Searching For New Physics In B Meson Decays

After the Big Bang matter and antimatter were present in equal amounts. Today, however, everything we see, is made almost entirely of matter. A fundamental ingredient to explain this asymmetry between matter and antimatter is the violation of the CP (Charge Parity) symmetry. CP violation has been observed but quantitatively not enough to explain the huge asymmetry. The Belle II experiment in Japan studies the particles produced in electron-positron collisions at the SuperKEKB collider, the highest intensity collider in the World, allowing a high precision measurements of the known sources of CP violation and looking for new ones. In my PhD, I worked on measuring the CP violation in B meson radiative decays (decays with photon in final state) particularly promising thanks to their high sensitivity to possible new processes. Following this analysis, I started a new measurement of the B mesons lifetime where I could verify the robustness of the tools used in the CP violation measurement.

This thesis presents two analyses of semileptonic b → sl+l− decays using Flavour Changing Neutral Currents (FCNCs) to test for the presence of new physics and lepton flavour universality, and the equality of couplings for different leptons, which on the basis of experimental evidence is assumed to hold in the Standard Model, free from uncertainties as a result of knowledge of the hadronic matrix elements. It also includes the angular analysis of Lambda_b->Lambda mumu decay and the RK* measurement, both of which are first measurements, not yet performed by any other experiment.

This thesis discusses different aspects of the analysis of charmless three-body B-meson decays. A study of a method of extracting the weak phase gamma using five decay modes under the assumption of flavour-SU(3) symmetry is performed. This study is carried out using BABAR-analysis results and consists in a complete proof of principle, including fully propagated uncertainties. Six possible solutions are obtained with an uncertainty below 11 degrees. It is also found that, when averaged over the entire Dalitz plane, the effect of SU(3) breaking on the analysis is only at the percent level. A second analysis presented in the manuscipt consists in the measurement of the branching fractions of the B0(s)->KShh' modes, where h(') represents a pion or a kaon. It uses data collected by the LHCb detector during run I and a part of run II, corresponding to an integrated luminosity of almost 5/fb. The B0s->KSKK decay mode is observed for the first time, with a significance of almost 7 standard deviations; the B0s->KSKK branching fraction with respect to B0->KSpipi is extracted. This analysis paves the road to the measurements of the branching fractions of the six B0(s)->KShh' decay modes with the whole run I and run II LHCb dataset.

This 2nd edition is an extensive update of "B Decays?. The revisions are necessary because of the extensive amount of new data and new theoretical ideas. This book reviews what is known about b-quark decays and also looks at what can be learned in the future.The importance of this research area is increasing, as evidenced by the approval of the luminosity upgrade for CESR and the asymmetric B factories at SLAC and KEK, and the possibility of experiments at hadron colliders.The key experimental observations made thus far, measurement of the lifetimes of the different B species, B0-B0 mixing, the discovery of ?Penguin? mediated decays, and the extraction of the CKM matrix elements Vub and Vcb from semileptonic decays, as well as more mundane results, are described in great detail by the experimentalists who have been closely involved with making the measurements. Theoretical progress in understanding b-quark decays using HQET and lattice gauge techniques are described by theorists who have developed and used these techniques.Synthesizing the experimental and theoretical information, several articles discuss the implications for the ?Standard Model? and how further tests can be done using measurements of CP violation in the B system.

The author explores the possibility of using CP violation in B decays to detect the presence of physics beyond the Standard Model. He first studies the possibility of new physics in the B-{anti B} mixing amplitude. He discusses a construction to extract information about the phase and magnitude of the new physics contribution, as well as the CKM parameters in a model independent way. He points out the difficulty of carrying through this program induced by hadronic uncertainties and discrete ambiguities, and suggests additional measurements to overcome these problems. He then studies the possibility of new physics contributions to the B meson decay amplitudes. The author emphasizes the sensitivity of the B → [phi]K{sub S} decay to these new contributions, and explains how this sensitivity can be quantified using experimental data on SU(3) related decays. Finally, he analyzes a number of models where the B decay amplitudes are modified.

Al llarg dels últims anys, s'han observat nombroses senyals que suggereixen l'existència de Física més enllà del Model Estàndard en desintegracions semi-leptòniques de mesons B, mediades per transicions del tipus b --> sll. Més concretament, l'experiment LHCb, juntament amb altres experiments al LHC i col·laboracions externes com Belle, han trobat desviacions en les mesures experimentals de diferents observables d'important rellevància respecte les seves prediccions teòriques en el Model Estàndard. Parcialment, aquesta situació no es sorprenent, ja que en un conjunt gran de dades un sempre espera tenir un subconjunt en tensió amb les seves prediccions. No obstant, la importància de les observacions actuals és que totes aquestes tensions formen un patró coherent apuntant cap a "una direcció privilegiada". Per aquesta raó, els anàlisis globals d'aquestes dades mostren una discrepància general de més de 5sigmes respecte el Model Estàndard Per estudiar aquestes tensions de manera consistent, treballem des del marc del Hamiltonià efectiu que descriu la dinàmica de les transicions b --> sll. Aquesta descripció, conceptualment senzilla, ens permet descriure qualsevol desintegració d'un mesó B, que procedeixi a través de l'anterior transició, en base a un conjunt reduït de paràmetres. Precisament, aquest fet és el fa possible analitzar la totalitat dels observables on s'observen tensions de manera eficient per mitjà de fits globals.En aquesta Tesi, presentem la última versió dels anàlisis globals esmentats. Primer, descrivim el context teòric general que ens serveix de base per tots els nostres càlculs, fent especial èmfasi en l'estimació dels errors corresponents. I després, exposem els nostres resultats amb tota profunditat de detalls, així com comentem també les seves possibles implicacions.

The Standard Model (SM) of particle physics has withstood thus far every attempt by experimentalists to show that it does not describe data. We discuss the SM in some detail, focusing on the mechanism of fermion mixing, which represents one of its most intriguing aspects. We discuss how this mechanism can be tested in b-quark decays, and how b decays can be used to extract information on physics beyond the SM. We review experimental techniques in b physics, focusing on recent results and highlighting future prospects. Particular attention is devoted to recent results from b decays into a hadron, a lepton and an anti-lepton, that show discrepancies with the SM predictions — the so-called B-physics anomalies — whose statistical significance has been increasing steadily. We discuss these experiments in a detailed manner, and also provide theoretical interpretation of these results in terms of physics beyond the SM.

While B° and B mesons are well studied at B-factories, large samples of B{sub s}° mesons are only available at the Tevatron so far. Since the B{sub s}° meson consists of quarks of the second and third generation it provides a complementary probe for searches for new physics effects. Results of the CDF and D0 experiments on the decays B{sub s}° →?+?− and B{sub s}° →?+?−? as well as on CP violation in B{sub s}° mixing and in interference between mixing and decay are presented.