Measurement of the mass of the W boson in e(+)e(-) collisions at root s=161 GeV

This letter describes the first observation of W boson pair production at a centre-of-mass energy root s = 161.3 GeV 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 sigma(WW) = 3.62(-0.82)(+0.93) +/- 0.16 pb. An analysis of the predicted M(W) dependence of the accepted cross-section, taking into account interference in the four-fermion production processes, yields M(W) = 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 from the beam energy uncertainty.