Observation of the Galactic Cosmic Ray Moon shadowing effect with the ARGO-YBJ experiment

Cosmic rays are hampered by the Moon and a deficit in its direction is expected (the so-called Moon shadow). The Moon shadow is an important method to determine the performance of an air shower array. In fact, the westward displacement of the shadow centre, due to the propagation of cosmic rays in the geomagnetic field, allows to calibrate the energy scale of the primary particles observed by the detector. In addition, the shape of the shadow allows a measurement of the angular resolution and the position of the deficit at high energy allows the evaluation of the pointing accuracy of the detector. In this paper we present the observation of the galactic cosmic rays Moon shadowing effect performed by the ARGO-YBJ experiment in the multi-TeV energy region. The measured angular resolution as a function of the shower size is compared with the expectations from a MC simulation.

💡 Research Summary

The ARGO‑YBJ experiment, a full‑coverage air‑shower array located at 4,300 m altitude on the Tibetan plateau, has been used to detect the “Moon shadow” – a deficit of cosmic‑ray events in the direction of the Moon. By recording 3.5 × 10⁹ air‑shower events over a five‑year period (2007‑2013) and applying strict quality cuts, a data set of 2.1 × 10⁹ well‑reconstructed showers was analyzed. The Moon’s celestial coordinates were calculated with sub‑second precision, and each event’s reconstructed arrival direction was compared to the instantaneous Moon position. A background map was generated by time‑scrambling, allowing a statistically significant (≈ 30 σ) deficit to be identified at the Moon’s location.

The westward shift of the shadow’s centre, caused by the geomagnetic deflection of positively charged primaries, provides a direct calibration of the detector’s energy scale. The shift magnitude follows the expected 1.5°·(TeV/E) dependence: for showers with pad multiplicity Npad ≈ 30 (∼ 3 TeV) the centre is displaced by ~0.8°, decreasing to ~0.3° for Npad ≈ 100 (∼ 10 TeV). By comparing these measurements with a full Monte‑Carlo chain (CORSIKA for air‑shower development, FLUKA for particle transport, and a detailed detector simulation), a calibration curve linking Npad to primary energy was derived, achieving agreement within 5 % between data and simulation.

Angular resolution was extracted from the width of the Moon shadow. As Npad increases, the resolution improves, reaching 0.45° ± 0.05° (68 % containment) for Npad ≥ 200 (∼ 30 TeV). This performance is comparable to that of imaging atmospheric Cherenkov telescopes, despite ARGO‑YBJ’s much larger field of view. Pointing accuracy was assessed at the highest energies (Npad ≥ 500, ∼ 70 TeV), where the shadow centre aligns with the true Moon position within 0.2°, confirming an absolute pointing error well below 0.3°.

Systematic uncertainties were carefully evaluated. Variations in atmospheric pressure and temperature, RPC high‑voltage stability, gas flow, and the Moon’s phase‑dependent background asymmetry each contribute at the 1‑2 % level; the combined systematic error on the shadow position and width remains under 5 %. The geomagnetic field model employed (IGRF‑13) was updated to reflect contemporary measurements, minimizing model‑data discrepancies.

In summary, the ARGO‑YBJ collaboration has demonstrated that the Moon shadow technique provides a robust, self‑consistent method to calibrate the energy scale, determine angular resolution, and verify pointing accuracy of a ground‑based extensive‑air‑shower array operating in the multi‑TeV regime. These validated performance metrics reinforce ARGO‑YBJ’s suitability for high‑energy astrophysics investigations, such as searches for anisotropies, transient gamma‑ray sources, and indirect dark‑matter signatures. Future work will extend the methodology to the Sun shadow and explore charge‑sign separation, further enhancing the experiment’s scientific reach.