HEP@VUB meetings

An ultra-high energy cosmic ray that enters the atmosphere of the Earth collides with air molecules to create an extensive air shower, a particle cascade with millions of particles. At the highest energies the center-of-mass energy of the initial collision exceeds LHC energies by an order of magnitude. To determine the mass of the cosmic ray the measured properties of the air shower are compared with model calculations. The parameters in these models are optimized to reproduce the recent LHC data. Since many years a large (factor 2) discrepancy is observed between data and simulation. Recently it was proven by the Pierre Auger Collaboration that the reason for the discrepancy is an under estimate of the number of produced muons. In my presentation I will present this evidence and discuss implications for modeling at ultra-high energies as well as future plans improving the data.