Etude de la désintégration B⁰→K*⁰μ⁺μ⁻ avec l’expérience LHCb : analyse angulaire et mesure du rapport RK

Rare beauty decays proceed mostly through the b→s Flavor Changing Neutral Current, which is possible only at loop level in the Standard Model. These FCNC processes are subject to GIM suppression leading to a rare decay. Therefore the b→s processes are good tools to look for New Physics phenomenon beyond the Standard Model. New Physics particle could become detectable by causing deviation from the Standard Model predictions for observables such as angular observables, branching ratio or CP asymmetries. This thesis present the angular analysis of the B⁰→K*⁰μ⁺μ⁻ decay with the whole dataset collected by lhcb during the first run of the lhc. The full set of the angular observables has been measured through a maximum likelihood fit, thanks to an improved selection and the 3 fb⁻¹ of data collected in 2011 and 2012 by lhcb. In addition, the analysis on the measurement of the ratio R_{K*} has also been presented and the results are expected by the end of 2015.Les désintégrations de quark b rares interviennent principalement via des courant neutres changeant la saveur, (Flavor Changing Neutral Current, FCNC en anglais). Le modèle Standard n'autorise les courants qu'au niveau des boucles. Ces processus FCNC sont sujets à une suppression de GIM conduisant à des processus rares. Par conséquent les désintégration b→s sont de très bon outils pour la recherche de phénomène de Nouvelle Physique au delà du Modèle Standard. Cette thèse présente l'analyse angulaire de la désintégration B⁰→K*⁰μ⁺μ⁻ avec l'ensemble des données collecté par l'expérience lhcb durant le run I du lhc. La complète série des observable angulaires a été mesuré via un ajustement d'une fonction de vraisemblance, grâce à une sélection amélioré et les 3 fb⁻¹ de donné collecté en 2011 et 2012 par lhcb. Par ailleurs, l'analyse du rapport R_{K*} est également présenté et les résultats sont attendu pour la fin de l'année 2015

Measurement of the CP asymmetry in B − → Ds − D 0 and B − → D − D 0 decays

Abstract: The CP asymmetry in B− → D− s D0 and B− → D−D0 decays is measured using LHCb data corresponding to an integrated luminosity of 3.0 fb−1 , collected in pp collisions at centre-of-mass energies of 7 and 8 TeV. The results are ACP (B− → D− s D0 ) = (−0.4 ± 0.5 ± 0.5)% and ACP (B− → D−D0 ) = (2.3 ± 2.7 ± 0.4)%, where the first uncertainties are statistical and the second systematic. This is the first measurement of ACP (B− → D− s D0 ) and the most precise determination of ACP (B− → D−D0 ). Neither result shows evidence of CP violation

A measurement of the CP asymmetry difference between Λc + → pK − K + and pπ−π+ decays

The difference between the CP asymmetries in the decays Λ + c → pK−K+ and Λ + c → pπ−π + is presented. Proton-proton collision data taken at centre-of-mass energies of 7 and 8 TeV collected by the LHCb detector in 2011 and 2012 are used, corresponding to an integrated luminosity of 3 fb−1 . The Λ + c candidates are reconstructed as part of the Λ 0 b → Λ + c µ −X decay chain. In order to maximize the cancellation of production and detection asymmetries in the difference, the final-state kinematic distributions of the two samples are aligned by applying phase-space-dependent weights to the Λ + c → pπ−π + sample. This alters the definition of the integrated CP asymmetry to A wgt CP (pπ−π +). Both samples are corrected for reconstruction and selection efficiencies across the five-dimensional Λ + c decay phase space. The difference in CP asymmetries is found to be ∆A wgt CP = ACP (pK−K+) − A wgt CP (pπ−π +) = (0.30 ± 0.91 ± 0.61) %, where the first uncertainty is statistical and the second is systemati

First observation of B + → Ds + K + K − decays and a search for B + → Ds + ϕ decays

A search for B+ → D+ s K+K− decays is performed using pp collision data corresponding to an integrated luminosity of 4.8 fb−1 , collected at centre-of-mass energies of 7, 8 and 13 TeV with the LHCb experiment. A significant signal is observed for the first time and the branching fraction is determined to be B(B + → D+ s K+K−) = (7.1 ± 0.5 ± 0.6 ± 0.7) × 10−6 , where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fraction of the normalisation mode B+ → D+ sD 0 . A search is also performed for the pure annihilation decay B+ → D+ s φ. No significant signal is observed and a limit of B(B + → D+ s φ) < 4.9 × 10−7 (4.2 × 10−7 ) is set on the branching fraction at 95% (90%) confidence level. Keywords: B physics, Branching fraction, Hadron-Hadron scattering (experiments), Rare deca

Search for the Lepton-Flavor-Violating Decays Bs0 →τ±μâ' and B0 →τ±μâ'

Results are reported from a search for the rare decays Bs0→τ±μâ' and B0→τ±μâ', where the τ lepton is reconstructed in the channel τ-→π-π+π-ντ. These processes are effectively forbidden in the standard model, but they can potentially occur at detectable rates in models of new physics that can induce lepton-flavor-violating decays. The search is based on a data sample corresponding to 3 fb-1 of proton-proton collisions recorded by the LHCb experiment in 2011 and 2012. The event yields observed in the signal regions for both processes are consistent with the expected standard model backgrounds. Because of the limited mass resolution arising from the undetected τ neutrino, the Bs0 and B0 signal regions are highly overlapping. Assuming no contribution from B0→τ±μâ', the upper limit B(Bs0→τ±μâ')<4.2×10-5 is obtained at 95% confidence level. If no contribution from Bs0→τ±μâ' is assumed, a limit of B(B0→τ±μâ')<1.4×10-5 is obtained at 95% confidence level. These results represent the first limit on B(Bs0→τ±μâ') and the most stringent limit on B(B0→τ±μâ')

Observation of the decay Λ b 0 → ψ(2S)pπ−

The Λ0 b baryon is the isospin-singlet ground state of a bound system of a beauty quark and two light quarks. The high production rate of b quarks at the Large Hadron Collider (LHC) [1-5], along with the excellent mass resolution and hadron-identification capabilities of the LHCb detector, give access to a variety of decay channels of the Λ0 b baryon, including multibody, rare, charmless and semileptonic decays [6-25]. The high signal yield of the Λ0 b → J/ψpK− decay [15] facilitated a precise measurement of the Λ0 b lifetime [26], while the relatively low energy released in the Λ0 b→ ψ(2S)pK− and Λ0 b→ χcpK− decays allowed for precise measurements of the Λ0 b mass [16, 22]. A six-dimensional amplitude analysis of the Λ0 b → J/ψpK− decay resulted in the observation of the Pc(4380)+ and Pc(4450)+ pentaquark states decaying into the J/ψp final state [27]. Later, these states were confirmed using a model-independent technique [28]. Subsequently, an analysis of Cabibbo-suppressed Λ0 b → J/ψpπ − decays found evidence for contributions from the Pc(4380)+ and Pc(4450)+ pentaquarks and from the Zc(4200)− tetraquark [29]

Search for the lepton-flavour violating decays B (s) 0 → e ± μ ∓

A search for the lepton-flavour violating decays B0 s → e ±µ ∓ and B0 → e ±µ ∓ is performed based on a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb−1 , collected with the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The observed yields are consistent with the background-only hypothesis. Upper limits on the branching fraction of the B0 s → e ±µ ∓ decays are evaluated both in the hypotheses of an amplitude completely dominated by the heavy eigenstate and by the light eigenstate. The results are B(B0 s → e ±µ ∓) < 6.3 (5.4) × 10−9 and B(B0 s → e ±µ ∓) < 7.2 (6.0) × 10−9 at 95% (90%) confidence level, respectively. The upper limit on the branching fraction of the B0 → e ±µ ∓ decay is also evaluated, obtaining B(B0 → e ±µ ∓) < 1.3 (1.0) × 10−9 at 95% (90%) confidence level. These are the strongest limits on these decays to dat

Measurement of CP asymmetry in Bs 0 → Ds ∓ K ± decays

Abstract: We report the measurements of the CP-violating parameters in B0 s → D∓ s K± decays observed in pp collisions, using a data set corresponding to an integrated luminosity of 3.0 fb−1 recorded with the LHCb detector. We measure Cf = 0.73 ± 0.14 ± 0.05, A∆Γ f = 0.39 ± 0.28 ± 0.15, A∆Γ f¯ = 0.31 ± 0.28 ± 0.15, Sf = −0.52 ± 0.20 ± 0.07, Sf¯ = −0.49 ± 0.20 ± 0.07, where the uncertainties are statistical and systematic, respectively. These parameters are used together with the world-average value of the B0 s mixing phase, −2βs, to obtain a measurement of the CKM angle γ from B0 s → D∓ s K± decays, yielding γ = (128 +17 −22) ◦ modulo 180◦ , where the uncertainty contains both statistical and systematic contributions. This corresponds to 3.8 σ evidence for CP violation in the interference between decay and decay after mixin

Measurements of the branching fractions of Λ c + → pπ − π +, Λ c + → pK−K+, and Λ c + → pπ −K+

Abstract: The ratios of the branching fractions of the decays Λ + c → pπ−π +, Λ + c → pK−K+, and Λ + c → pπ−K+ with respect to the Cabibbo-favoured Λ + c → pK−π + decay are measured using proton-proton collision data collected with the LHCb experiment at a 7 TeV centre-of-mass energy and corresponding to an integrated luminosity of 1.0 fb−1 : B(Λ + c → pπ−π +) B(Λ + c → pK−π+) = (7.44 ± 0.08 ± 0.18)%, B(Λ + c → pK−K+) B(Λ + c → pK−π+) = (1.70 ± 0.03 ± 0.03)%, B(Λ + c → pπ−K+) B(Λ + c → pK−π+) = (0.165 ± 0.015 ± 0.005)%, where the uncertainties are statistical and systematic, respectively. These results are the most precise measurements of these quantities to date. When multiplied by the worldaverage value for B(Λ + c → pK−π +), the corresponding branching fractions are B(Λ + c → pπ−π +) = (4.72 ± 0.05 ± 0.11 ± 0.25) × 10−3 , B(Λ + c → pK−K+) = (1.08 ± 0.02 ± 0.02 ± 0.06) × 10−3 , B(Λ + c → pπ−K+) = (1.04 ± 0.09 ± 0.03 ± 0.05) × 10−4

Search for CP violation using triple product asymmetries in Λb 0 → pK − π + π −, Λb 0 → pK − K + K − and Ξb 0 → pK − K − π + decays

Abstract: A search for CP and P violation using triple-product asymmetries is performed with Λ 0 b → pK−π +π −, Λ 0 b → pK−K+K− and Ξ0 b → pK−K−π + decays. The data sample corresponds to integrated luminosities of 1.0 fb−1 and 2.0 fb−1 , recorded with the LHCb detector at centre-of-mass energies of 7 TeV and 8 TeV, respectively. The CP- and Pviolating asymmetries are measured both integrating over all phase space and in specific phase-space regions. No significant deviation from CP or P symmetry is found. The first observation of Λ 0 b → pK−χc0(1P)(→ π +π −, K+K−) decay is also reported