the highest energies.Mass-sensitive observables accessible with the surface detectors alone are investigated. These are the signal risetime in the Cherenkov stations, the muon content, and the azimuthal signal risetime asymmetry.The mass sensitivity of these variables is demonstrated and their application  for composition studies is discussed.