This thesis presents a search for the rare B meson decay B -> pi l⁺l⁻ (l=e or mu) using 428 fb⁻¹ of data collected at the Upsilon(4S) resonance by the BaBar detector at the PEP-II asymmetric energy e⁺e⁻ storage rings located located at SLAC National Accelerator Laboratory in Menlo Park, California. Four exclusive modes are studied: B⁺ -> pi+ e⁺e⁻, B⁺ -> pi⁺ mu⁺mu⁻, B0 -> pi⁰ e⁺e⁻, and B0 -> pi⁰ mu⁺mu⁻. Branching fraction upper limits at the 90% confidence level are presented for the exclusive modes B⁺ -> pi⁺ mu⁺mu⁻ and B0 -> pi0 mu⁺mu⁻, and the combined mode B -> pi mu⁺mu⁻. / text

In this thesis, novel corrections to B → X˦ and B → Xl+l- decays, where X is a pseudoscalar or vector meson, are presented. These are the chromomagnetic matrix element, weak annihilation in a general four-quark operator basis and a long-distance charm loop contribution. The calculation of the chromomagnetic matrix element completes the calculation of matrix elements for all relevant effective weak operators in B → V decays, removing an infrared divergence which previous computations had treated in a very approximate way. It also encounters an interesting technical obstacle not previously seen in sum rule calculations, which is likely to be encountered regularly in future once higher order loop diagrams are calculated. The potential for this term to contribute to the CP asymmetry in D → V˦ in the presence of new physics is discussed. The improved computation of weak annihilation diagrams is applied to the analysis of isospin asymmetries in radiative and semi-leptonic B → (p;K(*)) decays, and the computation of long distance charm bubble terms is applied to produce an improved prediction for time-dependent CP asymmetries in various B → V decays.

This thesis presents two studies of charged charm meson decays using proton- proton collision data, corresponding to an integrated luminosity of 3 fb⁻¹, recorded by the LHCb experiment. A search for D_(s)^+ → π^+μ^+μ^- decays is performed. Rare decays of charged charm mesons have not previously been observed unlike their counterpart B meson and kaon decays. Such decays are c → u quark transitions and take place via loop diagrams within the Standard Model. Measurements of their branching fractions are an indirect test of New Physics, whose virtual contributions may enhance such quantities. No signals are observed and the 90% (95%) CL limits on the branching fractions, the most stringent to date, are found to be, B(D^+ → π^+μ^+μ^-) < 2.9 (3.3) × 10^-8, B(D_s^+ → π^+μ^+μ^-) < 1.76 (1.91) × 10^-7. The charge asymmetry measurement of D+ → π+π0 using π0 → e^+e^-γ decays is also described. Charge asymmetry in the Standard Model is expected to be small for charm meson decays. Any measured deviation from zero would be evidence of New Physics. This decay has not previously been observed at a hadron collider and the measurement acts as a 'proof of principle' study allowing a future analysis to examine the decay mode. No evidence of CP violation is found.

This dissertation documents a study of very rare B-meson decays at the LHCb experiment, using data taken during the first experiment run of the Large Hadron Collider (LHC) and during the second experiment run until September 2016. The LHCb experiment was designed to test the Standard Model of particle physics and to search for New Physics effects that go beyond the scope of the Standard Model through the decay of b hadrons produced in high energy proton-proton collisions at the LHC. The measurements described in this dissertation are made using data samples of proton-proton collisions with integrated luminosities of 1.0, 2.0 and 1.4fb⁻¹, collected at centre-of-mass energies of 7, 8 and 13 TeV, respectively. All results are consistent with the predictions of the Standard Model.

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No description available.

Two analyses are performed using data collected by the LHCb experiment during 2011. Both consider decays of charged B mesons reconstructed in the π±μ+μ− final state. Decays involving dimuons provide an experimentally clean signature, even in the high-background environment of the √s = 7 TeV proton-proton collisions at the LHC. The first analysis measures the CP asymmetry of B±→Ψπ± decays using 0.37 fb-1 of data, where the dimuon decays of two resonances are considered, J/ψ→μ+μ− and ψ(2S)→μ+μ−. The branching fraction is measured relative to the Cabibbo favoured B±→ΨK± mode. The second analysis uses 1 fb-1 of data to make the first observation of the non-resonant B±→π±μ+μ− decay. The branching fraction is measured relative to that of B±→K±μ+μ−, and measurements of the CP asymmetry and the ratio of CKM matrix elements Vtd/Vts are obtained. The branching fractions of the decays of interest are found to be B(B±→J/ψ π±) = (3.88 ± 0.11 ± 0.15) x 10-5, B(B±→ψ(2S) π±) = (2.52 ± 0.26 ± 0.15) x 10-5 and B(B±→π±μ+μ−) = (2.48 + 0.57 −0.52 ± 0.17) x 10-8, where the first uncertainty is related to the statistical size of the sample and the second quantifies systematic effects. The measured CP asymmetries in these modes are A CP (J/ψ π) = 0.005 ± 0.027 ± 0.011, A CP (ψ(2S) π) = 0.048 ± 0.090 ± 0.011 and A CP (μμπ) = -0.045 ± 0.220 ± 0.066, with no evidence of direct CP violation seen. The ratio of matrix elements is measured as Vtd/Vts = 0.274 + 0.031 − 0.028 ± 0.008, which is in agreement with previous results.

In this thesis, using a general model independent form of the effective Hamiltonian, the CP-violating asymmetries in the b -&gt / d l l transition, when one of the leptons is polarized, is investigated. The sensitivity of the CP-violating asymmetries on the new Wilson coefficients are analyzed. Next, in the frame work of the same formalism, the polarized lepton pair forward&amp / #8211 / backward asymmetries in B-&gt / K l l decay are studied. We present the general expression for the nine double&amp / #8211 / polarization forward&amp / #8211 / backward asymmetries. It is obtained that, the zero point position of the forward&amp / #8211 / backward asymmetries of the doubly&amp / #8211 / polarized lepton pair does not depend on long distance effects but depends on short distance dynamics. Furthermore, it is shown that the zero position of AFB is very sensitive to the sign of the new Wilson coefficients. When sign of the Wilson coefficients is positive (negative) the zero position of the forward&amp / #8211 / backward asymmetries shifts to the left(right) compared to the SM. Moreover, the dependencies of the nine double&amp / #8211 / polarization forward&amp / #8211 / backward asymmetries on new Wilson coefficients, and the correlation of the averaged nine double&amp / #8211 / polarization forward&amp / #8211 / backward asymmetries with branching ratio, have been studied. It is observed that, the study of the nine double&amp / #8211 / polarization forward&amp / #8211 / backward asymmetries can serve as a good test in establishing new physics beyond the Standard Model. Finally, we observed that there are exist such regions of new Wilson coefficients for which the nine double&amp / #8211 / polarization forward&amp / #8211 / backward asymmetries considerably depart from the SM result, while the branching ratio coincides with that of the SM prediction. In other words, new physics effects can be established by analyzing polarized forwardbackward asymmetry in this region of the new Wilson coefficients.

No description available.

Les désintégrations rares B0s → μ + μ-et B0 → μ + μ-sont des canaux de référence pour contraindre les modèles au-delà du Modèle Standard (BSM) avec un plus grand secteur de Higgs. Dans le SM, la fraction de branchement de ces désintégrations est prédite avec une bonne précision: B (B0 (s) → μ + μ-) = (3,2 ± 0,2) × 10-9 et B (B0 → μ + μ-) = (0,10 ± 0,01) × 10-10. Tout écart par rapport à ces valeurs peuvent donner des indications sur la physique BSM. Le cœur de cette thèse comporte deux thèmes principaux: le rejet du bruit de fond et l'extraction du signal. Nous avons optimisé un classificateur multivariée basée sur la décision des arbres technique permettant une réduction drastique du bruit de fond de B → h + h'-(h ≡ π, K) . Après le processus de sélection, environ 76% du fond combinatoire pour B0s → μ + μ-est enlevé, tout en gardant une efficacité de signal d'environ 92%. Une autre discrimination entre le signal et le fond est réalisé avec un autre classificateur multivariée optimisé pour un rejet de grand fond dans la région de l'efficacité de signal faible. Le travail présenté dans cette thèse décrit l'optimisation d'un classificateur d'arbres de décision qui supprime 99,9% du fond renforcé, après le processus de sélection ci-dessus, pour un rendement de signal de 50%. Nous avons proposé une méthode pour estimer les rendements de signaux présents dans notre échantillon de données en utilisant un ajustement extension maximale de vraisemblance. / The rare decays B0s→μ+μ− and B0→μ+μ− are benchmark channels to constrain models beyond the Standard Model (BSM) with a larger Higgs sector. In the SM, the branching fraction of these decays is predicted with a good accuracy: B(B0(s)→μ+μ−)=(3.2±0.2)×10−9 and B(B0→μ+μ−)=(0.10±0.01)×10−10. Any deviation from these values can lead to indications of physics BSM. The core of this thesis comprises two main topics: the background rejection and the signal yields extraction. We have optimized a multivariate classifier based on the boosted decision trees technique allowing for a drastic reduction of the B→h+h′− (h≡π,K) background. After the selection process, about 76% of the combinatorial background for B0s→μ+μ− is removed, while keeping a signal efficiency of about 92%. A further discrimination between signal and background is accomplished with another multivariate classifier optimized to have a large background rejection in the low signal efficiency region. The work presented in this thesis describes the optimization of a boosted decision trees classifier that suppresses 99.9% of the background, after the aforementioned selection process, for a signal efficiency of 50%. We have proposed a method to estimate the signal yields present in our data sample using an extended maximum likelihood fit.