Cosmic multi-muon events observed in the underground CERN-LEP tunnel with the ALEPH experiment

Multi-muon events have been recorded with the ALEPH-detector, located 140 m underground, in parallel with e+e− data taking. Benefiting from the high spatial resolution of the ALEPH tracking chambers narrowly spaced muons in high multiplicity bundles could be analysed. The bulk of the data can be successfully described by standard production phenomena. The multiplicity distribution favors, though not with very high significance, a chemical composition which changes from light to heavier elements with increasing energy around the “knee”. The five highest multiplicity events, with up to 150 muons within an area of ∼8 m2, occur with a frequency which is almost an order of magnitude above the simulation. To establish a possible effect, more of these events should be recorded with a larger area detector.