1. 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 = > 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:
[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.