OBSERVATION OF ANISOTROPY IN THE GALACTIC COSMIC-RAY ARRIVAL DIRECTIONS AT 400 TeV WITH ICECUBE

In this paper we report the first observation in the Southern hemisphereof an energy dependence in the Galactic cosmic-ray anisotropyup to a few hundred TeV. This measurement was performed using cosmic-ray-induced muons recorded by the partially deployed IceCube observatory between 2009 May and 2010 May. The data include a total of 33 x 10(9) muon events with a median angular resolutionof similar to 3 degrees. A skymap of the relative intensity in arrival direction over the Southern celestial skyis presented for cosmic-raymedian energies of 20 and 400 TeV. The same large-scale anisotropyobserved at median energies around 20 TeV is not present at 400 TeV. Instead, the high-energy skymap shows a different anisotropystructure including a deficit with a post-trial significance of -6.3 sigma. This anisotropyreveals a new feature of the Galactic cosmic-raydistribution, which must be incorporated into theories of the origin and propagation of cosmic rays.