Evidence of geomagnetic effect on the azimuthal distribution of EAS in the ARGO-YBJ experiment

The geomagnetic field affects the trajectories of the secondary charged particles of extensive air showers, causing their lateral distribution to be stretched. Thus both the density of the secondaries near the shower axis and the trigger efficiency of detector arrays decrease. The effect depends on the direction of the showers, thus involving the measured azimuthal distribution. This work concerns the non-uniformity of the azimuthal distribution studied by means of ARGO-YBJ data. The modulation is deeply investigated for different zenith angles. The influence of the geomagnetic field and detector effects are studied by means of a Monte Carlo simulation.