Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons

Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise. © 2015

Polarized Positive and Negative Muon Beams to perform DVCS Measurements at COMPASS

The high energies available at CERN, and the option of using either positive or negative polarized muon beams, make the fixed-target COMPASS set-up a unique place for studying GPDs, through Deeply Virtual Compton Scattering (DVCS). A GPD program is part of the Medium and Long Term Plans at COMPASS. This contribution presents the methodology and the goal of such experiments

Etude de la diffusion Compton virtuelle en régime profondément inélastique pour le dispositif expérimental COMPASS

De nouvelles distributions de partons généralisées (GPD), introduites très récemment (1996), apparaissent dans la description de la structure interne du proton. Elles complètent les informations apportées par les distributions de partons usuelles et lient ces dernières aux facteurs de forme. Elles sont le sujet d'études extensives tant au niveau théorique qu'expérimental. Le processus de Diffusion Compton Virtuelle en régime profondément inélastique (DVCS) est particulièrement adapté à la mesure de ces nouvelles distributions. Il est possible d'utiliser le faisceau de muons de haute énergie ( 100GeV) et le dispositif expérimental COMPASS au CERN pour mesurer le processus DVCS. Cette thèse est consacrée à l'étude des GPD et du DVCS en général et à sa mesure à COMPASS en particulier. Après une étude du cadre théorique des GPD, la situation expérimentale dans le monde est présentée, et les enjeux d'une expérience à COMPASS sont soulignés. La faisabilité d'une telle expérience est alors validée par une simulation. Cette expérience repose sur le dispositif déjà existant et nécessite la construction d'un détecteur de recul hermétique, principalement pour assurer de façon systématique l'exclusivité de la mesure. Le calcul des taux de comptage attendus pour 6 mois de Prises de données permet de confirmer l'intérêt des mesures pour l'étude des GPD. Des tests d'un prototype de détecteur de recul, basé sur la mesure de temps de vol et fonctionnant dans l'environnement créé par le faisceau de muons à COMPASS, sont décrits et analysés. Un pion de faible impulsion produit dans l'état final et s'ajoutant au processus DVCS sera très difficilement détectable. Il est essentiel de s'assurer que ce processus ne perturbera pas de manière significative la mesure du processus DVCS. Une estimation théorique, basée sur la dérivation d'un théorème de pion mou est présentée.Recently, new parton distributions called generalized parton distributions (GPD) appeared in the description of the internal structure of the nucleon. They enrich the pieces of information accessible through the usual parton distributions and connect the latter with the form factors. They are the subject of extensive theoretical as well as experimental studies. The Deeply Virtual Compton Scattering (DVCS) is the simplest process that gives access to these new distributions. It is possible to use the high energy muon beam (100GeV) and the experimental setup of COMPASS at CERN to measure the DVCS process. This PhD thesis is devoted to the studies of GPD and DVCS in general and to its measurement at COMPASS in particular. After a study of the theoretical framework of GPD, the experimental situation in the world is presented, and light is shed on the stake of a DVCS experiment at COMPASS. The feasibility of such an experiment is then validated through a simulation. This experiment takes benefit of the existing apparatus and requires the construction of an hermetic recoil detector, mainly to insure systematically the exclusivity of the measurement. The calculation of the counting rates expected for a 6 months data taking period allows to confirm the interest of the measurements for the study of GPD. Tests of a recoil detector prototype, based on a time of flight measurement and operating in the noisy environment of the COMPASS muon beam, are described and analyzed. A pion of weak impulsion produced in the final state and adding to the DVCS process will be very hard to detect. It is of essential importance to insure that this process will not significantly disrupt the measurement of the DVCS process. A theoretical estimate, based on the derivation of a soft pion theorem is presented.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Deeply virtual Compton scattering in the Hall A of Jefferson laboratory

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Aging effects in the COMPASS hybrid GEM-Micromegas pixelized detectors

International audienceLarge-size hybrid and pixelized GEM-Micromegas gaseous detectors (40x40 cm$^2$ active area) were developed and installed in 2014 and 2015 for the COMPASS2 physics program which started at the same time. That program involved in particular two full years of Drell-Yan studies using a high-intensity pion beam on a thick polarized target. Although the detectors were placed behind a thick absorber, they were exposed to an important flux of low energy neutrons and photons. The detectors were designed to drastically reduce the discharge rate, a major issue for non-resistive Micromegas in high hadron flux, by a factor of more than 100 compared to the former ones. A hybrid solution was chosen where a pre-amplifying GEM foil is placed 2 mm above the micromesh electrode. A pixelized readout was also added in the center of the detector, where the beam is going through, in order to track particles scattered at very low angles. The combination of the hybrid structure and the pixelized central readout allowed the detector to be operated in an environment with particle flux above 10 MHz/cm$^2$ with very good detection efficiencies and spatial resolution. The performance has remained stable since 2015 in terms of gain and resolution, showing the interest of hybrid structures associating a GEM foil to a Micromegas board to protect gaseous detectors against discharges and aging effect