The goal of the GBAR experiment is to determine the effect of gravity on antihydrogen atoms. The antihydrogen atoms are created by neutralising antihydrogen ions using laser pulses. The antihydrogen ions are produced after two positrons captures by antiprotons flying through a positronium cloud. In this scheme to produce one single antihydrogen atom 10e10 positrons have to be beamed on a nanoporous silica to yield the positronium cloud. The positrons are produced by a 9 MeV LINAC accelerating electrons into a tungsten target equipped with a mesh moderator. In this thesis we have studied and optimised the accumulation and trapping of positrons in two subsequent trapping devices. The LINAC based source providing 3e7 positrons per second, the particles have to be accumulated. They are first accumulated into a Buffer Gas Trap (BGT), a Penning trap, divided in 3 stages, with N2 and CO2, leading to inelastic collisions which insure the trapping and the cooling of the positrons. The positrons are then slowed in the first stage and accumulated in the second stage for 100 ms with a trapping rate of about 1.7e6 positrons per second, then they are transferred into the BGT's third stage. This accumulation and transfer procedure is repeated 10 times to finally provide a bunch of 1.5e7 positrons every 1.1s (a loss happens during this stacking operation and 100 ms are added for a final radial compression using the Rotating Wall technique, the trapping efficiency is then 5%). This new bunch is then ready to be sent and re-trapped into the High Field Trap. The High Field Trap is a 5 T multi-ring Penning trap allowing to trap large amounts of charged particle for hours. We first tested this trap with electrons by trapping about 5e9 of them. The experiments on the electrons lead to the conclusion that a better alignment of the electrodes with respect to the magnetic field still needs to be performed. However, an acceptable situation has been found allowing to re-trap the positrons with 66% efficiency. Then, accumulating the positrons bunches coming from the BGT, it was possible to accumulate 1e9 positrons in 1100. This is a really promising result for the GBAR experiment. For the future, it is about to do 10 times more, 10 times faster to collect the desired amount of positrons each time the ELENA decelerator provides a bunch of antiprotons (every 100 s). We also studied how it could be possible to use antimatter to propel a rocket. Indeed, the energy resulting from the antimatter-matter annihilation reaction has properties defying any other propellant. In our study, we focused on the proton-antiproton annihilation reaction in a high magnetic field in order to have the annihilation products aligned with the direction of the thrust. The theoretical model is named the beam cored engine. A simulator has been developed using GEANT4 to evaluate some parameters such the intensity of the field. According to our simulation, it is then possible to get a rocket with a specific impulse of about 0.5 c/g i.e., 1.5e7 s (with c the speed of light and g the earth's gravitational acceleration), which is outsized if it is compared to the most modern rocket (434 s for Vulcain, propelling Ariane 5). However, this model assumes the capability to produce and store a macroscopic number of antiprotons, which might be an insurmountable showstopper. Also, with this model, a large amount of gamma rays are produced and a solution to evacuate their energy has to be found.
S. Niang, Paris Saclay-University, France (2020). pdf
The main goal of the GBAR collaboration is to measure the Gravitational Behaviour of Antihydrogen at Rest. It is done by measuring the classical free fall of neutral antihydrogen, which is a direct test of the weak equivalence principle for antimatter. The first step of the experiment is to produce the antihydrogen ion and catch it in a Paul trap, where it can be cooled to μK temperature using ground state Raman sideband sympathetic cooling. The μK temperature corresponds to particle velocity in the order of 1 m/s. Once such velocity is reached, the antihydrogen ion can be neutralised and starts to fall. This allows reaching 1 % precision on the measurement of the gravitational acceleration g for antimatter with about 1500 events. Later, it would be possible to reach 10⁻⁵ - 10⁻⁶ precision by measuring the gravitational quantum states of cold antihydrogen. However, in order to measure the free fall, firstly the antihydrogen ion has to be produced. It is formed in the charge exchange reactions between antiproton/antihydrogen and positronium. Positronium and antihydrogen atoms can be either in a ground state or in an excited state. An experimental study of the cross section measurement for these two reactions is described in the presented thesis. The antihydrogen atom and ion production takes place in a cavity. The formation of one antihydrogen ion in one beam crossing requires about 5x10⁶ antiprotons/bunch and a few 10¹¹ Ps/cm⁻³ positronium density inside the cavity, which is produced with a beam containing 5x10¹⁰ positrons per bunch. The production of such intense beams with required properties is a challenging task. First, the development of the positron source is described. The GBAR positron source is based on a 9 MeV linear electron accelerator. The relatively low energy was chosen to avoid activation of the environment. The electron beam is incident on a tungsten target where positrons are created from Bremsstrahlung radiation (gammas) through the pair creation process. Some of the created positrons undergo a further diffusion in the tungsten moderator reducing their energy to about 3 eV. The particles are re-accelerated to about 53 eV energy and are adiabatically transported to the next stage of the experiment. Presently, the measured positron flux is at the level of 6x10⁷ e⁺/s, which is a few times higher than intensities reached with radioactive sources. Then, the thesis features a short description of the antiproton/proton beam preparations, finalised with a chapter about the expected antihydrogen atom and ion production yield. After the reaction, antiproton, antihydrogen atom, and ion beams are guided to the detection system. It is made to allow for detection from 1 to a few thousand antihydrogen atoms, a single antihydrogen ion and all 5x10⁶ antiprotons. It is especially challenging because antiproton annihilation creates a lot of secondary particles which may disturb measurements of single antihydrogen atoms and ions. The main part of the Thesis is the description of the expected background for the antihydrogen atom and ion detection. Additionally, the detection system allows measuring the cross sections for the symmetric reactions of a hydrogen atom and ion production through charge exchange between protons and positronium. The antihydrogen ion production part of the experiment was fully installed at CERN in 2018. The first tests with antiprotons from the ELENA decelerator were done. Currently, the experiment is being commissioned with positrons and protons, in order to perform the hydrogen atom and ion formation. The optimisation of the ion production with matter will help to be fully prepared for the next antiproton beam time in 2021.
B. Latacz, Paris Saclay-University, France (2019). pdf
The GBAR experiment aims to perform the first free-fall experiment with antihydrogen atoms to test the weak equivalence principle with antimatter. In a first step, antihydrogen ions are synthesized through a double charge exchange reaction and then cooled by laser to reach sufficiently low energies to observe the effects of gravity. The synthesis of anti-ions requires a very large number of antiprotons. We report in this manuscript a new electrostatic instrument for decelerating antiproton bunches from 100 keV to a few keV . This new technique aims to avoid losses caused by the use of degrader foils. Simulations based on a genetic algorithm made it possible to establish the most suitable electrostatic configurations. Preliminary tests carried out on a prototype are discussed. The technical characteristics of the deceleration system, particularly those related to UHV vacuum, high voltage and safety, are described. Finally, a secondary optical systems such as a switchyard or injection optics into a Penning trap are studied.
A. Husson, Paris Saclay-University, France (2018). pdf
The experiments NA64 and GBAR aiming to explore the still unanswered questions in physics of the existence of dark matter and the matter- antimatter asymmetry, respectively, are presented in the scope of this thesis. NA64 is an experiment at CERN searching for a new U0(1) gauge boson, A0 (dark photon) which may mediate the interaction of dark matter with ordinary matter via a very weak force. The experiment is sensitive to the still unexplored area of gamma-A0 mixing strength 10^-5 < e < 10^-3 and masses MA0 <= 100 MeV. The results from the first beam run are reported and new limits were set on the gamma-A0 mixing strength and the results exclude the invisibly decaying A0 with a mass <= 100 MeV as an explanation for the (g-2)µ anomaly. GBAR is an experiment set up at the AD hall at CERN aiming to measure the free-fall of anti-hydrogen with a relative precision of 1 % in the fi rst phase for a direct test of the equivalence principle for anti-matter. The experiment plans to use the ELENA anti-proton beam to produce H(bar)+ ions from its interaction with positronium, cool the ions down to 10 µK temperature and eventually detect the free-fall of H(bar) after photo-detaching the excess positron from the ion. The signal of detection is given by its annihilation producing pion tracks. The experiment is being set up at CERN and is expected to start a commissioning run in 2018. For both experiments a tracker is essential - NA64 requires precise tracking of the incoming particles to reconstruct their momentum and suppress background from the low energy beam tail. In GBAR tracking is required to track the pion tracks and reconstruct the vertex of the H(bar) annihilation and reject cosmic ray background. Multiplexed XY Resistive Micromegas modules chosen for the tracking requirements of both the experiments are presented. The use of multiplexed modules in high intensity environments was not explored so far, due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. The first performance results of multiplexed modules tested at the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 x10^5 e-/sec/cm2 is reported. At these rates, a factor 5 multiplexing introduces more than 50 % level of ambiguity. The results prove that by using the additional information of cluster size and integrated charge of the induced XY signal clusters the ambiguities can be reduced to a level below 2%. The expected performances of the GBAR Micromegas tracker is also summarized from the simulation of H(bar) annihilation done with Geant4, taking into account theinitial parameters of the atom, geometric acceptance and intrinsic resolution of tracker modules. The resolution of vertex reconstruction and estimation of the background rejection is also presented.
D. Banerjee, ETH Zurich, Switzerland (2017). pdf
The GBAR experiment relies on the production of antihydrogen positive ions to achieve its goal of measuring the gravitational acceleration of antimatter at rest. The ANTION project, included in the GBAR enterprise, is responsible for the production of these antimatter ions. Moreover, it also aims to measure the cross section of antihydrogen production throughout the collision of antiprotons and positronium atoms, as well as the matter cross sections of hydrogen and the hydrogen negative ion. These experiments imply the formation of a very dense positronium cloud, thus a large amount of positrons will be implanted on a positron/positronium converter material. This thesis reports the construction of a three stage buffer gas trap with the goal of trapping and accumulating positrons for the ANTION project. The combination of the Penning-type trap with a LINAC source constitutes a unique experimental setup. The trap was commissioned and optimized and is now fully operational. Trapping protocols were studied and the effect of the buffer and cooling gases on the positron trapping rate and lifetime was assessed. In order to assist the cross section measurement of hydrogen, a GEANT4 simulation was developed. It evaluates the time and spatial evolution of the ortho-positronium atoms in a cavity, where hydrogen production will take place. It was estimated that 2.7 hydrogen atoms are produced for proton impact energy of ~ 6keV, according to the cross sections computed with the Coulomb-Born Approximation model, and 1.6 hydrogen atoms for a proton impact energy of ~ 10keV, according to the two-center convergent close-coupling method. The simulations also allow the estimation of the background associated with the positron and para-positronium decay. In addition, a suggestion is proposed to increase the number of positronium atoms in the cavity. In parallel, the positron moderation efficiency of a commercially available 4H-SiC epitaxial layer was studied. A 65% moderation efficiency was observed for kiloelectronvolt implanted positrons. This result can be of interest to slow positron physics experiments by improving the brightness of positron beams, and in particular to GBAR as it can potentially increase the efficiency of positron trapping.
A. Leite, Paris-Saclay University, France (2017). pdf
This thesis is dedicated to cross section calculations involving the three body system (e-, e+, p) 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-Merkuriev 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.
M. Valdes Dupuy, Université de Strasbourg, France (2017). pdf
Etude et réalisation d’un faisceau de positons lents
This research thesis first proposes a presentation of the GBAR project (Gravitational Behaviour of Anti-hydrogen at Rest) within which this research took place, and which aims at performing the first direct test of the Weak Equivalence Principle on anti-matter by studying the free fall of anti-hydrogen atoms in the Earth gravitational field. The author presents different aspects of this project: scientific objective, experiment principle and structure, detailed structure (positron beam, positron trap, positron/positronium conversion, anti-proton beam, trapping, slowing down and neutralisation of anti-hydrogen ions). The author then reports the design of the positron beam: study of source technology, studies related to the fast positron source, design of the low positron line (approach, functions, simulations, technology). The two last chapters report the construction and the characterization of the slow-positron line.
N. Ruiz, Université Pierre et Marie Curie, Paris, France (2011). pdf
Piégeage de positons dans un piège de Penning Malmberg, en vue de leur accumulation avec un faisceau pulsé
The weak equivalence principle, a fundament of Einstein general relativity, states that gravitational mass and inertial mass are equal whatever the body. This equivalence principle has never been directly tested with antimatter. The GBAR (Gravitational Behaviour of Antimatter at Rest) experiment intends to test it by measuring the acceleration of ultra cold anti-hydrogens in free fall. The production of such anti-atoms requires a pulse of about 1010 positrons in a few tens of nanoseconds. This thesis focuses on the development of a new accumulation technique of positrons in a Penning-Malmberg trap in order to create this pulse. This new method is an improvement of the accumulation technique of Oshima et al.. This technique requires a non-neutral electron plasma to cool down positrons in the trap in order to confine them. A continuous beam delivers positrons and the trapping efficiency is about 0.4%. The new method needs a positron pulsed beam and the method efficiency is estimated at 80%. A part of this thesis was performed at Riken (Tokyo) on the trap of Oshima et al. to study the behavior of non-neutral plasmas in this type of trap and the first accumulation method. A theoretical model was developed to simulate the positron trapping efficiency. The description and the systematic study of the new accumulation technique with a pulsed positron beam are presented. They includes notably the optimization through simulation of the electromagnetic configuration of the trap and of the parameters of the used non-neutral plasmas.
P. Dupré, Université Pierre et Marie Curie, Paris, France (2011). pdf
Scattering of ultracold atoms from material surfaces is characterized by the reflection of the atomic matter wave from the attractive Casimir-Polder potential. In this thesis, the reflection probability is computed, taking into account the optical response of the material medium. This reflection probability is shown to be enhanced for slow atoms and weak potentials. A Liouville transformation of the Schrödinger equation is used show that scattering from a potential well can be reinterpreted as a collision with a potential barrier. This approach highlights the link between quantum reflection and the breakdown of the semiclassical approximation and clarifies the dependence of the reflection probability on the energy and potential strength. Our results have implications for the GBAR project at CERN, which will time the free fall of a cold antihydrogen atom onto a detector. We analyze the effect of quantum reflection on the GBAR detection scheme and propose to use quantum reflection to improve the accuracy of equivalence principle tests with antimatter.
G. Dufour, Université Paris VI, Pierre et Marie Curie, France (2015). pdf
The future CERN experiment called GBAR intends to measure the gravitational acceleration of antimatter on Earth using cold (neV) antihydrogen atoms undergoing a free fall. The experiment scheme first needs to cool antihydrogen positive ions, obtained thanks to two consecutive reactions occurring when an antiproton beam collides with a dense positronium cloud. The present thesis studies these two reactions in order to optimise the production of the anti-ions. The total cross sections of both reactions have been computed in the framework of a perturbation theory model (Continuum Distorted Wave ñ Final State), in the range 0 to 30 keV antiproton kinetic energy; several excited states of positronium have been investigated. These cross sections have then been integrated to a simulation of the interaction zone where antiprotons collide with positronium; the aim is to find the optimal experimental parameters for GBAR. The results suggest that the 2P, 3D or, to a lower extend, 1S states of positronium should be used, respectively with 2, less than 1 or 6 keV antiprotons. The importance of using short pulses of antiprotons has been underlined; the positronium will have to be confined in a tube of 20 mm length and 1 mm diameter.In the prospect of exciting the 1S-3D two-photon transition in positronium at 410 nm, a pulsed laser system had already been designed. It consists in the frequency doubling of an 820 nm pulsed titanium-sapphire laser. The last part of the thesis has been dedicated to the realisation of this laser system, which delivers short pulses (9 ns) of 4 mJ energy at 820 nm.
P. Comini, Université Paris VI, Pierre et Marie Curie, France (2014). pdf
The Gravitational Behaviour of Antihydrogen at Rest experiment - GBAR - is designed to perform a direct measurement of the weak equivalence principle on antimatter by measuring the acceleration (gbar) of antihydrogen atoms in free fall. Its originality is to produce H and H+ ions and use sympathetic cooling to achieve µK temperature. H and H+ ions are produced by the reactions : p + Ps → H and H + e-, and H + Ps → H and H+ + e-, where pbar is an antiproton, Ps stands for positronium (the bound-state of a positron and an electron), H and H is the antihydrogen and H and H+ the antiion associated. To produce enough Ps atoms, 2x1010 positrons must be impinged on a porous SiO2 target within 100ns. Such an intense flux requires the accumulation (collection and cooling) of the positrons in a particle trap. This thesis describes the injector being commissioned at CEA Saclay for GBAR. It consists of a Penning-Malmberg trap (moved from RIKEN) fed by a slow positron beam. A 4.3MeV linear accelerator shooting electrons on a tungsten target produces the pulsed positron beam, which is moderated by a multi-grid tungsten moderator. The slow positron flux is 104 e+/pulse, or 2x106 e+/s at 200Hz. This work presents the first ever accumulation of low-energy positrons produced by an accelerator (rather than a radioactive source) and their cooling by a prepared reservoir of 2x1010 cold electrons.
P. Grandemange, Université Paris Sud, Paris XI, France (2013). pdf