Multimuons in cosmic-ray events as seen in ALICE at the LHC

ALICE is a large experiment at the CERN Large Hadron Collider. Located 52 meters underground, its detectors are suitable to measure muons produced by cosmic-ray interactions in the atmosphere. In this paper, the studies of the cosmic muons registered by ALICE during Run 2 (2015--2018) are described. The analysis is limited to multimuon events defined as events with more than four detected muons ($N_\mu>4$) and in the zenith angle range $0^{\circ}<~\theta<~50^{\circ}$. The results are compared with Monte Carlo simulations using three of the main hadronic interaction models describing the air shower development in the atmosphere: QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3. The interval of the primary cosmic-ray energy involved in the measured muon multiplicity distribution is about $ 4 \times 10^{15}<~E_\mathrm{prim}<~ 6 \times 10^{16}$~eV. In this interval none of the three models is able to describe precisely the trend of the composition of cosmic rays as the energy increases. However, QGSJET is found to be the only model capable of reproducing reasonably well the muon multiplicity distribution, assuming a heavy composition of the primary cosmic rays over the whole energy range, while SIBYLL and EPOS-LHC underpredict the number of muons in a large interval of multiplicity by more than $20\%$ and $30\%$, respectively. The rate of high muon multiplicity events ($N_\mu>100$) obtained with QGSJET and SIBYLL is compatible with the data, while EPOS-LHC produces a significantly lower rate ($55\%$ of the measured rate). For both QGSJET and SIBYLL, the rate is close to the data when the composition is assumed to be dominated by heavy elements, an outcome compatible with the average energy $E_\mathrm{prim} \sim 10^{17}$~eV of these events. This result places significant constraints on more exotic production mechanisms.

 

Submitted to: JCAP
e-Print: arXiv:2410.17771 | PDF | inSPIRE
CERN-EP-2024-263
Figure group

Figure 1

TOF trigger efficiency $\varepsilon$ in Run 2 compared to the efficiency in Run 1 as a function of muon multiplicity, $N_\mu$.

Figure 3

Muon multiplicity distribution measured with the ALICE apparatus and obtained for the whole data sample of Run 2 corresponding to a live time of 62.5 days. The data points are grouped in multiplicity intervals with a width of five units($N_\mu=5-9, N_\mu=10-14, ...$), and are located at the center of each interval ($N_\mu=7, N_\mu=12, ...$). The vertical error bars representthe statistical uncertainties.

Figure 4

Measured muon multiplicity distribution compared with simulations from CORSIKA Monte Carlo generator usingQGSJET-II-04  (top), SIBYLL 2.3  (middle), and EPOS-LHC  (bottom) as hadronic interaction models for proton and iron primary cosmic rays. Iron points are slightly shifted to the right to avoid overlapping with the data points. The total uncertainties in the MC simulations are given by the vertical bars, while the boxes give the systematic uncertainties of the data and the vertical bars the statistical ones.

Figure 5

Average energy of the primary as a function of the muon multiplicity for proton and iron MC samples.

Figure 6

Ratio between the number of events obtained with the Monte Carlo simulation (QGSJET-II-04, SIBYLL 2.3, and EPOS-LHC) with respect to the data for the muon multiplicity distributions. The sky blue line represents unity.

Figure 8

Rate of HMM events (days to yield 1 event) for the data taken in Run 1 and Run 2 compared with the rates obtained with MC simulations with proton and iron samples for the three hadronic interaction models used. The green shaded band is the value of the rate for Run 2 with limits given by 1 standard deviation.