  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.

Instrumentation and Methods for Astrophysics;High Energy Astrophysical Phenomena