Measurement of the W mass at LEP2 from the WW cross-section

In 1996, the LEP e+ e- collider at CERN was operated for the first time above the kinematic threshold for the production of pairs of W bosons, at center-of-mass energies of 161 and 172 GeV. The W mass was determined from the measurement of the W pair cross-section at threshold, yielding Mw = 80.40:!:g:�� GeV for the average of the four LEP experiments. The first direct measurement of W leptonic and hadronic branching ratios was also performed: in the assumption of lepton universality, a value of B(W -4 hadrons) = 67.0 ± 2.0% was obtained.