Observation of W → μν decays in proton-antiproton collisions at √s = 1.8 TeV

The inclusive spectrum of charged particles resulting from the decays of stopped positively charged kaons, produced in proton-antiproton annihilations at rest, has been measured with good momentum resolution (δp/p = 1.5%) and high statistics (∼300,000 potential decays). The data were obtained with the Experiment PS183 magnetic pair spectrometer at LEAR, the Low Energy Antiproton Ring located at CERN, during two running periods: May 1985 and July-August 1986. Peaks corresponding to the reactions K+ → π+π0 and K+ → μ+νμ have been measured with high precision resulting in a determination of the relative branching ratio K+ → π+π0/K+ → μ+νμ = .3329 ± .0047 (statistical) ± .0017 (systematic). This measurement represents the most precise determination of this ratio to date