Etude des oscillations des mésons de beauté par l'analyse de leur évolution temporelle avec le détecteur DELPHI à LEP

Oscillations of $B^0_d$ and $B^0_s$ mesons have been studied at LEP with the DELPHI detector. In the Standard Model, the oscillation frequencies are linked with CP violation parameters, and the interest of their determination is presented in the first part. To reach a direct measurement of these oscillation frequencies, the B. proper time is reconstructed on an event-by-event basis .. After a description of the DELPHI detector, three chapters present the event selection and the algorithms used to reconstruct their geometrical and energetical characteristics. The energy reconstruction allows to measure the b quark fragmentation function, and its mean value is found to be : $x^b_E$ = 0.695 ± 0.015 (stat.)± 0.029. (syst.) Then, using one million hadronic $Z^0$ decays, a measurement of the $B^0_d$ oscillation frequency is derived from the selection of 240 events containing in opposite hemispheres two high Pt same sign leptons, one of them being accompanied by a D meson partially reconstructed in thtee charged tracks. A fit to the resulting proper .time distribution gives : $X_d$ = 0.69 ± 0.33 (stat.)± 0.14 (syst.) Finally, a combined study of kaon-lepton correlations and dilepton events shows which limit could be set in the near future on the value of the $B^0_s$ oscillatii>n frequency .