Study of muon bundles from extensive air showers with ALICE detector at the LHC

The ALICE experiment was mainly designed to study a new phase of mat- ter, the Quark-Gluon-Plasma (QGP), created in ultra-relativistic heavy-ion collisions. It is one of the four large experiments at the CERN Large Hadron Collider (LHC), located in a cavern 52 meters underground with 28 meters of overburden rock. This specific underground location and the excellent tracking capability of the ALICE Time Projection Chamber (TPC) have been the cornerstone for a long term program of cosmic ray physics. Between 2010 and 2013, during pauses in collider operations when there was no beam circulating in the LHC, ALICE collected approximately 22.6 million events with at least one reconstructed muon in the TPC, being these muons a component of the extensive air showers (EAS) created by cosmic ray interactions in the upper atmosphere. The total accumulated live time was 30.8 days. In this thesis, the muon multiplicity distribution was measured and com- pared with predictions from modern Monte Carlo models. A special atten- tion was dedicated to the study of high multiplicity events, containing more than 100 reconstructed muons, and corresponding to a muon areal den- sity ρμ > 5.9 ± 0.4 m−2. Similar events were studied in previous under- ground experiments, comprised also of accelerator based detectors, such as ALEPH and DELPHI during the LEP era. While these experiments were able to reproduce the measured muon multiplicity distribution with Monte Carlo simulations at low and intermediate multiplicities, their simulations failed to describe the frequency of the highest multiplicity events. The results of this thesis demonstrated that multi-muon events collected in ALICE are due to primary cosmic rays with mixed composition and energies above 1014 eV, where the average mass of the primary cosmic rays increases at larger energies. Additionally, the high multiplicity events observed in ALICE stem from primary cosmic rays with energies above 1016 eV. Our results successfully described the frequency of these events by assuming a heavy mass composition of primary cosmic rays in this energy range. The development of the resulting air showers was simulated using the latest versions of QGSJET (QGSJET II-03 and QGSJET II-04) to model hadronic interactions. The predictions of QGSJET II-04, whose parame- ters were tuned using the early LHC data, reproduced better the measured rate of the high multiplicity events. The reliability of the ALICE experiment to collect cosmic ray data, and to explain them with Monte Carlo simulations using the latest hadronic interaction models, is confirmed by this study. In particular, for the first time, the rate of high muon multiplicity events has been reproduced using conventional models. This result puts stringent limits on alternative, more exotic, and non-conventional muon production mechanisms.