Azimuthal modulation of cosmic ray flux as an effect of geomagnetic field in the ARGO-YBJ experiment

The geomagnetic field causes not only the East-West effect on the primary cosmic rays but also affects the trajectories of the secondary charged particles in the shower, causing their lateral distribution to be stretched along certain directions. Thus both the density of the secondaries near the shower axis and the trigger efficiency of a detector array decrease. The effect depends on the age and on the direction of the showers, thus involving the measured azimuthal distribution. Here the non-uniformity of the azimuthal distribution of the reconstructed events in the ARGO-YBJ experiment is deeply investigated for different zenith angles on the light of this effect. The influence of the geomagnetic field as well as geometric effects are studied by means of a Monte Carlo simulation.

💡 Research Summary

The paper investigates how the Earth’s magnetic field influences the azimuthal distribution of extensive air showers (EAS) recorded by the ARGO‑YBJ detector, a full‑coverage array located at 4300 m altitude in Tibet. While the geomagnetic field is well known to produce the East‑West effect on primary cosmic rays, the authors focus on a subtler effect: the Lorentz force acting on the secondary charged particles (electrons and positrons) as they travel a few kilometres through the atmosphere. This force displaces the particles laterally, stretching the shower footprint preferentially along certain directions. Consequently, the particle density near the shower core and the trigger efficiency of the detector become azimuth‑dependent.

The theoretical framework starts from the average lateral shift d = q h² B sin χ / (Eₑ cos² θ) (Eq. 1), where χ is the angle between the magnetic field vector B and the particle velocity, h the average path height, Eₑ the average electron energy and θ the zenith angle. The resulting azimuthal modulation of the event rate can be expressed as N(φ) = N₀