Mesure de la section efficace totalement leptonique et des rapports d'embranchement des bosons $W^{+-}$ dans le cadre de l'expérience ALEPH à LEP2

This thesis work is part of the precision tests done at LEP2. The W boson pairs are produced through e+e- collisions. The fully leptonic final states (lvlv) correspond to 10.5 % of the desintegrations. This channel is added to the hadronic and semi-leptonic channels, allowing a constraint on the Standard Model. Data recorded between 1996 and 1998 by the ALEPH detector are used to select the lvlv final states, caractarised by two energetic and acoplanar charged leptons and large transverse missing momentum. The fully leptonic cross-sections are determined in view of the measurement of the W's branching ratios. Contrary to the TEVATRON's measurements, the branching ratios are determined without universality constraint on W couplings. Presently, LEP obtains the best precision on the measurement of the W's branching ratios. No deviation was observed with respect to the Standard Model predictions.