Synthesis of antihydrogen from in‐flight charge exchange of decelerated antiprotons in positronium for the GBAR experiment

The GBAR experiment aims at measuring the gravitational acceleration of antimatter via the free fall of an antihydrogen atom. This is a test of the Weak Equivalence Principle which, although well verified at a precision level of 10⁻¹⁵ for matter, never had any conclusive experimental test with antimatter. The first step is the production of an antihydrogen ion which can be cooled down to μK temperatures. Then the additional charge is photodetached and one can observe the free fall of a neutral antihydrogen atom - "at rest" - under gravity only. The main challenge of GBAR is the production of the anti-ion from a double charge-exchange reaction of antiprotons on positronium Ps (a bound state of a positron and an electron). In the first reaction, an antiproton binds with a positron from positronium, to form an antihydrogen at a few keV. During the second reaction, the new antihydrogen atom reacts again with a positronium atom, to form a positive antihydrogen ion composed of 2 positrons and 1 antiproton. The first reaction, producing antihydrogen at low excitation states from impact of a 6 keV antiproton beam on a positronium cloud, is demonstrated in this thesis. The specific "in-flight production" scheme and the experimental setup used for data taking are described. From theoretical works, the optimal energy for the anti-ion production, considering the conjugation of both reactions' cross-sections, is around 6 keV. To further decelerate the 100 keV antiprotons delivered by the ELENA ring at CERN, GBAR developed a linear decelerator using a pulsed drift tube. This allows a very high efficiency compared to the commonly used degrader foils. This device was largely revised during this thesis. Various problems encountered and the developed solutions are described. A complete simulation of the GBAR antiproton line has been developed within the SIMION software and is presented here. The optimization of the line to transport the decelerated antiproton beam up to the reaction chamber where they mix with positronium is also detailed. Finally, the question of the cross-section for the antihydrogen synthesis reaction is discussed.