Multiple scattering measurement with laser events

Abstract

The Fluorescence Detector of the Pierre Auger Observatory performs a calorimetric measurement of the primary energy of cosmic ray showers. The level of accuracy of this technique is determined by the uncertainty in several parameters, including the fraction of the fluorescence and Cherenkov light reaching the detector after being scattered in the atmosphere through Rayleigh and Mie processes. A new method to measure this multiple scattering is presented. It relies on the analysis of the image of laser tracks observed by the fluorescence telescopes at various distances to characterize the scattering of light and its dependence on the atmospheric conditions. The laser data was systematically compared with a dedicated Geant4 simulation of the laser light propagation, allowing for any number of scatterings due to both Rayleigh and Mie processes, followed by a detailed simulation of the telescopes optics also based on Geant4.