Measurement of the mass of the W boson in e+e− collisions at s=161GeV

This letter describes the first observation of W boson pair production at a centre-of-mass energy s=161GeV in the OPAL detector at LEP. The analysis is sensitive to all expected W+W− decay channels. A total of 28 events have been selected for an integrated luminosity of 9.89±0.06 pb−1. This is consistent with the Standard Model expectation, including signal and background contributions. The W pair production cross-section is measured to be σWW = 3.62−0.82+0.93±0.16 pb. An analysis of the predicted MW dependence of the accepted cross-section, taking into account interference in the four-fermion production processes, yields MW = 80.40−0.41−0.10+0.44+0.09±0.10 GeV, where the first and second uncertainties are statistical and systematic, respectively, and the third arises form the beam energy uncertainty.