Measurement of the W mass by direct reconstruction in e+e−collisions at 172 GeV

The mass of the W boson is obtained from reconstructed invariant massdistributions in W-pair events. The sample of W pairs is selected from 10.65~pb$^{-1}$ collected with the ALEPHdetector at a mean centre-of-mass energy of 172.09 \GEV. The invariant massdistribution of simulated events are fitted to the experimental distributions and the following W masses are obtained: $WW \to q\ overline{q}q\ overline{q } m_W = 81.30 +- 0.47( stat.) +- 0.11(syst.) GeV/c^2$, $WW \to l\nu q\ overline{q}(l=e,\mu) m_W = 80.54 +- 0.47( stat.) +- 0.11(syst.) GeV/c^2$, $WW \to \tau\nu q\ overline{q} m_W = 79.56 +- 1.08( stat.) +- 0.23(syst.) GeV/C62$. The statistical errors are the expected errors for Monte Carlo samples of the same integrated luminosity as the data. The combination of these measurements gives: $m_W = 80.80 +- 0.11(syst.) +- 0.03(LEP energy) GeV/^2$.