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rpp2010-rev-w-mass - 1 THE MASS AND WIDTH OF THE W BOSON...

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– 1– THE MASS AND WIDTH OF THE W BOSON Revised March 2010 by M. W. Gr¨unewald (University Ghent), and A. Gurtu (Tata Inst.). The W mass and width definition used here corresponds to a Breit-Wigner with mass-dependent width. Until 1995, the production and study of the W boson was the exclusive domain of the pp colliders at CERN and Fermilab. W production at hadron colliders is tagged by a high p T lepton from W decay. Owing to unknown parton– parton effective energy and missing energy in the longitudinal direction, the experiments reconstruct only the transverse mass of the W , and derive the W mass from comparing the transverse mass distribution with Monte Carlo predictions as a function of M W . These analyses use the electron and muon decay modes of the W boson. Beginning in 1996, the energy of the LEP accelerator in- creased to above 161 GeV, the threshold for W –pair production. A precise knowledge of the e + e - center-of-mass energy enables one to reconstruct the W mass, even if one of them decays leptonically. At LEP two methods have been used to obtain the W mass. In the first method, the measured W –pair production cross sections, σ ( e + e - W + W - ), have been used to deter- mine the W
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