There is now convincing evidence that the Standard Model of electroweak and strong interactions is not the end of the story but only a low energy effective theory. In particular, new flavour physics is required to explain the fermion mass spectrum. Most of the proposed extensions of the Standard Model fail to meet this criterion. We may hope that the LHC or some future colliders could help to clarify the situation by discovering new particles or spotting some unexpected events. In the meantime, more precise measurements of masses and mixing parameters could also play an important role. In this work, we do not aim at finding a new mechanism that could explain this spectrum, but we rather assume that fermion masses and mixings are calculable in a yet-to-be-found more fundamental theory. Our goal is to glean as much information as possible from the observed fermion masses and mixings in order to find some hidden structures that could significantly lower the number of free parameters and help us to get some clues about what could be this fundamental theory. We analyse first the various parametrizations of the flavour mixing and single out a specific decomposition. The parameters of this decomposition can be independently and accurately computed if we impose some simple textures to the Yukawa couplings. We propose then a straightforward combination of these interesting textures which reproduces quite well the observed quark flavour mixing. We study then the properties of a successful mass relation for the charged leptons. We propose some generalizations of this relation in order to be valid also for the neutrinos and the quarks. One of them successfully combines the masses and mixings while another one describes the lepton masses via an accurate geometric description. Hopefully, these two studies lead to similar conclusions and allow us to speculate on some interesting properties for new flavour physics.

Das Standardmodell einer zusätzlichen sequentiellen Fermiongeneration (SM4) war 2012 auf Basis eines Fits an elektroschwache Präzisionsobservable und die Higgs-Signalstärken mit einer Signifikanz von 5.3 sigma ausgeschlossen worden. Komplementär dazu wurden in der vorliegenden Arbeit Fits des SM4 an eine Kombination eines typischen Satzes von Flavour-Observablen mit den Ergebnissen des zuvor durchgeführten Elektroschwachen Präzisionsfits durchgeführt. Im SM3-Kontext extrahierte Größen wurden gemäß ihrer Bedeutung im SM4 reinterpretiert und die angepassten theoretischen Ausdrücke angegeben. Die resultierenden Einschränkungen der CKM-Matrix des SM4, ihrer potentiell CP-verletzenden Phasen sowie der Masse des up-type-Quarks der 4. Generation t'' werden angegeben. Zum Vergleich des SM4 mit dem SM3 werden die erreichten chi^2-Werte genutzt. chi^2=15.53 im SM4 und 9.56 im SM3 passen fast vollkommen zu einer gleich guten Beschreibung der Experimente durch beide Modelle, wobei das SM3 aber sechs Freiheitsgrade mehr besitzt. Außerdem wurden die Vorhersagen des SM3 und des SM4 für die Dimyon-Ladungsasymmetrie ASL mit experimentellen Werten verglichen. Die Vorhersage des SM3 ist ca. 2 sigma vom experimentellen Wert entfernt, die des SM4 ca. 3 sigma.\par Die Ergebnisse deuten nicht darauf hin, dass die Signifikanz des 2012 erreichten Ausschlusses des SM4 durch die Hinzunahme von Flavour-Observablen zu den damals verwendeten elektroschwachen Präzisionsobservablen und Higgs-Querschnitten bedeutend verringert würde.\par Es konnte jedoch keine genaue quantitative Aussage über die Auswirkungen der Flavourobservablen auf diese Signifikanz getroffen werden, weil das Programm CKMfitter likelihood-ratio-Berechnung nur durchführen kann, wenn sich eines der untersuchten Modelle durch Fixierung von Parametern aus dem anderen ergibt (nested models), was hier nicht der Fall ist. / The Standard Model extended by an additional sequential generation of Dirac fermions (SM4) was excluded with a significance of 5.3 sigma in 2012. This was achieved in a combined fit of the SM4 to Electroweak Precision Observables and signal strengths of the Higgs boson. This thesis complements this excludion by a fit of the SM4 to a typical set of Flavour physics observables and the results of the previously performed Electroweak Precision fit. Quantities extracted in an SM3 framework are reinterpreted in SM4 terms and the adapted theoretical expressions are given. The resultant constraints on the SM4''s CKM matrix, its potentially CP-violating phases and the mass of the new up-type quark t'' are given. To compare the relative performance of the SM4 and the SM3, this work uses the chi^2 values achieved in the fit. The values of 15.53 for the SM4 and 9.56 for the SM4 are almost perfectly consistent with both models describing the experimental data equally well with the SM3 having six degrees of freedom more. The dimuon charge asymmetry ASL was not used as a fit input because the interpretation of its measurement was subject to debate at the time when the fits were produced, but its prediction in the fit was used as an additional test of the SM4. The SM3''s prediction differs from the experimental values by about 2 sigma, and the SM4''s prediction by about 3 sigma. \par In summary, these results do not suggest that any significant reduction of the 5.3 sigma exclusion could be achieved by combining the Electroweak Precision Observables and Higgs inputs with Flavour physics data. However, the exact effect of the Flavour physics input on the significance of the SM4''s exclusion cannot be given at this point because the CKMfitter software is currently not able to perform a statistically stringent likelihood comparison of non-nested models.

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.

L’étude de la décroissance β est un outil fantastique pour approfondir notreconnaissance de l’interaction faible décrite par le Modèle Standard. Ce modèle et laphysique au-delà peuvent être testés par des mesures précises de paramètres caractérisantces décroissances. Parmi ces tests, la vérification de l’hypothèse de la conservation ducourant vectoriel (CVC) et de l’unitarité de la matrice de mélange des quarks de Cabibbo-Kobayashi-Maskawa (CKM) sont d’un grand intérêt. Pour cela, les paramètres caractérisantles transitions β doivent être déterminés très précisément. Les meilleures précisions ont étéobtenues pour les transitions super-permises de Fermi de type 0+ → 0+. Cependant, il existed’autres types de décroissances pour réaliser ces tests, par exemple, les décroissances βmiroir. À ce jour, elles ne permettent pas d’atteindre les précisions obtenues avec lestransitions de type 0+ → 0+. Pour améliorer cela, de nouvelles mesures très précises desparamètres expérimentaux caractérisant ces transitions, comme la demi-vie et le rapportd’embranchement, sont nécessaires. C’est pourquoi une expérience a été réalisée àl’université de Jyväskylä en Finlande, afin d’étudier la décroissance β des noyaux miroir 23Mget 27Si et de mesurer ces paramètres. Les valeurs des demi-vies obtenues pour ces deuxnoyaux sont respectivement dix fois et sept fois plus précises que les moyennes de lalittérature. La valeur obtenue pour le rapport d’embranchement de 23Mg est trois fois plusprécise que la moyenne de la littérature, celui de 27Si étant déjà connu avec une très grandeprécision (0,02%), n’a pas été déterminé avec une meilleure précision. / Beta decays are a fantastic tool to study the weak interaction described by theStandard Model. This model and the physics beyond can be tested by precisemeasurements of nuclear β decays. Among these tests, the conserved vector current (CVC)hypothesis and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrixare of great interest. For these, the parameters of β transitions must be precisely determined.The highest precisions have been obtained with superallowed 0+ → 0+ Fermi β decays.However, there are other possibilities to make these tests, for example mirror β decays. Fornow, they do not allow to achieve the precision of the 0+ → 0+ transitions. To improve this,new accurate measurements of experimental parameters characterizing these transitions,such as the half-life and the branching ratio, are needed. For this purpose, an experimenthas been carried out at the University of Jyväskylä to study the mirror β decays of 23Mg and27Si and to measure these parameters. The half-life values obtained for these two nuclei are,respectively, ten times and seven times more precise than the literature averages. The valueof the branching ratio obtained for 23Mg is three times more precise than the literatureaverage; the one of 27Si has not been improved as it is already precisely known (0.02%).

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.