Abstract
ALICE is a general purpose experiment designed to investigate nucleus-nucleus collisions at the CERN Large Hadron Collider (LHC). Located 52 meters underground, with 28 meters of overburden rock, it has also been used to detect the muonic component of the extensive air showers produced by cosmic-ray interactions in the upper atmosphere. A program of cosmic-ray data taking, with specific triggers for atmospheric muons, was started in 2010 in periods when there is no beam circulating in the LHC. Several million events have been recorded to date. The large size and excellent tracking capability of the ALICE Time Projection Chamber are exploited to detect and reconstruct these muons. In this paper the analysis of the multiplicity distribution of the atmospheric muons detected by ALICE between 2010 and 2013 is presented, along with the comparison with Monte Carlo simulations. Special emphasis is given to the study of high multiplicity events containing more than 100 reconstructed muons. The comprehension of the frequency of these events was an unsolved problem since the pioneering studies performed by ALEPH and DELPHI experiments at LEP. In our work the ALICE measurements show that such high multiplicity events demand primary cosmic rays with energy above 1016 eV. Their frequency can be successfully described by assuming a heavy mass composition of primary cosmic rays above this energy and using the most recent interaction models to describe the development of the air shower resulting from the primary interaction.
Highlights
ALICE (A Large Ion Collider Experiment) [1] is a generalpurpose, Heavy-Ion detector at the CERN Large Hadron Collider (LHC)
The underground location of the experiment allowed ALICE to develop a cosmic-ray physics program, exploiting the excellent tracking capabilities of the Time Projection Chamber (TPC) to detect and reconstruct the muons produced in extensive air showers
These experiments concluded that the high multiplicity events occur with a frequency which is almost an order of magnitude above expectation, even when assuming that the primary cosmic rays are purely composed of iron nuclei
Summary
ALICE (A Large Ion Collider Experiment) [1] is a generalpurpose, Heavy-Ion detector at the CERN LHC. All results reported by them were consistent with standard hadronic interaction models, except the observation of high muon multiplicity events [2, 3] These experiments concluded that the high multiplicity events occur with a frequency which is almost an order of magnitude above expectation, even when assuming that the primary cosmic rays are purely composed of iron nuclei. A development of this program is possible at the LHC, where the experiments are expected to operate for many years with the possibility of collecting a very large sample of cosmic-ray data. In this context ALICE began a cosmicray physics program taking data between 2010 and 2013. The analysis of these data and in particular the study of the muon multiplicity distribution, with the purpose of addressing the problem of the rate of high muon multiplicity events, is discussed in this paper
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