A Measurement of the W Boson Mass with the ALEPH Detector

A measurement of the mass of the W boson in e+ e- collisions using data col- lected with the ALEPH detector at the LEP accelerator is presented. Approxi- mately 450 pb- 1 of data collected at centre of mass energies between 188.6 and 201.6 GeV during 1998 and 1999 is used for the measurement. The selection of fully hadronic WW decays using a neural network is described. The reconstruc- tion of the invariant mass from the selected events is presented, paying particular attention to the procedure of kinematic fitting and the improvements to the mass resolution obtained by selecting and reconstructing initial state radiation. The W mass is extracted from the resulting invariant mass distribution using the method of Monte Carlo reweighting. The evaluation of relevant systematic uncertainties is described. The W mass is measured to be Mw = 80.453 ± 0.090 Ge V / c 2 . This result is combined with results from ALEPH, the other LEP experiments, and further measurements to give a world average W mass of Mw = 80.436 ± 0.037 GeV /c 2 . These results are interpreted within the context of the Standard Model of particle physics and compared with the indirect measurement of the W mass: Mw = 80.386 ± 0.025 Ge V / c 2 .