Description of Cherenkov light production in extensive air showers

Abstract

The subject of this thesis is description of Cherenkov light production in extensive air showers for calculating analytically the Cherenkov light contribution in light profiles measured by air shower experiments using thefluorescence technique, in particular the Pierre Auger Observatory. The ansatz of the developed analytical Cherenkov model is based on parameterisations of the normalised electron energy spectra in shower age and the angular distribution of produced Cherenkov photons with respect to the shower axis. The former is shown to be universal for showers of energies larger than 10 1 8 eV, and thus neither depend on primary energy nor on primary particle type, and depends only on the shower age. The latter is motivated by the underlying energy dependent electron angular distribution and takes into account the resultant dependence on shower age, and the height dependence implicated by the energy threshold of the Cherenkov effect depending on the refractive index changing in the atmosphere. Compared to Monte Carlo simulations, this ansatz predicts the Cherenkov light production in extensive air showers of very high energies universally at high accuracy of a few percent within shower-to-shower fluctuations. By this new model, a significant and systematic improvement of prediction is achieved compared to the previous approach. Applying this new analytical description for Auger data analysis results in significant and systematic (depending on viewing angle) differences in reconstructed primary parameters. For this study, the whole available fluorescence data set has been reconstructed and the impact due to the new model is studied on an event-by-event basis. Apart from an improvement of systematic uncertainties in event reconstruction, a larger reconstruction efficiency is expected when using the parameterisation of Cherenkov light production introduced in this work.