Barate, R (reprint author), IN2P3, CNRS, LAPP, PHYS PARTICULES LAB, F-74019 ANNECY LE VIEUX, FRANCE.
In June 1996, the LEP centre-of-mass energy was raised to 161 GeV. Pair production of W bosons in e(+)e(-) collisions was observed for the first time by the LEP experiments. An integrated luminosity of 11 pb(-1) was recorded in the ALEPH detector, in which WW candidate events were observed. In 6 events both Ws decay leptonically. In 16 events, one W decays leptonically, the other into hadrons. In the channel where both Ws decay into hadrons, a signal was separated from the large background by means of several multi-variate analyses. The W pair cross-section is measured to be sigma(ww) = 4.23 +/- 0.73(stat.) +/- 0.19(syst.) pb. From this cross-section, the W mass is derived within the framework of the Standard Model: m(w) = 80.14 +/- 0.34(stat.) +/- 0.09(syst.) +/- 0.03(LEP energy) GeV/c(2) (C) 1997 Published by Elsevier Science B.V.