Etude des réactions d'annihilation proton-antiproton et contribution expérimentale à la polarimétrie hadronique.Study of antiproton-proton annihilation reactions and experimental contribution to hadron polarimetry
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Thursday, 7 September, 2017
Abstract This thesis work, set in the framework of hadron physics, reports on a phenomenological and an experimental study dedicated to nucleon internal structure, both related to hadron electromagnetic form factor measurements in the time-like and in the space-like regions. At the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, an experimental program aiming to determine the proton electromagnetic form factors in the time-like region is planned benefiting from the PANDA (anti-Proton Annihilation at Darmstadt) large acceptance detector through the annihilation reaction p̄p -> e+ e- and p̄p -> μ+ μ- in the energy range 2.25(1.5) ≤ √s(p L ) ≤ 5.47(15) GeV (GeV/c), where √s(p L ) is the total energy in the center of mass system (the beam momentum in laboratory frame). In such reactions, the difficulty will be to extract the signal of interest from the huge background coming from hadronic channels, mainly pion pair production. Phenomenological studies of meson pair production in p̄p annihilation is one of the subjects of this thesis. The goal is two-fold : to better evaluate the hadronic background and to improve our knowledge on the hadron internal structure as such reaction contains by itself useful information on the hadron quark content. For this purpose, an effective meson model has been developed to evaluate the cross section of p̄p annihilation into light meson pairs, taking into account mesonic and baryonic degrees of freedom, in the energy domain relevant to the PANDA experiment. A logarithmic form factor is introduced to account for the composite nature of the interacting hadrons. A comparison with the existing data for charged pion pair production and predictions for angular distributions and energy dependence are presented in the range 3.362(5) ≤ √s(p L ) ≤ 4.559(10.1) GeV (GeV/c). The model is applied to πp elastic scattering, using crossing symmetry, and to charged kaon pair production, on the basis of SU(3) symmetry. An exponential term is added to reproduce the energy dependence of the total cross section. The model is extended to different neutral channels π0π0, ηη, ηπ0 relying on SU(3) symmetry. The obtained angular distributions and energy dependences are presented and successfully compared to the existing data. In the space-like region, the experimental effort, started years ago at Jefferson Laboratory (Newport News, VA, USA) and dedicated to precise measurements of the proton electromagnetic form factor ratio based on the recoil polarization method in elastic ep scattering, will be pursued up to higher momentum transfer squared, above 10 GeV2, and will be also applied to neutron. This method requires intense and highly polarized electron beams, as well as an accurate measurement of the polarization of the recoil particle, the proton or the neutron. In order to design and optimize the polarimetry in the GeV region, the determination of the efficiency and the analyzing powers, that combine into the figure of merit, is mandatory to conceive a polarimeter, as polarization experiments are very lengthy. In this context I contributed to the ALPOM2 experiment (JINR Dubna, Russia), the only experiment right now able to measure proton and neutron analyzing powers in the momentum range of interest for Jefferson Lab experiments and allowing considering different types of target. Experimentally, in the framework of ALPOM2, from the Nuclotron polarized deuteron beam (up to 13 GeV energy), polarized proton and neutron beams at 3.0, 3.75 and 4.2 GeV/c were obtained by deuteron break-up and the charged fragments from the collisions with C, CH2, CH and Cu targets were measured. For neutrons, in the beam momentum range from 1 to 6 GeV/c, two processes have been considered to determine the analyzing powers: np -> np (zero charge exchanged) and the charge exchange reaction np -> pn. From the comparison of the figures of merit associated to each of these two processes, the charge exchange reaction appears to be more efficient for polarimetry at high energy. Neutron analyzing powers for the charge exchange reaction on CH, and Cu targets have been measured up to 4.2 GeV/c for the first time. For the proton, more precise analyzing power data have been obtained. Experimental results of preliminary analysis are presented.