Lateral distribution of cosmic ray muons underground: Results from the CosmoALEPH experiment

The CosmoALEPH experiment, located underground at the LEP e + e − storage ring at CERN at a depth of 320 m water equivalent, was used to study the chemical composition of primary cosmic rays up to 10 PeV e nergies from the measurement of high energy muons, created in extensive a ir showers by interactions of primary nuclei in the atmosphere. The Time Projection Chamber (TPC) and the Hadron Calorimete r of the ALEPH detector and six scintillator stations located at dis tances up to 1 km from each other were used to analyse the decoherence curve, m ultiplicity and transverse momentum distributions of energetic cosmic muo ns. The experimental data were compared with predictions from d ifferent Monte Carlo (MC) models and mass composition approaches. From a comparison between the measured decoherence distrib ution with CosmoALEPH and the MC predicted decoherence curves for prot on, helium and iron, a primary composition of (77 ± 11) % protons and (23 ± 11) % iron nuclei with a χ 2 - probability of 84 % was determined, based on the predic- tions of the VENUS model with the constant mass composition a pproach. The analysis of the decoherence curve, with consideration o f correlations be- tween the measured CosmoALEPH parameters, leads to a compos ition of (88 ± 8) % protons and (12 ± 8) % iron nuclei for cosmic rays with a χ 2 - probability of 53 %. The absolute comparison between the measured multiplicity and trans- verse momentum distributions in the TPC and those predicted by different Monte Carlo models results also in a dominant light composit ion. The ex- perimental data are in a good agreement with MC data lying bet ween proton and helium primaries. The results obtained for the primary composition of cosmic r ays up to the knee region are consistent with the results from other experimen ts.\