32ND I NTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011 Multiple scattering measurement with laser events PEDRO ASSIS 1 FOR THE PIERRE AUGER COLLABORATION 2 1 LIP - Laborat´ orio de Instrumentac ¸˜ ao e F´ ısica Experimental de Part´ ıculas, Av. Elias Garcia, 14-1 ◦ , 1000-149 Lisboa, Portugal 2 Observatorio Pierre Auger, Av. San Mart´ ın Norte 304, 5613 Malarg¨ ue, Argentina (Full author list: http://www.auger.org/archive/authors 2011 05.html) auger spokespersons@fnal.gov 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. Keywords: Laser; Multiple Scattering; Pierre Auger Observatory; Fluorescence detector 1 Introduction The Pierre Auger Observatory [1] in Argentina has 1660 surface detectors in a 3000 km 2 array that is overlooked by 27 fluorescence telescopes at four locations on its periph- ery. The Fluorescence Detector (FD) [2] telescopes mea- sure the shower development in the air by observing the fluorescence light. The FD offers optical shower detection in a calorimetric way and can be calibrated with very little dependence on shower models. The accuracy of the fluorescence technique is determined by the uncertainty in several parameters [3], among them, the fraction of shower light (both from fluorescence and Cherenkov processes) that reaches the detector after being multiply scattered in the atmosphere. The multiple scat- tering (MS) component, which has to be estimated and in- cluded in the reconstruction analysis to correctly derive the cosmic ray properties, depends on the atmospheric condi- tions, in particular on the Rayleigh and Mie scattering pro- cesses. The atmospheric conditions at the Auger site are monitored by several devices [4]. In particular, the observatory is equipped with a set of laser systems that can shoot laser pulses into the atmosphere to be seen by the FD, allow- ing us to measure atmospheric conditions and monitor the performance of the telescopes. One of them is the Central Laser Facility (CLF) [5], a unit placed about 30 km from the FD sites emitting energy calibrated pulses of wave- length λ = 355 nm. In addition, a roving laser system, emitting vertical laser pulses at λ = 337 nm, can be posi- tioned in front of the telescopes at distances of a few km. Using the large amount of laser data and profiting from the negligible width of the beam we have developed a method to extract the transverse distribution of light in the FD cam- eras, from which it is possible to access the MS parameters. Data is compared to a dedicated Geant4 simulation of light propagation in the atmosphere. 2 Extraction of the transverse light profile The emitted laser pulses propagate upwards in the atmo- sphere. The direct light seen by the telescopes results from a first Rayleigh scattering, as illustrated in figure 1. The light from the first scattering can suffer additional scatter- ings and contribute to the signal recorded at large angles with respect to the direct light component. The method employed to extract this transverse light profile from laser data relies on the principle that identical laser events can be averaged to extract information inaccessible on an event- by-event basis. For each acquisition time slot, the recorded image is trans- lated into a distribution of the number of detected photons as a function of the angular distance, ζ , to the direction of the direct light.