Physical Review D
The magnetized MINOS Near Detector, at a depth of 225 mwe, is used to measure the atmospheric muon charge ratio. The ratio of observed positive to negative atmospheric muon rates, using 301 days of data, is measured to be 1.266 +/- 0.001(stat)(-0.014)(+0.015)(syst). This measurement is consistent with previous results from other shallow underground detectors and is 0.108 +/- 0.019(stat + syst) lower than the measurement at the functionally identical MINOS Far Detector at a depth of 2070 mwe. This increase in charge ratio as a function of depth is consistent with an increase in the fraction of muons arising from kaon decay for increasing muon surface energies.
Adamson, P., Andreopoulos, C., Auty, D. J., Ayres, D. S., Backhouse, C., Barr, G., ... & Bock, G. J. (2011). Measurement of the underground atmospheric muon charge ratio using the MINOS Near Detector. Physical Review D, 83(3), 032011.