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Les désintégrations B+- -> KSpi+pi0 ont été extraites des données enregistrées par le détecteur BABAR auprès de l'accélérateur PEP2 du SLAC et mesurées en mettant en œuvre une analyse en amplitude (analyse de Dalitz). La désintégration recherchée fait partie d une classe de processus susceptibles de mettre en évidence une physique au-delà du modèle standard et du formalisme de Cabibbo, Kobayashi et Maskawa. L'originalité de ce travail réside dans la mise au point d un algorithme d'ajustement des événements utilisant la méthode du maximum de vraisemblance étendue applicable sur un lot non discrétisé. Des outils statistiques et de mathématique appliquée originaux ont été développés pour gérer le faible rapport signal sur bruit d'un canal avec un pi0 dans l état final et auquel une désintégration charmée contribue sans aucun facteur d interdiction. Faute de statistique, la méthode n'atteint pas la sensibilité requise pour tester le modèle standard en mesurant les phases fondamentales. Elle permet par contre de préciser les rapports d embranchement et les asymétries de CP directes des canaux qui contribuent à la désintégration étudiée.

Precision measurements of superallowed Fermi β-decay allow for stringent tests of the magnitude of isospin-symmetry-breaking effects in nuclei, the validity of the conserved vector current hypothesis, and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. A high-precision measurement of the branching ratio for the superallowed β+ decay of 74Rb has been performed at the Isotope Separator and ACcelerator (ISAC) facility at TRIUMF. The 8π spectrometer, an array of 20 HPGe detectors, was used to detect γ-rays emitted following the Gamow-Teller and non-analogue Fermi β-decays of 74Rb. PACES, an array of 5 Si(Li) detectors, was used to detect emitted conversion electrons, while half of SCEPTAR, a close-packed array of 10 plastic scintillators, was used to detect emitted β particles. In this experiment, 23 excited states were identified in 74Kr which were populated following the β-decay of 74Rb. A total of 58 γ-rays were identified following the 8.241(4)x108 detected β-particles. An observed non-superallowed branching ratio of 0.396(7)% was determined from the intensity of the identified γ-rays, while the unobserved non-superallowed intensity was calculated to be 0.05(5)%, leading to a superallowed branching ratio of 99.55(5)%. The superallowed branching ratio is now the most precise experimental quantity in the determination of the superallowed ft-value of 74Rb. Combining the half-life and Q-value with the superallowed branching ratio measured in this work leads to a superallowed ft-value of 3082.6(66) s. Finally, comparisons between the superallowed ft-value, the world average Ft-value, and the non-analogue Fermi branching ratio are made to provide future guidance in the refinement of the theoretical models required to describe the crucial isospin-symmetry-breaking term in superallowed β-decay.

In this analysis the decay B- --> D*° e- nu is measured. The underlying data sample consists of about 226 million B-meson pairs accumulated on the Y(4S) resonance by the BABAR detector at the asymmetric e+e- collider PEP II. The reconstruction of the decay uses the channels D*° --> D° pi°, D° --> K- pi+ and pi° --> gamma gamma. The neutrino is not reconstructed. Since the rest frame of the B meson is unknown, the boost w of the D*° meson in the B meson rest frame is estimated by w'. The w' spectrum of the data is described in terms of the partial decay width dGamma/dw given by theory and the detector simulation translating each spectrum dGamma/dw into an expectation of the measured w' spectrum. dGamma/dw depends on a form factor F(w) parameterizing the strong interaction in the decay process. To find the best descriptive dGamma/dw a fit to the data determines the following two parameters of dGamma/dw: (i) F(1)|Vcb|, the product between F at zero D*°-recoil and the Quark mixing parameter |Vcb|; (ii) rho^2, a parameter of the form factor F(w). The former parameter scales the height of dGamma/dw and rho^2 varies the shape of it. The determined values of F(1)|Vcb|, rho^2 and the branching fraction BF(B- --> D*° e- nu) are F(1)|Vcb| = (35.8 +- 0.5 +- 1.5) x 10e-03, rho^2 = (1.08 +- 0.05 +- 0.09) and BF(B- --> D*° e- nu) = (5.60 +- 0.08 +- 0.42)%, where the uncertainties are statistical and systematic, respectively. The value of BF(B- --> D*° e- nu) has been determined by an integration of dGamma/dw over the allowed w range using the fitted values of F(1)|Vcb| and rho^2.

In this analysis the decay B- --> D*° e- nu is measured. The underlying data sample consists of about 226 million B-meson pairs accumulated on the Y(4S) resonance by the BABAR detector at the asymmetric e+e- collider PEP II. The reconstruction of the decay uses the channels D*° --> D° pi°, D° --> K- pi+ and pi° --> gamma gamma. The neutrino is not reconstructed. Since the rest frame of the B meson is unknown, the boost w of the D*° meson in the B meson rest frame is estimated by w'. The w' spectrum of the data is described in terms of the partial decay width dGamma/dw given by theory and the detector simulation translating each spectrum dGamma/dw into an expectation of the measured w' spectrum. dGamma/dw depends on a form factor F(w) parameterizing the strong interaction in the decay process. To find the best descriptive dGamma/dw a fit to the data determines the following two parameters of dGamma/dw: (i) F(1)|Vcb|, the product between F at zero D*°-recoil and the Quark mixing parameter |Vcb|; (ii) rho^2, a parameter of the form factor F(w). The former parameter scales the height of dGamma/dw and rho^2 varies the shape of it. The determined values of F(1)|Vcb|, rho^2 and the branching fraction BF(B- --> D*° e- nu) are F(1)|Vcb| = (35.8 +- 0.5 +- 1.5) x 10e-03, rho^2 = (1.08 +- 0.05 +- 0.09) and BF(B- --> D*° e- nu) = (5.60 +- 0.08 +- 0.42)%, where the uncertainties are statistical and systematic, respectively. The value of BF(B- --> D*° e- nu) has been determined by an integration of dGamma/dw over the allowed w range using the fitted values of F(1)|Vcb| and rho^2.