Étude de la désintégration semileptonique des mésons $\bar{B}^0_d$ en méson $D*^+}$ et mesure de $|V_{cb}|$ auprès de l'expérience DELPHI au LEP

Abstract : Semileptonic decays $\bar{B}^0_d -> D*^{+}l-\nu_l$ were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A measurement of the Standard Mode1 parameter |Vcb| has been performed using these decays. In addition, a search for production of narrow orbitally excited D mesons in semileptonic B decays is also presented. The measurement of IVcb] is based on the precise determination of the $q^2 = (m_b - m_D*)^2$ spectrum of the reconstructed events. An exclusive reconstruction of $\bar{B}^0_d -> D*^{+}l-\nu_l$ decays was done in the channel $D*^+ -> d^0\pi^+$ followed by $D^0 -> k^-\pi^+$ and $D^0 -> K^-\pi^+\pi^-^-\pi^+$ . The distribution was fitted over the full momentum transfer range to extract the product of IVcb| times the normalization of the decay form factor. Using HQET predictions of the from factor the value of IVcb| was extracted. This thesis begins by a description of the role of the IVcb/ matrix element in the Standard model. The theoretical tools used to determine IVcb| are also described. A brief overview of the performances of the DELPHI detector is given in the second part. Then the different steps of the analysis are detailed: event selection, background study, Br($\bar{B}^0_d -> D*^{+}l-\nu_l$ measurement which is the first step of IVcb| measurement, determination of a q2 estimator. The fitting procedure is then discussed. The obtained results are: Br($\bar{B}^0 -> D*^{+}l-\nu_l) = (5,17$\pm 0,49(stat) \pm 0,71(syst))% $|V_{cb}| = (38,7\pm 3,4(stat) \pm 2,9(syst) \pm 1,2(th)) x 10^{-3}$ A good agreement with previous measurements from LEP and B factories (CESR,DESY) is found. The search for narrow orbitally excited charm mesons in semileptonic B decays has led to the measurement of the brandling fraction Br(b -> $D^0_{J}lX) x Br(D^0_J -> D*^{+}\pi-) = (6.0^{+1,7}_{-1,5}(stat) \pm 0,8(syst))) x 10^{-3}$ where $D^0_J$ corresponds to the sum of the two narrow states $D^0_1$ and $D^0_2$. This measurement indicates a non negligible production of narrow excited states in semileptonic B decays.
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High Energy Physics - Experiment [hep-ex]. Université Louis Pasteur - Strasbourg I, 1997. French


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Pierre Alexandre Fischer. Étude de la désintégration semileptonique des mésons $\bar{B}^0_d$ en méson $D*^+}$ et mesure de $|V_{cb}|$ auprès de l'expérience DELPHI au LEP. High Energy Physics - Experiment [hep-ex]. Université Louis Pasteur - Strasbourg I, 1997. French. <tel-00393079>

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