0755 Azimuthal modulation of cosmic ray flux as an effect of geomagnetic field in the ARGO-YBJ experiment P. Bernardini1, A. D'Amone1, H.H. He2, G. Mancarella1, L. Perrone3, A. Surdo4, on behalf of the ARGO-YBJ Collaboration 1 - INFN and Dipartimento di Fisica dell’Università del Salento, Lecce, Italy 2 - Key Laboratory of Particle Astrophysics, Institute of High Energy Physics (CAS), Beijing, P.R. China 3 - INFN and Dipart. di Ingegneria dell’Innovazione dell’Università del Salento, Lecce, Italy 4 - INFN, Lecce, Italy 0755 Fig. 3 – Coefficient g1 versus zenith angle. Function g1=k1sin 2q is used for the superimposed fit. The geomagnetic field enlarges the Extensive Air Showers (EAS) in the East-West direction[1]. Therefore the particle density is reduced and also the trigger efficiency is lower. The effect depends on the sine of the angle (c) between magnetic field and particle momentum (Fig. 1). As a consequence an azimuthal modulation is expected[2] in the EAS flux where f1 = f2 = -fB ( fB=71.89° is the magnetic field azimuth in the ARGO-YBJ reference frame). In addition g1 µ sin 2q and g2 µ sin2 q (q is zenith). The fit of the azimuthal distribution (Fig. 2) and the dependence of g1 on q (Fig. 3) confirm what expected for the first harmonic. Fig. 1 - Absolute value of sin c versus local angular coordinates (q and f) of the arrival direction of a particle in the ARGO-YBJ reference frame. c is the angle between the magnetic field and the particle momentum. The fit results for the second harmonic do not agree with the geomagnetic effect. A possible explanation is a combined detector effect due to the difference of the detector density along x and y-axes. Therefore the second harmonic can be split in a geomagnetic (2B) and a detector component (2A) with phase -90°. The azimuthal distributions of 3 subsamples (a for q < 20°, b for 20° < q < 40°, g for 40° < q < 60°) can be fitted all together by means of a single function (i=a,b,g) The new fit is better than the previous one (c2/ndf = 3.95 -> 2.34) and the parameters are what expected: Conclusion – The geomagnetic effect on EAS has been verified together with the excellent angular reconstruction of the detector (expected fB=71.89°, measured fB = 71.86° ± 0.44°). [1] G. Cocconi, Phys. Review, 1954, 93:646-647 [2] H.H. He et al. Proc. of 29th ICRC, 2005, Pune Fig. 2 – Azimuthal distribution of EAS sample. Fit with double harmonic is superimposed.