Top Physics at LHC

An overview of the top quark physics with the ATLAS and CMS detectors at the LHC is presented, with a focus on the early data taking phase. Due to the cental role played by the top quark in the Standard Model and the fact that LHC is a top factory, various aspects are adressed trough the different top quark production modes and top properties. In addition, the top quark can be used as a basic tool for calibrations

Production électrofaible du quark top au Run II de l'expérience DØ

Le travail présenté dans cette thèse porte sur l'étude de la production électrofaible du quark top dans les collisions proton-antiproton à s=1.96 TeV. Ce mode de production du quark top n'a jamais été observé. Les données analysées ont été collectées au cours du Run II de l'expérience DØ auprès du collisionneur Tevatron du Fermilab et représentent une luminosité intégrée d'environ 370 pb-1. La désintégration du quark top dans le Modèle Standard produit systématiquement un quark beau de grande impulsion. L'identification des jets issus de l'hadronisation d'un quark beau joue ainsi un rôle central dans l'analyse. L'important temps de vol des hadrons beaux qui se traduit par des particules chargées de grand paramètre d'impact relativement au vertex primaire de la collision. Les paramètres d'impact des traces associées à un jet sont convertis en une probabilité pour le jet de provenir du vertex primaire. Cet algorithme atteint une efficacité d'étiquetage des jets beaux de 45% pour environ un taux da mauvaise identification de 0.5%. Au Tevatron, la production électrofaible du quark top est dominée par deux processus (voies s et t), avec des états finals légèrement différents. La signature recherchée consiste en 2 à 4 jets, dont au moins un jet de quark beau, un lepton chargé (électron ou muon) et de l'énergie manquante provenant d'un neutrino. Cet état final est dominé par les deux principaux bruits de fond : la production associée d'un boson W et de jets et la production d'une paire de quarks top. Une analyse multivariable est mise en oeuvre pour séparer le signal de ces fonds. La sensibilité de l'analyse ne permet pas encore la mise en évidence du signal et des limites supérieures à 95% de niveau de confiance sur les sections efficaces de production de 5 pb (voie s) et 4.3 pb (voie t) ont été déterminées.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Electroweak parameters of the z0 resonance and the standard model

Contains fulltext : 124399.pdf (publisher's version ) (Open Access

Differential effects of methyl jasmonate on growth and division of etiolated zucchini cotyledons

Measurements of the jet energy calibration and transverse momentum resolution in CMS are presented, performed with a data sample collected in proton-proton collisions at a centre-of-mass energy of 7TeV, corresponding to an integrated luminosity of 36pb-1. The transverse momentum balance in dijet and γ/Z+jets events is used to measure the jet energy response in the CMS detector, as well as the transverse momentum resolution. The results are presented for three different methods to reconstruct jets: a calorimeter-based approach, the "Jet-Plus-Track" approach, which improves the measurement of calorimeter jets by exploiting the associated tracks, and the "Particle Flow" approach, which attempts to reconstruct individually each particle in the event, prior to the jet clustering, based on information from all relevant subdetectors

Transverse momentum and pseudorapidity distributions of charged hadrons in pp collisions at (s)\sqrt(s) = 0.9 and 2.36 TeV

Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at sqrt(s) = 0.9 and 2.36 TeV. The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between -2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dN(charged)/d(eta) for |eta| < 0.5, are 3.48 +/- 0.02 (stat.) +/- 0.13 (syst.) and 4.47 +/- 0.04 (stat.) +/- 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in p-pbar and pp collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date

Search for a light charged Higgs boson in top quark decays in pp collisions at sqrt(s) = 7 TeV

Submitted to the Journal of High Energy Physics; see paper for full list of authorsResults are presented on a search for a light charged Higgs boson that can be produced in the decay of the top quark to charged H and b quark and which, in turn, decays into tau and tau neutrino. The analysed data correspond to an integrated luminosity of about 2 inverse femtobarns recorded in proton-proton collisions at sqrt(s) = 7 TeV by the CMS experiment at the LHC. The search is sensitive to the decays of the top quark pairs t anti-t to charged Higgs W b anti-b and t anti-t to charged Higgs b anti-b. Various final states have been studied separately, all requiring presence of a tau lepton from charged Higgs decays, missing transverse energy, and multiple jets. Upper limits on the branching fraction B(t to charged Higgs b) in the range of 2-3% are established for charged Higgs boson masses between 80 and 160 GeV, under the assumption that B(charged Higgs to tau anti-tau neutrino) = 1

A New Boson with a Mass of 125 GeV Observed with the CMS Experiment at the Large Hadron Collider

The Higgs boson was postulated nearly five decades ago within the framework of the standard model of particle physics and has been the subject of numerous searches at accelerators around the world. Its discovery would verify the existence of a complex scalar field thought to give mass to three of the carriers of the electroweak force-the W+, W-, and Z 0 bosons-as well as to the fundamental quarks and leptons. The CMS Collaboration has observed, with a statistical significance of five standard deviations, a new particle produced in proton-proton collisions at the Large Hadron Collider at CERN. The evidence is strongest in the diphoton and four-lepton (electrons and/or muons) final states, which provide the best mass resolution in the CMS detector. The probability of the observed signal being due to a random fluctuation of the background is about 1 in 3 x 106. The new particle is a boson with spin not equal to 1 and has a mass of about 1.25 giga-electron volts. Although its measured properties are, within the uncertainties of the present data, consistent with those expected of the Higgs boson, more data are needed to elucidate the precise nature of the new particle