Search for large-scale cosmic-ray anisotropy with the first detection units of KM3NeT/ARCA

Although KM3NeT/ARCA was primarily built to detect high-energy neutrinos from astrophysical sources, it also provides a valuable opportunity to study cosmic rays in the TeV to PeV energy range. This is made possible by the detection of down-going atmospheric muons produced in cosmic-ray air showers, the same particles that usually represent background in neutrino searches. In this study, we focus on identifying large-scale anisotropies in the arrival directions of these cosmic rays, using data from ARCA’s first detection units. To separate true anisotropies signals from instrumental effects, the East-West method, which correctly removes the count variations not associated with real anisotropies, has been applied. The distribution of event counts in 20-minute sidereal time bin is analyzed to search for a dipole component. The amplitude of the first harmonic shows a significance of \( 1.7\,\sigma \), hence, an upper limit on the dipole amplitude at \( 2.0 \times 10^{-3} \) is set at 99% confidence level. The corresponding phase of the dipole is found to be \( -40^\circ \pm 26^\circ \).