The Atmospheric Muon Charge Ratio at the MINOS Near Detector

The magnetized MINOS near detector can accurately determine the charge sign of atmospheric muons, this facilitates a measurement of the atmospheric muon charge ratio. To reduce the systematic error associated with geometric bias and acceptance we have combined equal periods of data obtained with opposite magnetic field polarities. We report a charge ratio of $1.2666\pm0.0015(stat.)^{+0.0096}_{-0.0088}(syst.)$ at a mean E$_{μ,0}cos(θ)$=\unit[63]{GeV}. This measurement is consistent with the world average\cite{Hebbeker}\cite{Haino}\cite{Achard} but significantly lower than the earlier observation at the MINOS far detector\cite{Adamson}. This increase is shown to be consistent with the hypothesis that a greater fraction of the observed muons arise from kaon decay within the cosmic ray shower.