The Muon Puzzle in cosmic-ray induced air showers and its connection to the LHC

Hans Dembinski

Observations of cosmic rays with PeV energies and higher rely on indirect measurements of
extensive air showers (EAS) in the Earth’s atmosphere. Inference of the properties of the initial cosmic ray requires precise physical models of the air shower. Evidence for an excess of GeV muons in EAS with respect to model predictions has been reported by many experiments over the last 20 years. This excess was recently confirmed in a meta analysis of measurements from eight air shower experiments for all currently available hadronic interaction models. The excess is referred to as the Muon Puzzle, because no satisfactory explanation exists why hadronic interaction models tuned to LHC data under-predict muon production in air showers. New theoretical insights obtained in the last decade clarified the connection between fundamental properties of hadronic interactions and muon production in air showers, which led to precise projections what measurements are needed at the LHC to shed light on this issue and give theorists the reference data to solve the muon puzzle. This talk discusses the Muon Puzzle, why it is a major obstacle in high-energy cosmic ray research and how it is limiting the precision of diffuse gamma ray and neutrino flux observations. In the second part we discuss what dedicated measurements at the LHC need to be performed to make progress on this front.