Measurement of the W mass and width in e$^+$e$^-$ collisions at 189 GeV

Abstract.

The mass of the W boson is determined in e$^+$e$^-$ collisions at LEP by the direct reconstruction of W decays in $\mathrm{W}\mathrm{W}\rightarrow\mathrm{q\bar{q}}\mathrm{q\bar{q}}$ and $\mathrm{W}\mathrm{W}\rightarrow\mbox{$\ell\nu \mathrm{q\bar{q}}$}$ events, supplemented by measurements using the kinematic properties of the leptons in the \(\mathrm{W}\mathrm{W}\rightarrow\mbox{$\ell\nu $}\mbox{$\ell\nu $}$ decay channel. The main sample of W pairs is selected from an integrated luminosity of 174 pb$^{-1}$ collected with the ALEPH detector in 1998 at a centre-of-mass energy of 188.63 GeV. The combined result from all channels is

\[ m_\mathrm{W} = 80.432 \pm 0.072{\mathrm{(stat.)}} \pm 0.041{\mathrm{(syst.)}} \pm 0.019{\mathrm{(FSI)}} \pm 0.017{\mathrm{(LEP)}} \mathrm{GeV}/c^2, \]

where FSI represents the possible effects of final state interactions in the $\mathrm{q\bar{q}}\mathrm{q\bar{q}}$ channel. In a second two-parameter fit to the $\mathrm{q\bar{q}}\mathrm{q\bar{q}}$, $\mathrme\nu \mathrm{q\bar{q}}$ and $\mu\nu \mathrm{q\bar{q}}$ channels, where the W mass and width are decoupled, the average W width is found to be {$2.24 \pm 0.20{\mathrm{(stat.)}} \pm 0.13{\mathrm{(syst.)}}$ GeV/$c^2$}, consistent with the Standard Model prediction. The combination of the mass measurement presented in this paper together with those derived previously from the W pair cross section at 161 and 172 GeV and direct reconstruction at 172 and 183 GeV gives

\[ m_\mathrm{W} = 80.418 \pm 0.061{\mathrm{(stat.)}} \pm 0.040{\mathrm{(syst.)}} \pm 0.019{\mathrm{(FSI)}} \pm 0.017{\mathrm{(LEP)}} \mathrm{GeV}/c^2.\]