Mise au point de détecteurs Micromegas pour le spectromètre CLAS12 au laboratoire jefferson

Cette thèse présente mon travail de recherche accompli depuis 2010 pour développer les détecteurs Micromegas du spectromètre CLAS12 qui sera installé dans le hall B du laboratoire Jefferson aux Etats-Unis. Les Micromegas sont des détecteurs gazeux robustes, rapides et bon marché. Ils doivent cependant être adaptés à l'environnement spécifique de CLAS12 car les défis sont nombreux : présence d'un champ magnétique fort, éloignement de l'électronique, fonctionnement avec un taux de hadrons élevé, nécessité de courber les détecteurs, espace disponible restreint. Ma thèse a commencé par des tests de détecteurs en faisceau au CERN qui ont permis d'estimer que le taux de décharges dans les Micromegas de CLAS12 serait de quelques Hertz. Une part importante de ce document est ainsi consacrée à l'étude de plusieurs méthodes innovantes dont l'objectif est de minimiser le temps mort dû aux décharges. J'ai donc mené des tests intensifs portant sur l'optimisation du filtre haute tension de la microgrille, l'introduction d'une feuille de GEM dans un Micromegas ou encore l'utilisation de Micromegas dits résistifs. Ces derniers donnant d'excellents résultats, des prototypes à l'échelle 1, dont l'un fabriqué par un industriel, ont été testés. La mécanique et le point de fonctionnement (gaz, tensions, géométrie...) des détecteurs ont ensuite été validés par des tests en laboratoire. Toutefois, afin de s'assurer un meilleur rapport signal sur bruit, des optimisations de la microgrille du détecteur ont été menées. Le CEA Saclay étant également responsable du développement de l'électronique des Micromegas pour CLAS12, j'ai comparé ses performances avec une autre électronique, vérifier sa résolution temporelle et déterminer le rapport signal sur bruit lorsque des limandes de 2 m connectent le détecteur à l'électronique. Les progrès réalisés dans le cadre de CLAS12 ont par ailleurs initié d'autres projets. J'ai ainsi effectué des simulations basées sur des pseudo-données pour valider la faisabilité d'une expérience portant sur les mésons exotiques pour laquelle nous avons proposé un trajectographe composé de Micromegas.This thesis presents my work performed since 2010 to develop Micromegas detectors for the CLAS12 spectrometer that will be installed in the Hall B of Jefferson Laboratory (USA). The Micromegas are robust, fast and cheap gaseous detectors. Nevertheless, they must be adapted to the specific CLAS12 environment as there are many challenges to face : presence of a strong magnetic field, off-detector frontend electronics, high hadrons rate, necessity to curve the detectors, few space available. My PhD started by beam tests at CERN that allowed to evaluate the spark rate in CLAS12 Micromegas at a few Hertz. An important part of this document is therefore devoted to the study of several innovative methods to minimize the dead time induced by sparks. Thus, I have performed intensive tests on the optimization of the micromesh high voltage filter, with on Micromegas equipped with a GEM foild or on resistive Micromegas. The latter giving excellent results, full scale prototypes, one of which built by a company, have been tested. The mechanics and the working point (gas, voltages, geometry...) of the detectors have then be validated by laboratory tests. However, to ensure a better signal over noise ratio, the micromesh has been optimized. The CEA Saclay being also responsible for the development of the electronics for CLAS12 Micromegas, I have compared its performance with another electronics, verify its time resolution and determine the signal over noise ratio when 2 m long cables are connecting the electronics to the detector. The progress realized in the context of CLAS12 have furthermore triggered other projects. So, I have carried out simulations based on pseudo-data to validate the feasibility of a meson spectroscopy experiment for which we have proposed a Micromegas based tracker.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

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

Nucleon Structure Studies by Lepton Deep Inelastic Scattering

International audienceWe present physics results obtained recently on the structure of the nucleon using lepton beams. The subjects discussed are: i) nucleon longitudinal spin structure with quark and gluon helicity measurements, ii) Semi-inclusive deep inelastic (SIDIS) results on hadron multiplicities for quark fragmentation function determination, iii) Transverse Momentum Dependent Distributions via SIDIS reactions and iv) Generalized Parton Distributions studies via Deep Virtual Compton Scattering and Meson Production. Data were taken at JLab (6 GeV electrons), HERMES (27 GeV electrons), and COMPASS (160/ 190 GeV muons)

COMPASS measurement of pion and kaon multiplicities and extraction of quark fragmentation functions into pions

We present preliminary COMPASS results on pion and kaon multiplicities produced in semi inclusive deep inelastic scattering of 160 GeV muons off an isoscalar ($^{6}$LiD) target. The results constitute an impressive data set of more than 400 points in p and 400 in K, covering a large x,Q$^{2}$ and z domain in a fine binning, which will be used in future QCD fits at next to leading order to extract quark fragmentation functions. We show results of a first leading order fit performed to extract the favored and unfavored quark fragmentation functions into pions $D^{\pi}_{fav}$ and $D^{\pi}_{unfav}$

COMPASS results on proton longitudinal spin structure function g1 and quark fragmentation functions

We present a few recent highlights from the COMPASS experiment at CERN, related to the nucleon spin and structure: the proton spin structure function g1 p(x) measured using 200 GeV polarized muons, together with a QCD fit of g1 world data; pion and kaon multiplicities in DIS in view of constraining quark fragmentation functions. The statistical precision on g1p is improved by a factor of ~2 at low x. A reevaluation of the Bjorken sum rule based on COMPASS data alone confirms its validation within 9% accuracy. An NLO QCD fit to g1 world data leads to a value of the quark spin contribution to the nucleon spin ΔΣ, between 0.26 and 0.36 at Q2= 3 (GeV/c)2, the size of the uncertainty being driven by the unknown shape of the polarized parton distributions. The results on pion and kaon multiplicities produced in DIS of muons off an isoscalar target constitute an impressive data set of more than 400 points in π and 400 in K, covering a large kinematical domain in (x, z, Q2), which will be used in future NLO QCD fits to extract quark fragmentation functions into π and K. A LO fit is performed to extract the favoured and unfavoured quark fragmentation functions into pions

Pion and kaon SIDIS multiplicities from COMPASS

International audienceWe present preliminary COMPASS results on pion and kaon multiplicities produced in semi- inclusive deep inelastic scattering of 160 GeV muons off an isoscalar ($^6$LiD) target. The results constitute an impressive data set of more than 600 points for pions and 600 for kaons, covering a large $x, Q^2$ and $z$ domain in a fine binning. The data are available for QCD fits to extract quark fragmentation functions. We show results of a first leading order fit performed to extract the favored and unfavored quark fragmentation functions into pions $D_{\mathrm{fav}}$ $^{\pi}$ and  $D_{\mathrm{unfav}}$ $^{\pi}$. The results from the sum of the $z$-integrated multiplicities $M(\pi^+) + M(\pi^-)$ and $M(K^+) +M(K^-)$ are presented versus $x$ and compared to previous experiments. At high $x$, the kaon data indicate a value of the non-strange quark fragmentation function significantly larger than obtained by the earlier QCD DSS fit. Overall, the new kaon data will strongly impact the flavour decomposition of kaon fragmentation functions. Recent additional data were added at high $z$ on the ratio of $K^-$ to $K^+$ multiplicities. For $z>0.8$, the ratio falls well below expectations from LO and NLO pQCD calculations. The ratio also shows a strong and unexpected dependence on the missing mass of the single kaon production process. This suggest to take into account the phase space available for hadronisation in further theoretical calculations

Measurement of the gluon polarization Δg/g\Delta g/g from Open Charm at COMPASS

We have measured the gluon polarization in the nucleon by detecting charm production via D$^{0}$ meson decay to charged K and $\pi$ in polarized muon scattering off a longitudinally polarized deuteron target. The dominant process for charm production is the photon gluon fusion into a charm anti-charm quark pair. By using all deuteron statistics from COMPASS accumulated between 2002 and 2006, we extract double spin asymmetries in bins of the transverse momentum and the energy of the D$^{0}$ meson and we perform a leading order analysis of the data to extract the gluon polarization $=-$0.49 $\pm$ 0.27 (stat) $\pm$ 0.11 (syst) at a QCD scale $\mu^{2}=$ 13 GeV$^{2}$ and at a gluon momentum fraction $=$0.11