W mass measurement at LEP

In 1998, the four experiments of LEP, i.e. ALEPH, DELPHI, L3 and OPAL, collected data of about 175 /pb per experiment at the center-of-mass energy of 189 GeV. Using these data, the mass of W boson was directly measured by reconstructing the decay products of two W bosons from the e+e- collisions. The W mass measurement was combined personally with the results obtained from data at 161, 172, and 183 GeV. This yielded the private LEP2 average of Mw = 80.350 +/- 0.056 GeV.