A precision measurement of the CKM angle γ from tree-level processes is one of the principal goals of the LHCb experiment. The results from this study are compared to predictions from two different theoretical models and from the default LHCb tuning of the PYTHIA Monte Carlo event generator, and the results shown to be in good agreement. The cross-section results are also compared to an independent LHCb measurement. LHCb analyses rely on the ability to identify kaons and pions with a high efficiency and low mis-identification rate, achieved by two Ring Imaging Cherenkov (RICH) detectors. To ensure optimal performance of the RICH detectors, the time alignment of the Level-0 (L0) front-end electronics modules has been optimised using a combination of a pulsed laser system installed in the LHCb cavern and pp collision data. After the time-alignment procedure, the L0 modules have been time-aligned to within approximately ° 1ns across both detectors.

This thesis presents a measurement of the charm mixing parameters x_D and y_D in prompt D⁰&rarr; K ⁰<sub style='position: relative; left: -.6em;'>S</sub> &pi;^&plus; &pi;^&minus; decays using 1 fb^-1 of data collected by the LHCb experiment during 2011. Mixing in charm is predicted to be small within the Standard Model, but there are significant uncertainties associated with calculating the long range contributions to the decay. Recent measurements made by LHCb and others have confirmed that mixing in charm exists at a rate of less than 1 &percnt;. With LHCb due to collect more data and Belle II being commissioned, the reduction of systematic uncertainties will become increasingly important. The D⁰&rarr; K ⁰<sub style='position: relative; left: -.6em;'>S</sub> &pi;^&plus; &pi;^&minus; decay provides sensitivity to both the magnitude and relative sign between the mixing parameters. It is also one of the few channels that can measure x_D directly. It is therefore crucial to study this mode in detail as more data becomes available. The work presented in this thesis utilises a model-independent description of the K ⁰<sub style='position: relative; left: -.6em;'>S</sub> &pi;^&plus; &pi;^&minus; Dalitz plot decay for the first time in the context of charm mixing. Previous mixing measurements with this final state have used a Dalitz plot amplitude model, and the associated systematic uncertainty is not straight forward to estimate or control. In its place, this analysis uses external, statistically-limited measurements of the strong-phase difference between D⁰ and &macr;<em style='position: relative; left: -.6em;'>_D⁰ obtained by CLEO as input. In addition, a data-driven technique is used to correct for decay time biases induced by the selection removing any systematic effects due to extracting this from simulated data. As the amount of available data increases, both of these techniques will become vital to improving our understanding of mixing in charm. In the CP convention used by Babar and adopted for this thesis, the measured mixing parameters are x_D = -(0.863 &plusmn; 0.527 (stat.) &plusmn; 0.171 (syst.)) &percnt;, y_D = -(0.026 &plusmn; 0.463 (stat.) &plusmn; 0.134 (syst.)) &percnt;. Both x_D and y_D are consistent with the current world averages.

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)^&plus; &rarr; &pi;^&plus;&mu;^&plus;&mu;^- 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 &rarr; 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&percnt; (95&percnt;) CL limits on the branching fractions, the most stringent to date, are found to be, B(D^&plus; &rarr; &pi;^&plus;&mu;^&plus;&mu;^-) &LT; 2.9 (3.3) × 10^-8, B(D_s^&plus; &rarr; &pi;^&plus;&mu;^&plus;&mu;^-) &LT; 1.76 (1.91) × 10^-7. The charge asymmetry measurement of D+ &rarr; &pi;+&pi;0 using &pi;0 &rarr; e^&plus;e^-&gamma; 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.

Les désintégrations charmées rares interviennent principalement via des courants neutres changeant la saveur (FCNC). Le Modèle Standard (SM) n'autorise les courants qu'au niveau des boucles. Dans les désintégrations du charme, les FCNC sont sujets à une très efficace suppression de GIM. Des processus très rares sont donc à considérer. Ils sont d'excellents outils pour la recherche Nouvelle Physique (NP) au-delà du SM. Les particules de NP pourraient devenir détectables en écartant des observables telles que des rapports de branchement et des asymétries CP et angulaires prévisions de le SM. Le sujet principal de cette thèse est la mesure du rapport de branchement D0->K-π+ρ/ω(->µ+µ-). Il sera précieux en particulier en tant que mode de normalisation lors de l'étude de toutes les désintégrations D0 -> h-h’+ µ+µ- : D0 -> K-π+µ+µ-, D0 -> π+π-µ+µ-, D0 -> K+K-µ+µ- et D0 -> K+π-µ+µ-. En utilisant 2/fb de données collectées par LHCb en 2012, nous mesurons: B(D0 -> K-π+ ρ/ω (->µ+µ-)) = (4.37± 0.12(stat.) ±0.53(syst.)) ×10^-6. C'est la toute première mesure de ce mode. Nous avons également étudié la sensibilité qu'attendra LHCb dans les modes D0 -> h-h’+ µ+µ- pour la mesure de rapport des branchements totaux et partiels, et pour celle d'asymétries, avec les échantillons de données qui seront collectés d'ici 10 ans. Par ailleurs, nous avons déterminé les incertitudes systématiques touchant les recherches de désintégrations à trois corps, D+(s) -> π+µ+µ- et D+(s) -> π-µ+µ+, effectués par LHCb dans les données recueillies en 2011 (1/fb). Enfin, les tests effectués sue les prototypes des cartes d'électronique embarquée qui assureront la lecture des calorimètres de l'expérience le LHCb mise à jour sont présenté dans cette thèse. / Rare charm decays proceed mostly through the c -> u Flavor Changing Neutral Current (FCNC), which is possible only at loop level in the Standard Model (SM). In charmed decays, FCNCs are subject to a very efficient GIM suppression, leading to very rare processes. Consequently, rare charm decays are good tools to probe to New Physics (NP) beyond the SM. NP particles could become detectable by causing observables such as branching ratios and CP or angular asymmetries to deviate from the SM predictions. The main subject of this thesis is the measurement of the branching ratio of the D0 -> K-π+ ρ/ω (->µ+µ-) mode. It will be precious in the future, in particular as a normalization mode in the study of all: D0 -> h-h’+ µ+µ- decays D0 -> K-π+µ+µ-, D0 -> π+π-µ+µ-, D0 -> K+K-µ+µ- and D0 -> K+π-µ+µ-. Using 2/fb of 2012 LHCb data we find: B(D0 -> K-π+ ρ/ω (->µ+µ-)) = (4.37± 0.12(stat.) ±0.53(syst.)) ×10^-6. This is the first measurement of this mode. We also determined sensitivities to total and partial branching fractions and asymmetries in D0 -> h-h’+ µ+µ- decays with future LHCb datasets. In addition, the systematic uncertainties affecting the searches for the 3-body decays, D+(s) -> π+µ+µ- and D+(s) -> π-µ+µ+, carried out by LHCb based on the data collected in 2011 (1/fb). Finally, the results of the tests of front-end electronic board for the Upgrade of LHCb are presented.

Le mélange des quarks est décrit dans le modèle standard de la physique des particules par le mécanisme de Cabibbo-Kobayashi-Maskawa (CKM). À ce jour, l'angle gamma du triangle d'unitarité est un des paramètres de ce mécanisme mesuré avec la moins bonne précision. La mesure de cet angle sert de référence pour le modèle standard, puisqu'elle peut être réalisée sans contribution significative de nouvelle physique. La précision actuelle de la meilleure mesure directe de gamma est d'environ 10°, alors que les ajustements globaux des paramètres CKM, potentiellement sujets à une contribution de nouvelle physique, déterminent cet angle à quelques degrés près. Par conséquent, une mesure directe précise de cette quantité est nécessaire pour contraindre d'avantage le triangle d'unitarité de la matrice CKM et ainsi tester la cohérence de ce modèle. Cette thèse présente une mesure de gamma par une analyse de Dalitz du canal B0->DK*0, avec une désintégration du méson D en K0Spipi. Elle est basée sur les 3 fb⁻¹ de données enregistrés par LHCb pendant le Run I du LHC, à une énergie de collision proton-proton dans le centre de masse de 7 et 8 TeV. Ce canal est sensible à gamma par l'interférence entre les transitions b->u et b->c. La mesure des observables de violation de CP réalisée est x- = -0.09 ^{+0.13}_{-0.13} ± 0.09 ± 0.01 , x+ = -0.10 ^{+0.27}_{-0.26} ± 0.06 ± 0.01 , y- = 0.23 ^{+0.15}_{-0.16} ± 0.04 ± 0.01 , y+ = -0.74 ^{+0.23}_{-0.26} ± 0.07 ± 0.01 , où le première incertitude est statistique, la deuxième est l'incertitude systématique expérimentale et la troisième est l'incertitude systématique venant du modèle de Dalitz. Une interprétation fréquentiste de ces observables donne rB0 = 0.39 ± 0.13 , deltaB0 = ( 186^{+24}_{-23} )° , gamma = ( 77^{+23}_{-24})° , où rB0 est le module du rapport des amplitudes des désintégrations supprimées et favorisées et deltaB0 la différence de phase forte entre ces deux désintégrations. Par ailleurs, un travail sur l'optimisation de la reconstruction des photons pour la mise à niveau du détecteur LHCb est aussi présenté. Lors du Run III du LHC, la luminosité instantanée reçue par LHCb sera augmentée d'un facteur cinq, générant un plus grand recouvrement entre les cascades se développant dans le calorimètre électromagnétique. L'étude montre que l'effet de ce recouvrement entre les gerbes est limité en réduisant la taille des clusters utilisés pour la détection des photons, tout en évitant une diminution significative de l'énergie reconstruite. Avec des corrections adaptées, la nouvelle reconstruction développée améliore la résolution en masse de 7 à 12%, suivant la région du calorimètre considérée. / Quark mixing is described in the standard model of particle physics with the Cabibbo-Kobayashi-Maskawa mecanism. The angle gamma of the unitarity triangle is one of the parameters of this mecanism that is still determined with a large uncertainty. It can be measured without significant contribution of new physics, making it a standard model key measurement. The current precision of the best direct measurement of gamma is approximately 10°, whereas the global fits of the CKM parameters determine this angle up to a few degrees. Therefore precise measurement of this quantity is needed to further constrain the Unitarity Triangle of the CKM matrix, and check the consistency of the theory. This thesis reports a measurement of gamma with a Dalitz analysis of the B0->DK*0 channel where the D meson decays into K0Spipi, based on the 3 fb⁻¹ of proton-proton collision data collected by LHCb during the LHC Run I, at the centre-of-mass energy of 7 and 8 TeV. This channel is sensitive to gamma through the interference between the b->u and b->c transitions. The CP violation observables are measured to be x- = -0.09 ^{+0.13}_{-0.13} ± 0.09 ± 0.01 , x+ = -0.10 ^{+0.27}_{-0.26} ± 0.06 ± 0.01 , y- = 0.23 ^{+0.15}_{-0.16} ± 0.04 ± 0.01 , y+ = -0.74 ^{+0.23}_{-0.26} ± 0.07 ± 0.01 , where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is the systematic uncertainty due to the Dalitz model. A frequentist interpretation of these observables leads to rB0 = 0.39±0.13 , deltaB0 = ( 186^{+24}_{-23} )°, gamma = ( 77^{+23}_{-24} )° , where rB0 is the magnitude of the ratio between the suppressed and favoured decays and deltaB0 the strong phase difference between these two decays. In addition, the work performed on the optimisation of the photon reconstruction for the upgraded LHCb detector is reported. During LHC Run III, the LHCb instantaneous luminosity will be increased by a factor five, implying a larger shower overlap in the electromagnetic calorimeter. The study shows that reducing the cluster size used in the photon reconstruction limits the effect of the overlap between the showers, without inducing a significant energy leakage. With some dedicated corrections, the new cluster reconstruction improves the Bs->Phi gamma mass resolution by 7 to 12%, depending on the calorimeter region.

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.

Using 3 fb-1 of pp collision data collected at √s = 7 and 8 TeV by the LHCb experiment in the 2011 and 2012 data taking periods, the decays B⁰s → ϕπ+π- and B⁰d → ϕπ+π- have been studied in the π+π- invariant mass range below 1600 MeV/c². The B⁰s,d → ϕπ+π- branching fractions are determined to be: B(B⁰s → ϕπ+π-;mππ < 1600) = [3:72 ± 0:18 ± 0:38 ± 0:38] x 10-6 B(B⁰d → ϕπ+π-,mππ < 1600) = [1:75 ± 0:25 ± 0:42 ± 0:14] x 10-7 where the first uncertainty is statistical, the second is systematic, and the third comes from the normalisation mode B⁰s → ϕϕ. From the π+π- mass distribution and fits to angular distributions, the resonant decay mode B⁰s → ϕf₀(980) is observed and the branching fraction is measured to be: B(B⁰s → ϕf₀(980); f₀(980) → π+π-) = [1:23 ± 0:15 ± 0:12 ± 0:12] x 10-6 The fit also requires contributions from B⁰s → ϕf2(1270). A search for a P-wave contribution from B⁰s → ϕρ⁰(770) finds evidence at ~ 4σ but confirmation will require more data. An upper limit of the B⁰s → ϕρ⁰(770) decay branching fraction at 90% C.L. is measured to be: B(B⁰s → ϕρ⁰(770)) < 4 x 10-7.

This thesis describes two pieces of work. The first is a study of the resolution of the LHCb vertex locator throughout Run 1. The second comprises analyses to measure the charm mixing and CP violation observables A_{Gamma} and y_{CP}.An estimate of the resolution of the LHCb vertex locator is required for use in the track fits. A method to measure the resolution with collision data has been developed and tested. The performance of the sub-detector throughout Run 1 of the LHC has been assessed. A significant degrading of the resolution has been seen. The effects of this on the track reconstruction has been examined with little change in the measured quantities being observed. The measurement of indirect CP violation in neutral D meson transitions has been measured through the observables A_{Gamma} and y_{CP}, using 1fb^{-1} of pp collisions with a centre of mass energy 7TeV, collected by the LHCb detector in 2011.A_{Gamma} describes the CP asymmetry of the lifetime of the D^0 decaying to a CP eigenstate (KK or pipi). The analysis documented here yields A_{Gamma} = (-0.17+-0.54)x10^{-3} when the measurements are combined. This is the world's best measurement and represented a factor of four improvement over the previous best result. The observable y_{CP} compares the effective lifetimes of the Cabibbo favoured decay D^0→Kpi and the transition to a CP eigenstate (KK or pipi). The unblinded result obtained in this document, averaged over both final states is y_{CP} = (5.61+-1.56)x10^{-3}. This result is commensurate with the world average central value within 1.25 standard deviations and has significance of 3.6 standard deviations relative to zero.

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