The cosmic ray muon charge ratio Measurements and calculations at energies relevant for the atmospheric neutrino anomaly

The electromagnetic calorimeter set up WILLI, operated in NIPNE-HH Bucharest, has been modified for measurements of the charge ratio in the atmospheric muon flux. The experimental method is based on the observation of the reduced effective lifetime of negative muons, if stopped in matter, as compared to the lifetime of positive muons. The method which gives accurate results and avoids the difficulties and systematic errors of magnetic spectrometers and the technical procedures are studied in detail and demonstrated with developing a compact and flexible measuring device. Aluminium is used as optimal absorber material, being a compromise of maximising the reduction of the lifetime due to nuclear capture and the rate of stopped muons observed by delayed coincidences with the decay electrons. The present studies are focused to an energy range of the muons which provides important information for the discussion of the so-called atmospheric neutrino problem and for studies of the atmospheric neutrino and antineutrino fluxes. The results obtained by two measuring periods are: R(#mu#"+/#mu#"-) = 1.30 #+-# 0.05 (stat) for muons of 0.86 GeV/c of vertical incidence (mean zenith angle 26 ) and R(#mu#"+/#mu#"-) 1.27 #+-# 0.01 (stat) for muons of 0.52 GeV/c of vertical incidence (mean zenith angle 19 ). For the discussion in context with the information existing in the literature, a comprehensive data basis and survey on the ''world knowledge'' about muon charge ratio spectra are prepared. These results are compared with results of extensive simulation calculations using the Monte Carlo air shower simulation code CORSIKA. Various hadronic interaction models used as generator for the hadron - air interactions are scrutinised. Serious discrepancies with the model GHEISHA, generally used for calculating low energy interactions, are evidenced. Some reasons for that observation are indicated. Predictions of the VENUS and the FLUKA models are presented for an energy range of primary cosmic rays relevant for the low energy atmospheric muon flux. (orig.)SIGLEAvailable from TIB Hannover: ZA 5141(6368) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman