Measurement of the W mass in e+e- collisions at production threshold

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 σww = 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: mw = 80.14 ± 0.34(stat.) ± 0.09(syst.) ± 0.03(LEP energy) GeV/c2 © 1997 Published by Elsevier Science B.V.