The cosmic ray induced muon spectrum measured with the L3 detector

This thesis presents a measurement of the cosmic ray induced muon spectrum and charge ratio in the energy range from about 40 GeV to 1 TeV. The measurement is performed with the L3+Cosmics detector at CERN, Geneva, on a sample of events from the 1999 data taking. This measurement makes use of the about 200 m^2 of scintillators and DAQ added to the L3 detector during 1998 and the early spring of 1999. The analysis of the muon momentum spectrum and charge ratio is performed in bins of the zenith angle. The vertical flux is measured within a half opening angle of 10 degrees. Potential sources of systematic uncertainties are studied in detail. The normalization of the vertical flux is determined with an uncertainty of 3.7%, while the average vertical charge ratio is determined with an uncertainty of only 1.4%. The uncertainty on the shape of the flux is dominated by the uncertainty on the variation of the overburden below 100 GeV, resulting in a uncertainty of about 3%. Above 100 GeV, where the overburden is less important, the uncertainty on the shape drops to about 1%, slowly rising to about 8% at 1 TeV. This is mainly due to the uncertainty in the chamber alignment. The shape of the charge ratio is at low energies determined up to about 2%. This uncertainty rapidly rises to about 13% at 300 GeV due to uncertainties in the chamber alignment. The uncertainty on the absolute overburden results in a 500 MeV uncertainty on the momentum scale. The measured spectrum above 100 GeV is in good agreement with the prediction from CORT, but disagrees with the world average both in normalization and shape. Below 100 GeV the measured flux is larger than both the CORT prediction and the world average. This could be due to an overestimate of the overburden. The measured charge ratio is in good agreement with the world average, while the prediction from CORT is systematically higher. The obtained accuracy is not high enough to observe the small variation of the charge ratio with momentum predicted by CORT. The zenith angle dependence of the flux and charge ratio is presented in 14 bins in cos(Zenith) from vertical down to about 53 degree. The estimated uncertainty on the flux varies substantially, due to the very different sampling of the detector in different bins. It ranges from about 3% to about 12%. The estimated uncertainty on the average charge ratio only varies between 0.7% and 3%. CORT describes well the measured zenith angle dependence of both the flux and the charge ratio.