Measurement of the Muon Production Depths at the Pierre Auger   Observatory
  The muon content of extensive air showers is an observable sensitive to the primary composition and to the hadronic interaction properties. The Pierre Auger Observatory uses water-Cherenkov detectors to measure particle densities at the ground and therefore is sensitive to the muon content of air showers. We present here a method which allows us to estimate the muon production depths by exploiting the measurement of the muon arrival times at the ground recorded with the Surface Detector of the Pierre Auger Observatory. The analysis is performed in a large range of zenith angles, thanks to the capability of estimating and subtracting the electromagnetic component, and for energies between $10^{19.2}$ and $10^{20}$ eV. 
Laura Collica (for the Pierre Auger Collaboration)
astro-ph.HE hep-ex