The mass of the W boson is obtained from reconstructed invariant mass distr
ibutions in W-pair events. The sample of W pairs is selected from 57 pb(-1)
collected with the ALEPH detector in 1997 at a centre-of-mass energy of 18
3 GeV. The invariant mass distributions of reweighted Monte Carlo events ar
e fitted separately to the experimental distributions in the q (q) over bar
q (q) over bar and all l nu q (q) over bar channels to give the following
m(W)(hadronic) = 80.461 +/- 0.177 (stat.) +/- 0.045 (syst.) +/- 0.056 (theo
m(W)(semileptonic) = 80.326 +/- 0.184 (stat.) +/- 0.040 (syst.) GeV/c(2)
where the theory error represents the possible effects of final state inter
actions. The combination of these two measurements, including the LEP energ
y calibration uncertainty, gives
m(W) = 80.393 +/- 0.128 (stat.) +/- 0.041 (syst.) +/- 0.028 (theory) +/- 0.
021 (LEP) GeV/c(2)
(C) 1999 Published by Elsevier Science B.V. All rights reserved.