1,721,350 research outputs found
Study of the Higgs self coupling in ATLAS detector at LHC. Study of performances of the timing detector for the high luminosity phase of LHC
No full textUne particule ayant une masse de 125 GeV a été observée en 2012 grâce au grand collisionneur de hadrons (Large Hadron Collider - LHC) par les collaborations ATLAS et CMS. Cette particule fut associée au boson de Higgs ou boson BEH, dont l'existence fut prédite en 1964 par François Englert, Robert Brout et Peter Higgs. Cette particule permit de valider l'existence du mécanisme BEH, expliquant l'origine de la masse des particules découverte dès lors et de la brisure de symétrie électrofaible.Depuis sa découverte, il est devenu crucial de sonder les différentes propriétés que l'on confère au boson de Higgs. En particulier, l'auto-couplage du boson de Higgs est une des propriétés les plus attendues et une mesure de ce paramètre permettrait d'obtenir une mesure directe du potentiel de Higgs dans le vide. Cette mesure se réalise grâce à la production de di-Higgs au LHC, en exploitant différents canaux de désintégrations.Cette thèse porte sur l'étude de la désintégration de la paire de boson de Higgs en deux leptons légers de même charge (appelé signature ou 2lSS), dans le cadre du Run2 du LHC correspondant à une luminosité intégrée de 139 fb⁻¹ et une énergie de centre de masse de √s = 13TeV. L'étude utilise des simulations Monte-Carlo et vise à développer une stratégie basée sur le machine Learning afin de maximiser la distinction entre le signal (2lSS issues de la désintégration de la paire de Higgs) et les bruits de fond (ensemble des processus produisant la signature). Une étude sur l'estimation des bruits de fond ainsi que sur l'estimation des incertitudes systématiques y sont aussi présentées.Enfin, bien que la mesure de l'auto-couplage puisse être contrainte par le Run2, sa mesure directe est attendue pour la phase de haute luminosité du LHC (HL-LHC). Cette phase implique une augmentation de la luminosité instantanée par un facteur 5, nécessitant une actualisation du détecteur ATLAS afin de garantir des performances comparables à celles du Run2, malgré l'augmentation des radiations et de l'effet d'empilement. De ce fait, un nouveau détecteur en temps à haute granularité (HGTD) sera ajouté. Une étude portant sur l'électronique de lecture de ce futur détecteur est présentée dans cette thèse, déterminant les performances en banc de tests ou lors de tests d'irradiations.A bosonic particle with a mass equal to 125GeV was observed in 2012, by ATLAS and CMS collaborations at the Large Hadron Collider (LHC). This particle was associated with the Higgs Boson or BEH boson, predicted fifty years before its discovery by François Englert, Robert Brout and Peter Higgs. This particle validates the BEH mechanism, explaining the origin of the mass of known particles and the electroweak symmetry breaking.Since its discovery, it has become crucial to probe the various properties of the Higgs boson such as the Higgs self-coupling. The success in probing Higgs self-coupling will bring another probe of the standard model and will provide a direct measurement of the Higgs field potential in the vacuum. This measure is performed through a global analysis of the di-Higgs (HH) production at LHC, decaying into various channels.This thesis focuses on the study of the decay of the Higgs boson pair into two light leptons with the same charge (referred to as 2lSS) within the context of the Run2 at LHC, providing an integrated luminosity of 139 fb⁻¹ and a centre of mass energy of √s = 13 TeV. The study based on Monte-Carlo simulations aims to develop a machine learning-based strategy to discriminate the signal (2lSS originating from the decay of the Higgs boson pair ) from the background (all other processes producing 2lSS events). An analysis of background noise estimation and systematic uncertainty estimation is also presented in this work.Finally, although the measurement of the self-coupling can be constrained by Run2, its direct measurement is expected in the High-Luminosity phase of the LHC (HL-LHC). This phase involves a five-fold increase in instantaneous luminosity, requiring an upgrade of the ATLAS detector to ensure performance comparable to Run2 performances, despite the increase of radiation and pile-up effects. As a result, a new high-granularity timing detector (HGTD) will be added. A study on the readout electronics of this future detector is presented in this thesis, determining its performance in the test bench and during irradiation tests
Evidence for the associated production of ttH with the ATLAS detector
No full textLa découverte du boson de Higgs au LHC en 2012 est la plus récente confirmation de la validité du modèle standard, théorie décrivant les particules élémentaires et leurs interactions fondamentales. De nombreuses études ont été mises en place pour étudier les différentes caractéristiques de cette particule récemment découverte. Ce travail se concentre sur la recherche du processus ttH dans l’expérience ATLAS, pour réaliser une première mesure directe du couplage de Yukawa du boson de Higgs au quark top, paramètre important dans les modèles de physique au-delà du modèle standard. La première partie de ce travail a porté sur l’automatisation de l’étalonnage du calorimètre hadronique à tuiles du détecteur ATLAS (TileCal ) par un système laser. Une description de ce système qui permet un étalonnage régulier de toutes les cellules du calorimètre est tout d’abord présentée. Ensuite, l’algorithme d’automatisation de ce processus d’étalonnage, qui a été écrit lors de ce travail, est décrit. Le but final de cet algorithme est de faciliter et accélérer la correction des canaux dont le gain dérive. Une deuxième partie concerne la recherche du processus ttH par l’étude des états finals multi-leptons et particulièrement avec deux leptons de même charge électrique et au moins 4 jets dont 1 étiqueté b. L’estimation des bruits de fond instrumentaux et le traitement statistique réalisé sont décrits en détail dans le document pour deux versions de l’analyse. Une première version correspondant aux données de l’année 2015 et du début 2016, soit une luminosité intégrée de 13.2f b −1 de données, aboutit à une précision insuffisante pour conclure sur la présence du processus ttH. Une deuxième version de l’analyse, optimisée avec l’utilisation d’analyses multivariées sur l’ensemble des données 2015 et 2016, se conclut par une observation du processus ttH lors de la combinaison de l’ensemble des états finals ttH. La signification statistique observée est alors de 4.2σ pour une signification statistique attendue de 3.8σ. Ce résultat est donc en accord avec la prédiction du modèle standard.Discovery of Higgs boson at LHC in 2012 is the most recent confirmation of the validity of Standard Model, theory describing elementary particles and their interactions. Many analysis now target the extraction of properties of the newly-discovered particle. A direct measurement in the ATLAS experiment of the top Yukawa coupling, one of these properties, is targeted in this work through ttH process. This coupling is of particular interest because of its strong sensitivity to New Physics. The first part of the work is about the automation of the calibration of the hadronic tile calorimeter of the ATLAS detector. A detailed description of the laser system used for a regular calibration of the calorimeter is done as well as of the calibration itself. Then more details on the algorithm written for the automation of the calibration are given. The final goal of this algorithm is to ease and fasten the calibration of channels with gain variation. The second part is dedicated to the search for ttH process through multilepton final states with emphasis on final state with two same-sign leptons, at least four jets and at least 1 b-tagged jet. Estimation of reducible backgrounds and statistical treatment of the analysis are detailed. A first version of the analysis with 13.2f b −1 , corresponding to 2015 and mid-2016 LHC data, ends with a final precision too low to extract any conclusion on the tt̄H process. In a second version of the analysis, improvements are made using multivariate analysis and adding more signal regions. The results from the multilepton analysis is combined with results from other ttH analysis, targeting other Higgs decays, to attain a final observed sentivity of 4.2σ. Thus an evidence for ttH production can be claimed from this combination. The final results give good agreement with Standard Model prediction
Search of events with two same-sign top quarks with the ATLAS detector. Calibration of the ATLAS hadronic calorimeter with a laser system
No full textLe Modèle Standard de la physique des particules permet de décrire le comportement des particules et leurs interactions. L’une d’elles, le quark top, est la particule élémentaire la plus lourde connue à ce jour. Cette propriété lui confère un rôle privilégié dans les théories dites de Nouvelle Physique, il pourrait se coupler préférentiellement aux nouvelles particules prédites par ces modèles.Cette thèse a été menée auprès du détecteur ATLAS installé au LHC. La première partie de ce travail a porté sur l’étalonnage du calorimètre hadronique, une partie du détecteur servant entre autres à mesurer l’énergie des hadrons produits lors des collisions. Ce calorimètre est étalonné grâce à un système laser, qui envoie des pulses de lumière dans les 9852 canaux du calorimètre. Cette étude a permis de corriger les canaux dont le gain dérive en appliquant des facteurs correctifs aux données. La deuxième partie de cette thèse a été consacrée à la recherche d’évènements à deux quarks top de même charge. La topologie avec deux leptons de même charge permet d’avoir une signature claire et présente peu de bruits de fond prédits par le Modèle Standard. Deux versions de l’analyse sont présentées : l’une avec une partie des données enregistrées par ATLAS en 2012, la deuxième plus optimisée avec le lot complet de données de 2012. Ces optimisations ont améliorées d’un facteur 3 la sensibilité sur le signal. N’ayant pas observé d’excès dans les données, une limite sur la section efficace de production d’évènements à deux quarks top de même charge a été calculée dans le cadre de deux modèles effectifs.The Standard Model of particle physics is used to describe the behavior of the particles and their interactions. One of them, the top quark, is the heaviest elementary particle. This property gives it a favored role in the New Physics theories, it could interact preferentially with new particles predicted by these models.This thesis was performed using the ATLAS detector at LHC. The first part of this work was on the calibration of the ATLAS hadronic calorimeter, part of the detector which is used to measure the energy of the hadrons produced during the collisions. The hadronic calorimeter is calibrated with a laser system, which sends pulses to the 9852 calorimeter’s channels. With this study, drifting channels were corrected by applying correction factors on the data.The second part of this thesis was dedicated to the search of events with two same-sign top quarks. The topology with a same-sign dilepton pair is used to have a clear signature and show only a few Standard Model backgroud processes. Two versions of this analysis are discussed : one of them with a part of the data collected in 2012 and the other one, with a lot of optimizations and with the complete dataset of 2012. These optimizations permits to improve the sensitivity of the signal by a factor 3. Without any excess observed in the data events, it is possible to calculate a limit on the cross section of the same-sign top quarks pair and interpreted in the case of two effective models
ATLAS Calorimeters: Run-2 performance and Phase-II upgrade
No full textThe ATLAS detector was designed and built to study proton-proton collisions produced at the LHC at centre-of-mass energies up to 14 TeV and instantaneous luminosities up to 10^{34} cm^{−2} s^{−1}. A liquid argon (LAr)-lead sampling calorimeter is employed as electromagnetic calorimeter and hadronic calorimter, except in the barrel region, where a scintillator-steel sampling calorimeter (TileCal) is used as hadronic calorimter. This presentation will give first an overview of the detector operation and data quality, as well as the achieved performance of the ATLAS calorimetry system. Additionally, the upgrade projects of the ATLAS calorimeter system for the high luminosity phase of the LHC (HL-LHC) will be presented. For the HL-LHC, the instantaneous luminosity is expected to increase up to L ≃ 7.5 × 10^{34} cm^{−2} s^{−1} and the average pile-up up to 200 interactions per bunch crossing. The major R&D item is the upgrade of the electronics for both LAr and Tile calorimeters in order to cope with longer latencies of up to 60 us. The expected radiation doses will exceed the qualification range of the current readout system. The status on the R&D of the low-power ASICs (pre-amplifier, shaper, ADC, selializer and transmitters) and readout electronics for all the design options will be discussed. Moreover, a High Granularity Timing Detector (HGTD) is proposed to be added in front of the LAr calorimeters in the end-cap region (2.4 <|eta|< 4.2) for pile-up mitigation at Level-0 trigger level and offline reconstruction. The HGTD will correlate the the energy deposits in the calorimeter to different proton-proton collision vertices by using TOF information with high time resolution (30 pico-second per readout cell) based on the Silicon sensor technologies. The current test beam results will be presented as well
results from ATLAS and CMS
No full textAfter the discovery of a Higgs boson, the measurements of its properties are at the forefront of research. The observation of the associated production of a Higgs boson and a pair of top quarks is of particular importance as the Yukawa coupling is large and can probe for physics beyond the Standard Model. Analyses of data taken by the ATLAS and CMS experiments recorded from 13 TeV proton-proton collisions are discussed. The production was analysed in various final states and used to constraint the overall production rate and CP properties of the top-Higgs coupling.
Higgs Precision Measurements with ATLAS
No full textAfter the discovery of the Higgs boson, precise measurements of its properties are of particular importance to understand the true nature of the found particle, which could be the Standard Model predicted Higgs, or indeed offering to be a portal for new physics beyond the Standard Model. In this talk measurement of the mass, cross sections and the couplings of the Higgs boson in bosonic and fermionic decay channels with the ATLAS detector will be presented and compared with Standard Model predictions
Calibration and monitoring of the ATLAS Tile calorimeter
No full textThe ATLAS Tile Calorimeter (TileCal) is the central section of the hadronic calorimeter of the ATLAS experiment and provides important information for reconstruction of hadrons, jets, hadronic decays of tau leptons and missing transverse energy. This sampling calorimeter uses steel plates as absorber and scintillating tiles as active medium. The light produced by the passage of charged particles is transmitted by wavelength shifting fibres to photomultiplier tubes (PMTs). PMT signals are then digitized at 40~MHz and stored on detector and are only transferred off detector once the first level trigger acceptance has been confirmed. The readout is segmented into about 5000 cells (longitudinally and transversally), each of them being read out by two PMTs in parallel. To calibrate and monitor the stability and performance of each part of the readout chain, a set of calibration systems is used. The TileCal calibration system comprises Cesium radioactive sources, laser, charge injection elements and an integrator based readout system. Combined information from all systems allows to monitor and equalize the calorimeter response at each stage of the signal production, from scintillation light to digitisation. Calibration runs are monitored from a data quality perspective and used as a cross-check for physics runs. Data quality in physics runs is monitored extensively and continuously. Problems are reported and immediately investigated. The data quality efficiency achieved was 99.6\% in 2012, 100\% in 2015 and 98.9\% in 2016. Based on LHC Run~1 experience, several calibration systems were improved for Run~2. The lessons learned, the modifications, and the LHC Run~2 performance, between 2015 and 2017, are discussed, including the calibration, stability, absolute energy scale, uniformity and time resolution. These results show that the TileCal performance is within the design requirements and has given essential contributions to reconstructed objects and physics results
ATLAS Tile calorimeter calibration and monitoring systems
No full textThe ATLAS Tile Calorimeter (TileCal) is the central section of the hadronic calorimeter of the ATLAS experiment and provides important information for reconstruction of hadrons, jets, hadronic decays of tau leptons and missing transverse energy. This sampling calorimeter uses steel plates as absorber and scintillating tiles as active medium. The light produced by the passage of charged particles is transmitted by wavelength shifting fibres to photomultiplier tubes (PMTs). PMT signals are then digitized at 40 MHz and stored on detector and are only transferred off detector once the first level trigger acceptance has been confirmed. The readout is segmented into about 5000 cells (longitudinally and transversally), each of them being read out by two PMTs in parallel. To calibrate and monitor the stability and performance of each part of the readout chain, a set of calibration systems is used. The TileCal calibration system comprises Cesium radioactive sources, laser, charge injection elements and an integrator based readout system. Combined information from all systems allows to monitor and equalize the calorimeter response at each stage of the signal production, from scintillation light to digitisation. Calibration runs are monitored from a data quality perspective and used as a cross-check for physics runs. Data quality in physics runs is monitored extensively and continuously. Problems are reported and immediately investigated. The data quality efficiency achieved was 99.6% in 2012, 100% in 2015 and 98.9% in 2016. Based on LHC Run 1 experience, several calibration systems were improved for Run 2. The lessons learned, the modifications, and the LHC Run 2 performance, between 2015 and 2017, are discussed, including the calibration, stability, absolute energy scale, uniformity and time resolution. These results show that the TileCal performance is within the design requirements and has given essential contributions to reconstructed objects and physics results
Top Associated Production in ATLAS and CMS
No full textThe high center-of-mass energy of proton-proton collisions and the large available datasets at the CERN Large Hadron Collider allow to study rare processes of the Standard Model with unprecedented precision. Measurements of rare SM processes provide new tests of the SM predictions with the potential to unveil discrepancies with the SM predictions or provide important input for the improvement of theoretical calculations. In this contribution, total and differential measurements of top-quark production in association with a photon, Z or W boson are shown using data taken with the ATLAS experiment at a center-of-mass-energy of 13 TeV. These measurements provide important bounds on the electroweak couplings of the top quark and constrain backgrounds that are important in searches for Higgs production and for new phenomena beyond the SM
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