Cosmic ray composition from multiple muon data with the KGF underground detector

The Kolar Gold Field (KGF) experiment was operated for about 6 years at a depth of about 6045 hg/cm${}^2$ using a calorimetric, fine-grain detector and recorded 307 multiple muon events in addition to about 23000 single muons. The mean sea level energy of muons arriving vertically at this depth being around 7 TeV, one has a sensitive probe for the chemical composition of primary cosmic rays at energies around the knee in the energy spectrum. Predictions based on several rigidity dependent composition models are compared with observed multiplicity spectrum. The data strongly support a mixed chemical composition with medium to heavy primaries contributing substantially beyond ${10}^{15}$ eV. The proposed model explains all the features of the data and is consistent with the results from direct measurements as well as the all particle energy spectrum.