Muon bundles from cosmic ray showers detected by DELPHI at LEP

The DELPHI detector located at the LEP accelerator has been used to measure multi-muon bundles originating from cosmic ray interactions. Two sub-detectors, the Hadron Calorimeter and the Time Projection Chamber (TPC), are used for this purpose. The 1999 and 2000 data were analysed over a wide range of muon multiplicities. The muon multiplicity distribution is compared with predictions of Monte-Carlo simulations based on CORSIKA/QGSJET. The Monte-Carlo does not describe the large multiplicity part of data. Even the extreme assumption of cosmic rays composed of pure iron hardly predicts the flux of high multiplicity events. The sensitivity of multiplicity distribution to QGSJET internal parameters is studied.