The East-West effect of the muon charge ratio at energies relevant to the atmospheric neutrino anomaly

The charge ratio of atmospheric muons is rather sensitive to effects of the geomagnetic field and to the hadronic interaction. Experimental information about this quantity is very useful for tuning the ingredients of models used for calculations of atmospheric neutrino fluxes. For experimental observations, and in particular regarding the asymmetry in the charge ratio of muons from the east and West directions (East-West effect) a rotatable device has been installed in NIPNE-HH (National Institute of Physics and Nuclear Engineering -- Horia Hulubei) Bucharest (44 deg. 26'N, 26 deg. 04' E, 85 m above sea-level at a vertical cut-off rigidity of 5.6 GV). The detector is mounted in a rotatable frame, consisting of a stack of 16 modules, 90 x 90 cm2, formed by plastic scintillator layers (3 cm thick) and aluminum support (1.2 cm thick), surrounded by 4 lateral veto counters. The measurements are based on observation of the life time of muons stopped in the absorber layers of the detector. At a mean zenith angle of 35 deg. the East West asymmetry in the ratio has been found to decrease from 0.25 to 0.20, in the momentum range 0.35 - 0.50 GeV/c