Measurement of the angular spectrum of cosmic rays at Belle II

Cosmic muons, produced in atmospheric particle showers initiated by primary cosmic rays, constitute the dominant component of the secondary cosmic radiation at sea level. Due to their high penetration power, they are well suited for detector calibration and alignment studies, while simultaneously representing a relevant background during regular beam operation.

This thesis presents a study of the angular distribution of cosmic muons recorded with the Belle II detector during beam-off operation without magnetic field. The goal is to determine the exponent nnn of the cosmic muon flux parameterized as

\(I(\alpha) = I(0^\circ) \cos^n(\alpha)\)

and to evaluate whether Belle II reproduces the expected angular dependence.

To extract the unfolded flux, several corrections are applied. A solid angle correction accounts for geometrical acceptance effects, while detector inefficiencies and bin migration are treated using an unfolding procedure. Additional correction factors are derived to mitigate reconstruction and trigger effects. The corrected simulation and data distributions are fitted with a cosine power-law function.

The resulting exponents are \(n = 1.90^{+0.19}_{-0.18} \) for simulation and \(n = 1.946^{+0.055}_{-0.054} \)  for data, showing agreement within uncertainties and consistency with literature values. Remaining limitations arise from statistical constraints and incomplete treatment of systematic uncertainties, particularly those related to reconstruction, trigger performance, and environmental effects.

Overall, the study demonstrates that the Belle II detector is capable of reproducing the characteristic angular dependence of the cosmic muon flux and provides a basis for more refined future analyses.