No description available.

Ma thèse s’est déroulée dans le cadre de la collaboration AEgIS, une des expériences étudiant l’antimatière au CERN. L’objectif final est de mesurer l’effet de la gravité sur un faisceau froid d’antihydrogène (Hbar). AEgIS se propose de créer les Hbar froids par échange de charges entre un atome de Positronium (Ps) excité (état de Rydberg) et un antiproton piégé : 〖Ps〗^*+ pbar → (H^*)⁻ + e⁻. L’étude de la physique du Ps est cruciale pour AEgIS, et demande des systèmes lasers adaptés. Pendant ma thèse, ma première tâche a été de veiller au bon fonctionnement des systèmes lasers de l’expérience. Afin d’exciter le positronium jusqu’à ses états de Rydberg (≃20) en présence d’un fort champ magnétique (1 T), deux lasers pulsés spectralement larges ont été spécialement conçu. Nous avons réalisé la première excitation par laser du Ps dans son niveau n=3, et prouvé une excitation efficace du nuage de Ps vers les niveaux de Rydberg n=16-17. Ces mesures, réalisées dans la chambre à vide de test d’AEgIS, à température ambiance et pour un faible champ magnétique environnant, sont la première étape vers la formation d’antihydrogène. Le prochain objectif est de répéter ces résultats dans l’enceinte du piège à 1 T, où les antihydrogènes seront formés. Pour autant, malgré l’excitation Rydberg des Ps pour accroître la section efficace de collision, la production d’antihydrogène restera faible, et la température des H bar formés sera trop élevée pour toute mesure de gravité. Pendant ma thèse, j’ai installé au CERN un autre système laser prévu pour pratiquer une spectroscopie précise des niveaux de Rydberg du Ps. Ce système excite des transitions optiques qui pourraient convenir à un refroidissement Doppler : la transition n=1 ↔ n=2. J’ai étudié la possibilité d’un tel refroidissement, en procédant à des simulations poussées pour déterminer les caractéristiques d’un système laser adapté La focalisation du nuage de Ps grâce au refroidissement des vitesses transverses devrait accroitre le recouvrement des positroniums avec les antiprotons piégés, et ainsi augmenter grandement la production d’Hbar. Le contrôle du refroidissement et de la compression du plasma d’antiprotons est aussi essentiel pour la formation des antihydrogènes. Pendant les temps de faisceaux d’antiprotons de 2014 et 2015, j’ai contribué à la caractérisation et l’optimisation des procédures pour attraper et manipuler les antiprotons, afin d’atteindre des plasmas très denses, et ce, de façon reproductible. Enfin, j’ai participé activement à l’élaboration d’autre projet à l’étude AEgIS, qui vise aussi à augmenter la production d’antihydrogène : le projet d’un refroidissement sympathique des antiprotons, en utilisant un plasma d’anions refroidis par laser. J’ai étudié la possibilité de refroidir l’ion moléculaire C₂⁻, et les résultats de simulations sont encourageants. Nous sommes actuellement en train de développer au CERN le système expérimental qui nous permettra de faire les premiers tests de refroidissement sur le C₂⁻. Si couronné de succès, ce projet ne sera pas seulement le premier résultat de refroidissement par laser d’anions, mais ouvrira aussi les portes à une production efficace d’antihydrogènes froids. / My Ph.D project took place within the AEgIS collaboration, one of the antimatter experiments at the CERN. The final goal of the experiment is to perform a gravity test on a cold antihydrogen (Hbar) beam. AEgIS proposes to create such a cold Hbar beam based on a charge exchange reaction between excited Rydberg Positronium (Ps) and cold trapped antiprotons: 〖Ps〗^* + pbar → (H^*)⁻ + e⁻. Studying the Ps physics is crucial for the experiment, and requires adapted lasers systems. During this Ph.D, my primary undertaking was the responsibility for the laser systems in AEgIS. To excite Ps atom up to its Rydberg states (≃20) in presence of a high magnetic field (1 T), two broadband pulsed lasers have been developed. We realized the first laser excitation of the Ps into the n=3 level, and demonstrated an efficient optical path to reach the Rydberg state n=16-17. These results, obtained in the vacuum test chamber and in absence of strong magnetic field, reach a milestone toward the formation of antihydrogen in AEgIS, and the immediate next step for us is to excite Ps atoms inside our 1 T trapping apparatus, where the formation of antihydrogen will take place. However, even once this next step will be successful, the production rate of antihydrogen atoms will nevertheless be very low, and their temperature much higher than could be wished. During my Ph.D, I have installed further excitation lasers, foreseen to perform fine spectroscopy on Ps atoms and that excite optical transitions suitable for a possible Doppler cooling. I have carried out theoretical studies and simulations to determine the proper characteristics required for a cooling laser system. The transverse laser cooling of the Ps beam will enhance the overlap between the trapped antiprotons plasma and the Ps beam during the charge-exchange process, and therefore drastically improve the production rate of antihydrogen. The control of the compression and cooling of the antiproton plasma is also crucial for the antihydrogen formation. During the beam-times of 2014 and 2015, I participated in the characterization and optimization our catching and manipulation procedures to reach highly compressed antiproton plasma, in repeatable conditions. Another project in AEgIS I took part aims to improve the formation rate of ultracold antihydrogen, by studying the possibility of a sympathetically cooling of the antiprotons using a laser-cooled anion plasma. I investigated some laser cooling schemes on the C₂⁻ molecular anions, and the simulations are promising. I actively contribute to the commissioning of the test apparatus at CERN to carry on the trials of laser cooling on the C₂⁻ species. If successful, this result will not only be the first cooling of anions by laser, but will open the way to a highly efficient production of ultracold antihydrogen atoms.

La mesure récente de l'angle de mélange theta-13, à laquelle l'expérience Double Chooz contribue, a ouvert la voie aux futures expériences de la physique des neutrinos. Dans ce manuscrit, la caractérisation de certains bruits de l'expérience sont décrits. Les muons cosmiques qui s'arrêtent et se désintègrent dans le détecteur sont mal reconstruits, résultant en distorsion de la distribution temporelle des signaux laquelle peut être utilisée pour identifier ce type de fond. Les neutrons rapides créés par spallation par les muons cosmiques produisent de nombreux protons de recul qui peuvent entraîner un décalage dans la distribution temporelle des signaux et ainsi être identifiés. Ces distributions temporelles ont aussi été utilisées pour identifier la formation de l'état d'orthopositronium en observant et en mesurant un délai entre l'ionisation du positron et l'annihilation de celui-ci, pouvant permettre une séparation positron-électron. / The measurement of the theta-13 mixing angle, to which the Double Chooz experiment contributed, paves the way to future findings in neutrino physics. In this manuscript, we describe the characterization of some Double Chooz backgrounds. Cosmic muons that stop and decay in the detector are characterized by anisotropic emission of the scintillation light, causing the vertex to be poorly reconstructed. The resulting pulse shape distortion can be used to tag and remove such background. Fast spallation neutrons producing multiple recoil protons may produce a similar distortion in the pulse shape and can also be tagged. Pulse shapes are also used to identify the formation of ortho-positronium. The tagging of such electron-positron bound state is made possible by the induced distortion in the pulse shape due to the delay in the positron annihilation, and can be used for an electron-positron separation.

No description available.

AEgIS is an experiment at CERN where the goal is to directly measure the gravitational force on antimatter by producing antihydrogen. The antihydrogen will be produced by a charge exchange reaction using laser excited positronium and cold antiprotons. Having a well-characterized positron plasma with at least 108 positrons and knowing how it can be controlled is essential for the positronium production. This thesis is based on the goals of AEgIS experiment and describes the positron plasma manipulations being used in AEgIS in order to achieve the required plasma properties for the experiment. The positron system is made up by a source, a Surko trap and a Penning-Malmberg trap. This system was first optimized to increase the number of positrons. The plasma was then moved to the main traps of the experiment where it was systematically characterized in terms of lifetime, cooling efficiency and compression. Positron plasma compression in time, trapping and cooling was tested for the first time in AEgIS using a buncher and Penning-Malmberg traps respectively. In this thesis, it is shown that a compression of more than 50 % in time of the positron cloud using a buncher can be achieved. It is also shown that trapping and cooling with an efficiency of nearly 100 % in the main traps using a “V” shaped potential trap was successful. On top of that, the lifetime inside this “V” shaped potential trap was observed to be longer than 30 minutes.

The production and control of antihydrogen at very low temperatures provided a key tool to test the validity for the antimaterial of the fundamental principles of the interactions of nature such as the weak principle of equivalence (WEP), and CPT symmetry (Charge, Parity, and Time reversal). The work presented in this dissertation studies the collisions of electrons and positrons in strong magnetic fields that generate magnetobound positronium (positron-electron system temporarily bound due to the presence of a magnetic field) and its possible role in the generation of antihydrogen.

No description available.

The positron is the antiparticle of the electron. In general, for any particle there exists a corresponding antiparticle. The two are identical except for the charges, i.e. electric charge, leptonic number, muonic number, ..., which are equal in module but opposite sign. Examples are the electron and the positron, the first has negative electric charge, while the second has positive charge. Similarly for proton, which is positive charged, and the antiproton, which is negative charged. In the case of photons, they are their own antiparticle. When a particle and its antiparticle interact, they are destroyed in a process called annihilation which converts all their mass into energy following Einstein’s equation E=mc2. The inverse is also possible, a high-energy event creates a particle-antiparticle couple, this is called pair production. Needless to say, the products have total mass less than the one corresponding to initial energy from Einstein’s equation. For this reason, from an annihilation event, a cascade of particle-antiparticle pairs is generated, the mass of the created particle and antiparticle is less than the sum of the mass of the original particles. Still, in the annihilation process, the momentum and angular momentum of the initial particle-antiparticle system is conserved. The annihilation of a stationary positron-electron pair generates two photons. Due to the conservation of momentum, the two photons are emitted in opposite direction both with 511keV energy. The direction of emission is random. Due to their light mass, few MeV gamma-rays are capable of producing positron with pair production. In fact, the positrons are the most available antiparticle in the universe, the characteristic 511keV annihilation photons have been observed in active galactic nuclei [1], in the sun [2], and even in thunderstorm clouds on Earth [3]. The antiparticles are not easily available, the observable universe is mainly composed of matter, so any interaction would result in the annihilation. From here one of the main unanswered questions of modern physics: given the big bang was a high energy event, it should have generated matter and antimatter in equal quantity, however this symmetry is not observed in the universe around us. High-energy photons capable to produce positron-electron pairs can be generated in a controlled environment here on Earth with the use of LINACs [4] or nuclear reactor [5]. Moreover, positrons can be generated by radioisotope decay. The β^+ decay transforms a proton in a neutron in the atom nucleus, the process frees a positron, other than an electronic neutrino. This makes positrons the easiest available antiparticle and the first to be discovered and studied.In the 1920s, special relativity and quantum mechanics were two of the pillars of modern physics. One of the first attempts to combine the two was Dirac’s equation [6]. Dirac tried to explain the behavior of spin one-half particles like the electron when moving at relativistic speed, however the resulting equation admits free-particle solutions with positive and negative energies. Obviously, negative energies are not physically possible. An explanation proposed by Dirac involved a sea of particle [7]. The positive energy solution of the equation represents a particle excited from the sea, the hole left by this process corresponds to the negative energy solution. Then for any particle there is a correspond hole, called antiparticle. In the 1930s, Anderson was studying the behavior of cosmic rays interacting with a cloud chamber in the presence of a magnetic field [8]. Between the photographed particles, he demonstrated for the first time the existence if a particle with mass and charge equal in absolute value to the electron but positively charged. He called this particle positron, following studies confirmed the positron is the antiparticle of the electron. Nowadays, the positrons have found two main applications: in the medical field and in material studies. In medicine, β^+ radioisotopes are used as tracer for the individuation of cancer in patient with Positron Emission Tomography (PET). By detecting the two counterpropagating annihilation gamma-rays, it is possible to reconstruct the annihilation spot, and so the distribution of the absorption of the molecules with the radioisotope in the body. Cancerous cells have a different metabolism with respect normal cell, so their absorption of particular molecules is amplified. By selecting the correct molecular vector and radioisotope, the area in the patient body affected by the cancer is highlighted by the PET. In the case of material science, positrons are implanted in the material with energies up to tens of kiloelectronvolts. Interacting with the material, the positrons lose energy and diffuse in the material surrounding few tens of nanometers until they annihilate with an electron in the material, or they escape from the material surface. This makes the positrons a good probe because the information on the electrons transmitted outside the material by the annihilation gammas. How much time the positrons live in the material depends on the electron density. The presence of defects in the atomic structures creates spaces with lower electron density where the positron can live longer. This is studied with the Positron Annihilation Lifetime Spectroscopy (PALS). Because the positrons are generally consider thermalized at the annihilation, they have much less energy than the electron in the material. Then we can obtain information on the electron from any deviation in the direction and energy of the two annihilation photons form the case of stationary particles. The deviation in direction of the two photons is studied with Angular Correlation Annihilation Radiation (ACAR), the annihilation gammas energy with Doppler Broadening Spectroscopy (DBS). From the study of positron interaction with the matter, the bound state of the positron and the electron was discovered for the first time in the 1950s [9]. This bound state is called positronium (Ps) and it is the lightest bound matter-antimatter system. It is a hydrogen-like atom, with the positron substituting for the proton, this gives it particular properties [10]. The Ps is not stable, and, in its ground level, it is divided based on the total spin S in para- (S=0) and ortho- (S=1) positronium (p- and o- Ps, respectively) with different behaviors. Para-positronium is in a singlet spin state S=0 and m=0, where m is the projection of the spin on the z-axis. It tends to annihilate in two counterpropagating photons with 511keV energy and it has a vacuum lifetime of 125ps. Ortho-positronium corresponds to the triplet of spin states S=1 and m=-1, 0, +1. It annihilates mainly into three gamma-rays with a lifetime of 142ns in vacuum. This longer lifetime makes it possible to manipulate the o-Ps level with laser excitation [10], bringing it in longer lived levels for the study of its properties. In both the case of p-Ps and o-Ps, the conservation of energy and momentum in the annihilation fixes the gamma-rays direction and energy. For p-Ps like for free positrons, the two photons have a fixed energy and direction of one respect to the other, however the emission direction is random. The three photons resulting from the annihilation of o-Ps are emitted on a plane, called annihilation plane, with a wide range of energies and direction, the inclination of the annihilation plane is randomly distributed. In this discussion, we did not consider the conservation of the angular momentum in the annihilation process. This brings a constrain in the direction of polarization of the annihilation gamma-rays. For positronium in the ground level, the total angular momentum is given by the spin. For para-positronium and free positrons, the spin conservation means the two photons are entangled in the polarization state: the polarization of a gamma-ray is orthogonal to the other [11,12]. For ortho-positronium, the three gamma polarizations are genuinely multiparticle entangled, however the exact entanglement state depends on the emission direction of the three [13]. The correlation in the annihilation radiation of the two annihilation gammas was first experimentally studied in the 1940s [14–16]. Only a decade later, the experimental results demonstrated for the first time the existence of entanglement [17]. The entanglement in the case of three gammas has not yet been experimentally demonstrated. This is due to the complexity in the realization of a detector capable of measuring the polarization of three high-energy photon at the same time and of a source of positronium in a spin selected state in a free-field environment. This work thesis is centered on the design and study of an apparatus with the objective of study the entanglement of gamma-rays polarization generated by the annihilation of ortho-positronium. This apparatus is called PSICO (Positronium Inertial and Correlation Observations) apparatus, and it is under construction at the Antimatter laboratory (AML) of the University of Trento.At the center of the PSICO apparatus is the realization of a dense bunched positron beam capable of implanting the positrons in a positron/positronium converter in a field-free region with energy up to tens of kiloelectronvolt. No other positron beamline in the literature satisfies all these requirements. The creation of the PSICO positron beamline is based on four steps: the creation of a monoenergetic continuous positron beam, the trapping of the positrons in a buffer-gas trap (BGT) [18,19] and the generation of a dense bunched beam, the extraction of the bunched beam from the magnetic field of the trap, the acceleration, time-compression, and focalization of the positron bunches into a target in a free field region. To each step corresponds a part of the PSICO positron beamline. During this thesis work all fours parts have been designed, the last three parts are now under construction, the first part has been completed and commissioned. The creation of the monoenergetic continuous positron beam in the first step of the PSICO apparatus requires a radioactive source, a moderator, and a magnetic transport system. The design of this part of the apparatus is based on a commercial solution from First Point Scientific [20]. The source generates continuously positrons, which are emitted with a wide energy distribution. So, a moderator is required for the creation of a monoenergetic beam. The transport system guides the beam to the next section while eliminating the non-moderated positrons that are anyway emitted from the moderator. In the implementation, a sodium-22 source is used for its long half-life of 2.6 years. A solid noble gas moderator is used for the moderation process due to its high efficiency [21]. The design of the magnetic transport is based on raytracing simulation in order to optimize the speed-selection and transport of the positrons from the moderator with the minimal number of magnets. Once constructed, the first part of the PSICO apparatus can generate a continuous positron beam with up to 50 000 positrons per second with a total efficiency from the source to the end of the magnetic transport higher than 0.15%. Three solid noble gas moderators were tested: Neon, Argon, and Krypton. The advantages and disadvantages of the moderator realized with the three gases have been studied. The commissioning of the continuous positron beam has been completed by measuring the dimension of the beam spot, energy distribution, and polarization at the end of the magnetic transport system for the three gases. The measured beam dimensions are compatible with the transport simulation at the same position. The second part of the PSICO apparatus consists of the buffer-gas trap. The BGT is a modified Penning trap, where a 700G magnetic field confines radially the positrons which are accumulated in an electrostatic potential well along the magnet axis. When the positrons enter the trap, they are cooled by inelastic scattering with gas introduced into the chamber until they fall to the bottom of the potential well. The design of the PSICO BGT is based on a commercial design from First Point Scientific with changes in the magnet and terminal electron for the optimal release of the positron bunches from the BGT and their extraction from the trap magnetic field in the third stage of the PSICO beamline. From the simulation with the new trap design, the positron bunches are formed in a region with a field homogeneity ΔB\/B better than 0.1%, and the bunches are released from the trap with a temporal width less than 5ns [22].The extraction of magnetic field is done in the third step of the PSICO apparatus immediately after the trap. In the literature, a few configurations for the magnetic field extraction of positrons from the BGT have been proposed and implemented [23–25]. However, the present design is the first one where the positron extraction from the magnetic field is performed directly at the exit of the BGT. According to the simulations of our design, 60% of the positron can be extracted from the magnetic field of the trap [22].In order to perform the fourth step, a buncher-elevator followed by four lenses has been designed. After the extraction from the BGT, the positrons enter the buncher-elevator whose potential is shaped in 5.5 ns [26]. The final potential is given by a constant value superimposed by a parabolic potential. The parabolic potential has a height of 1kV at the start of the buncher-elevator and the vertex at its end, it is required for the time compression of the bunch. The constant potential value gives, instead, the majority of the implantation energy to the positrons and it can reach up to 21kV [22]. The positron bunch is then focused by the last four lenses onto the target in a spot smaller than 5mm in diameter and temporal width lower than 2ns. This will be the first positron beam from a BGT capable to operate at high implantation energy with a target in a field-free region. These four parts complete the dense bunched positron beam. For the study of the entanglement in the polarization of the o-Ps annihilation photons, a good positron/positronium converter is needed. In the literature, there are example of good converters [27,28], however they work in reflection geometry, i.e. they emit Ps on the same side where positrons are implanted. This presents some limitation in the manipulation and study of positronium in vacuum. Positron/positron converter capable of emitting Ps on the other side of the implantation can be found in the literature, however their efficiency is low [29]. So, a new kind of converter has been studied for the production of positronium in transmission [30]. It consists of silicon membranes of few microns of thickness where pass through nanochannels have been electrochemically etched. This kind of converters have shown a conversion efficiency of at least 16%. The last element needed for the entanglement measurement is the detector. This needs to be capable of measuring the polarization of the gamma-rays at the same time. The only way of measuring the polarization of hundreds of kiloelectronvolt photons is by applying the Compton scattering. For each annihilation gamma, the detector records the electron scattered from the Compton scattering and the scattered photons. The direction of the scattered photon depends on the polarization of the annihilation gammas. To reconstruct the three polarizations, the detector records six events, this requires a complex detection system. After an extensive study of literature, we are oriented into the use of modules of plastic scintillators originally developed for PET measurements by a group of the Jagiellonian University (Krakow, Poland) [31]. Two modules have been tested with the continuous positron beam from the first part of the apparatus. Just two modules were enough to reconstruct the beam spot with an uncertainty of few centimeters [31]. Using more modules, a higher precision can be obtained. Thanks to all the work done in the design, testing, and implementation of the different components of the PSICO apparatus it makes possible in the near future the realization of the first test of entanglement in the polarization of the three gamma-rays generated by annihilation of ortho-positronium.

Cette thèse est consacrée au calcul de sections efficaces de réactions impliquant le système à trois corps (e − , e + , p̄) à des énergies représentatives de l’expérience GBAR. Deux approches théoriques ont été utilisées. La première, appelée méthode des canaux couplés, permet de traiter le système dans un cadre théorique plus simple. La deuxième, basée sur le formalisme rigoureux des équations de Faddeev-Merkuriev, a permis le calcul explicite des sections efficaces. Une des difficultés majeures provient de la dégénérescence accidentelle du premier état excité des atomes d’antihydrogène et de positronium. Le traitement de cette dégénérescence a été réalisé dans un premier temps dans le formalisme de canaux couplés avant d’être adapté au code des équations de Faddeev-Merkuriev. Dans ce document, nous discutons les sections efficaces dans le contexte de l’expérience GBAR et interprétons les phénomènes résonnants mis en évidence, les résonances de Feshbach et les oscillations de Gailitis-Damburg. / This thesis is dedicated to cross section calculations involving the three body system (e − , e + , p̄) at representative energies for the GBAR experiment. Two different theoretical formalisms have been used. The first one, the close coupling method, allows to study the system in a more simple and schematic theoretical frame. The second, based on the mathematically rigorous formalism of the Faddeev-Merkuriev equations, is used to compute the explicit cross sections. One of the major difficulties comes from the accidental degeneracy of the antihydrogen and positronium atoms first excited states. The treatment of this degeneracy has been realised, in a first time, with the close-coupling formalism before being adapted to the Faddeev-Merquriev equations code. In this document, we discuss the cross sections in the GBAR experiment frame and we construe the highlighted resonant phenomena, the Feshbach resonances and the Gailitis-Damburg oscillations.

A minimum-norm method has been developed for solving the coupled integro-differential equations describing the scattering of positrons by one-electron targets in which the rearrangement channels for positronium formation have been explicitly included. The minimum-norm method, applied to this application for the first time in this thesis, is an enhancement of a previously reported least-squares method which has enabled the extension to a significantly larger basis consisting of up to 26 states on the direct centre, including pseudostates, and 3 states on the positronium. The method has been applied here to e+-H and e+-He+ scattering; cross-sections have been produced for the latter over a range of energies up to 250 eV. The basis was found to be large enough to produce smooth cross sections and little evidence of pseudoresonance structure was found. The results are the first converged cross sections to be calculated for e+-He+ scattering using the coupled channel approximation. Results for e+-H scattering compare well with the work of other authors. A highly efficient parallel code was developed for solving the largest coupling cases. The results prove the minimum-norm approach to be an accurate and reliable method for large-scale coupled channel calculations involving rearrangement collisions. Also in this thesis, the capture of slow antiprotons by atomic hydrogen and positronium has been simulated by the Classical Trajectory Monte Carlo (CTMC) method. Statistically accurate cross sections for protonium and antihydrogen formation have been obtained and the energy dependence of the process established. Antihydrogen formation from antiproton collisions with positronium in the presence of a laser has also been simulated with the CTMC method and the effects of laser polarisation, frequency and intensity studied. Enhancements of the antihydrogen formation cross section were observed and it is suggested that more sophisticated calculations should be undertaken