Measurement of the W-boson mass with data taken by the DELPHI experiment at the LEP collider of CERN.

The experiments which have been performed until now indicate that our Universe can be described by 6 elementary particles, the fermions. The weak force is explainedd by the exchange of a Z or W-boson between two fermions. One of the essential ingredients inthe description of weak interactions is the precise measurement od the mass of the W-boson. To this aim one records electron-positron interactions at the LEP collider of CERN. Since 1996 data are taken at centre-of-mass energies ranging from 172 tot 206 Gev. The VUB group analyses the data taken by the DELPHI experiment. The final state of interest is the production of a pair of W bosons. Special statistical methods are investigated to determine the W-boson mass with a precison of 3/10000. Special attention is paid to the reduction of the systemic errors due to interactions between the decay products of the two produced W-bosons.