Mesure de l'angle gamma de la matrice CKM dans les désintégrations B⁰->DK*⁰ en utilisant la méthode de Dalitz dans l'expérience LHCb au CERN et optimisation de la reconstruction des photons pour l'upgrade du détecteur LHCb

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.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

Measurement of the forward Z boson production cross-section in pp collisions at s=13\sqrt{s} = 13 TeV

A measurement of the production cross-section of Z bosons in pp collisions at $\sqrt{s} = 13$ TeV is presented using dimuon and dielectron final states in LHCb data. The cross-section is measured for leptons with pseudorapidities in the range $2.0 \eta 4.5$, transverse momenta $p_\text{T} 20$ GeV and dilepton invariant mass in the range $60 m(\ell\ell) 120$ GeV. The integrated cross-section from averaging the two final states is \begin{equation*}\sigma_{\text{Z}}^{\ell\ell} = 194.3 \pm 0.9 \pm 3.3 \pm 7.6\text{ pb,}\end{equation*} where the first uncertainty is statistical, the second is due to systematic effects, and the third is due to the luminosity determination. In addition, differential cross-sections are measured as functions of the Z boson rapidity, transverse momentum and the angular variable $\phi^*_\eta$

Novel fully-heterogeneous GNN designs for track reconstruction at the HL-LHC

Data from the LHC detectors are not easily represented using regular data structures. These detectors are comprised of several species of subdetectors and therefore produce heterogeneous data. LHC detectors are highly granular by design so that nearby particles may be distinguished. As a consequence, LHC data are sparse, in that many detector channels are not active during a given collision event. Graphs offer a flexible and efficient alternative to rectilinear data structures for representing LHC data. Accordingly, graph-based machine learning algorithms are becoming increasingly popular for a large number of LHC physics tasks [1, 2]. This popularity, and the corresponding potential for substantial increase in physics output, are illustrated on the cover of a recent issue [3] of the CERN Courier magazine. The graphs used in almost all practical applications at the LHC so far are homogeneous, i.e. each node is assigned the same features, and each edge is assigned the same features. In other words, the power of graphs to represent sparse data has been exploited in applications for the LHC, but the potential of graphs to represent heterogeneous data has not. The pink graph on the cover of the CERN Courier [3] can be seen as an illustration of this limitation: all nodes are pink, regardless of their position in the detector. We present novel fully-heterogeneous GNN designs and apply them to simulated data from a tracking detector that resembles the trackers that will be used at the HL-LHC. It contains a pixel subsystem that provides 3D hits and a strip subsystem that provides 2D hits. We present a new design which solves the degraded performance observed in the strip detector in the first GNNbased tracking studies presented by the ATLAS Collaboration [4]

Les droits disciplinaires des fonctions publiques : « unification », « harmonisation » ou « distanciation ». A propos de la loi du 26 avril 2016 relative à la déontologie et aux droits et obligations des fonctionnaires

The production of tt‾ , W+bb‾ and W+cc‾ is studied in the forward region of proton–proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98±0.02 fb−1 . The W bosons are reconstructed in the decays W→ℓν , where ℓ denotes muon or electron, while the b and c quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions.The production of $t\overline{t}$, $W+b\overline{b}$ and $W+c\overline{c}$ is studied in the forward region of proton-proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98 $\pm$ 0.02 \mbox{fb}^{-1}. The $W$ bosons are reconstructed in the decays $W\rightarrow\ell\nu$, where $\ell$ denotes muon or electron, while the $b$ and $c$ quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions

Physics Performance of the ATLAS GNN4ITk Track Reconstruction Chain

Particle tracking is vital for the ATLAS physics programs. To cope with the increased number of particles in the High Luminosity LHC, ATLAS is building a new all-silicon Inner Tracker (ITk), consisting of a Pixel and a Strip subdetector. At the same time, ATLAS is developing new track reconstruction algorithms that can operate in the HL-LHC dense environment. A track reconstruction algorithm needs to solve two problems: track finding for building track candidates and track fitting for obtaining track parameters of those track candidates. Previously, we developed GNN4ITk, a track-finding algorithm based on a Graph Neural Network (GNN), and achieved good track-finding performance under realistic HL-LHC conditions. Our GNN pipeline relied only on the 3D spacepoint positions. This work introduces heterogeneous GNN models to fully exploit the subdetector-dependent features of ITk data, improving the performance of our GNN4ITk pipeline. In addition, we interfaced our pipeline to the standard ATLAS track-fitting algorithm and data model. With that, the GNN4ITk pipeline produces full-fledged track candidates that can be used for any downstream analyses and compared with the other track reconstruction algorithms

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

New algorithms for identifying the flavour of B⁰mesons using pions and protons

Two new algorithms for use in the analysis of pp collision are developed to identify the flavour of B0mesons at production using pions and protons from the hadronization process. The algorithms are optimized and calibrated on data, using B0→D-π+ decays from pp collision data collected by LHCb at centre-of-mass energies of 7 and 8 TeV . The tagging power of the new pion algorithm is 60% greater than the previously available one; the algorithm using protons to identify the flavour of a B0meson is the first of its kind.</p

Observation of the B0 → ρ0ρ0 decay from an amplitude analysis of B0 → (π+π−)(π+π−) decays

Proton–proton collision data recorded in 2011 and 2012 by the LHCb experiment, corresponding to an integrated luminosity of 3.0 fb−1 , are analysed to search for the charmless B0→ρ0ρ0 decay. More than 600 B0→(π+π−)(π+π−) signal decays are selected and used to perform an amplitude analysis, under the assumption of no CP violation in the decay, from which the B0→ρ0ρ0 decay is observed for the first time with 7.1 standard deviations significance. The fraction of B0→ρ0ρ0 decays yielding a longitudinally polarised final state is measured to be fL=0.745−0.058+0.048(stat)±0.034(syst) . The B0→ρ0ρ0 branching fraction, using the B0→ϕK⁎(892)0 decay as reference, is also reported as B(B0→ρ0ρ0)=(0.94±0.17(stat)±0.09(syst)±0.06(BF))×10−6

Observation of the decay B0s→ψ(2S)K+π−

The decay B¯s0→ψ(2S)K+π− is observed using a data set corresponding to an integrated luminosity of 3.0 fb−1 collected by the LHCb experiment in pp collisions at centre-of-mass energies of 7 and 8 TeV. The branching fraction relative to the B0→ψ(2S)K+π− decay mode is measured to be B(B¯s0→ψ(2S)K+π−)B(B0→ψ(2S)K+π−)=5.38±0.36(stat)±0.22(syst)±0.31(fs/fd)%, where fs/fd indicates the uncertainty due to the ratio of probabilities for a b quark to hadronise into a Bs0 or B0 meson. Using an amplitude analysis, the fraction of decays proceeding via an intermediate K⁎(892)0 meson is measured to be 0.645±0.049(stat)±0.049(syst) and its longitudinal polarisation fraction is 0.524±0.056(stat)±0.029(syst) . The relative branching fraction for this component is determined to be B(B¯s0→ψ(2S)K⁎(892)0)B(B0→ψ(2S)K⁎(892)0)=5.58±0.57(stat)±0.40(syst)±0.32(fs/fd)%. In addition, the mass splitting between the Bs0 and B0 mesons is measured as M(Bs0)−M(B0)=87.45±0.44(stat)±0.09(syst) MeV/c2

Measurement of CP violation parameters and polarisation fractions in Bs0→J/ψK‾∗0 {\mathrm{B}}_{\mathrm{s}}^0\to \mathrm{J}/\psi {\overline{\mathrm{K}}}^{\ast 0} decays

The first measurement of ${C\!P}$ asymmetries in the decay ${B_s^0\to J/\psi \overline{K}^{*}(892)^{0}}$ and an updated measurement of its branching fraction and polarisation fractions are presented. The results are obtained using data corresponding to an integrated luminosity of $3.0\,fb^{-1}$ of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of $7$ and $8\,\mathrm{TeV}$. Together with constraints from ${B^0\to J/\psi \rho^0}$, the results are used to constrain additional contributions due to penguin diagrams in the ${C\!P}$-violating phase ${{\phi}_{s}}$, measured through ${B_s^0}$ decays to charmonium.The first measurement of CP asymmetries in the decay ${B}_s^0\to J/\psi {\overline{\mathrm{K}}}^{\ast }{(892)}^0$ and an updated measurement of its branching fraction and polarisation fractions are presented. The results are obtained using data corresponding to an integrated luminosity of 3.0 fb^{−}^{1} of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of 7 and 8 TeV. Together with constraints from B$^{0}$ → J/ψ ρ$^{0}$, the results are used to constrain additional contributions due to penguin diagrams in the CP -violating phase ϕ$_{s}$ , measured through B$_{s}^{0}$ decays to charmonium.The first measurement of ${C\!P}$ asymmetries in the decay ${B_s^0\to J/\psi \overline{K}^{*}(892)^{0}}$ and an updated measurement of its branching fraction and polarisation fractions are presented. The results are obtained using data corresponding to an integrated luminosity of $3.0\,fb^{-1}$ of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of $7$ and $8\,\mathrm{TeV}$. Together with constraints from ${B^0\to J/\psi \rho^0}$, the results are used to constrain additional contributions due to penguin diagrams in the ${C\!P}$-violating phase ${{\phi}_{s}}$, measured through ${B_s^0}$ decays to charmonium