Measurement of the atmospheric muon spectrum from 20 to 3000 GeV

The absolute muonflux between 20 GeV and 300 GeV is measured with the L3 magnetic muonspectrometer for zenithangles ranging from 0 degree to 58 degrees. Due to the large exposure of about 150 m2 sr d, and the excellent momentum resolution of the L3 muonchambers, a precision of 2.3% at 150 GeV in the vertical directionis achieved. The ratio of positive to negative muonsis studied between 20 GeV and 500 GeV, and the average vertical muoncharge ratio is found to be 1.285 +- 0.003 (stat.)+- 0.019 (syst.).