The top quark has been discovered in 1995 by CDF and D0 collaborations in proton-antiproton collisions at the Tevatron. The amount of data recorded by both experiments makes it possible to accurately measure the properties of this very massive quark. This thesis is devoted to the measurement of the top pair production cross-section via the strong interaction, in a final state composed of two electrons, two particle jets and missing transverse energy. It is based on a 1 fb{sup -1} data set collected by the D0 experiment between 2002 and 2006. The reconstruction and identification of electrons and jets is of major importance in this analysis, and have been studied in events where a Z boson is produced together with one or more jets. The Z+jets process is indeed the dominant physics background to top pair production in the dielectron final state. The primary goal of this cross-section measurement is to verify Standard Model predictions. In this document, this result is also interpreted to indirectly extract the top quark mass. Moreover, the cross-section measurement is sensitive to new physics such as the existence of a charged Higgs boson. The selection established for the cross-section analysis has been used to search …

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This study analyzes students' use of the French second-person pronouns tu (T) and vous (V) in small-group (2-3 students) inter-learner online chat sessions. The influence of internal linguistic factors (i.e., turn type and morphosyntactic environment) on learners' appropriate vs. inappropriate use of these pronouns is considered. The study also investigates the influence of Instructional Level on tu-vous use and the extent to which students from different instructional levels provide various types of peer assistance (e.g., lexical, morphosyntactic, and sociolinguistic/pragmatic) . Pronoun use was extremely unstable for learners of all levels, and a Kruskal-Wallis analysis revealed that Instructional Level did not significantly affect appropriate T/V use overall. Instructional Level and Syntax did, however, significantly affect interrogative T/V use, as shown through multivariate analyses. Peer-assisted performance was limited to lexical retrieval. Pedagogical recommendations are presented for teaching and learning second-person pronouns in French.

The authors present the study of the charm-pair correlations produced in photon-nucleon interactions at <E{sub {gamma}}> = 175 GeV/c, by the Fermilab fixed target experiment E831/FOCUS. The E831/FOCUS experiment produced and reconstructed over one million charm particles. This high statistics allows the reconstruction of more than 7000 charm-pair mesons D{bar D}, 10 times the statistic of former experiments, and also allows to get, for the first time, about 600 totally reconstructed charm-pairs in the DD{sub s} and D{Lambda}{sub c} channels. They were able to study, with some detail, the kinematical correlations between the charm and anticharm particle forming a pair, in the square transverse momentum (p{sub T}{sup 2}), azimuthal angle difference ({Delta}{phi}), rapidity difference ({Delta}y) and the charm-pair mass variables. They observe some correlation for the longitudinal momenta, and a significant correlation for the transverse momenta of the charm and anticharm particles. They compare the experimental distributions with theoretical predictions based on the photon-gluon fusion model (PGF), for the production of c{bar c} quarks, and the standard Lund hadronization model. These models are implemented by the PYTHIA Monte Carlo event generator. The PYTHIA program allows the inclusion, in the simulation, of non-perturbative effects that have been shown to be important …