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We report on the first production of an antihydrogen beam by charge exchange of 6.1 keV antiprotons with a cloud of positronium in the GBAR experiment at CERN. The 100 keV antiproton beam delivered by the AD/ELENA facility was further decelerated with a pulsed drift tube. A 9 MeV electron beam from a linear accelerator produced a low energy positron beam. The positrons were accumulated in a set of two Penning–Malmberg traps. The positronium target cloud resulted from the conversion of the positrons extracted from the traps. The antiproton beam was steered onto this positronium cloud to produce the antiatoms. We observe an excess over background indicating antihydrogen production with a significance of 3–4 standard deviations.

We report on the first production of an antihydrogen beam by charge exchange of 6.1 keV antiprotons with a cloud of positronium in the GBAR experiment at CERN. The 100 keV antiproton beam delivered by the AD/ELENA facility was further decelerated with a pulsed drift tube. A 9 MeV electron beam from a linear accelerator produced a low energy positron beam. The positrons were accumulated in a set of two Penning–Malmberg traps. The positronium target cloud resulted from the conversion of the positrons extracted from the traps. The antiproton beam was steered onto this positronium cloud to produce the antiatoms. We observe an excess over background indicating antihydrogen production with a significance of 3–4 standard deviations.

The production and cooling of the bar H+ ion is the key point of the GBAR experiment (Gravitational Behaviour of Antihydrogen at Rest), which aims at performing the free fall of antihydrogen atoms to measure bar g, the acceleration of antimatter on Earth. bar H+ ions will be obtained from collisions between a positronium cloud and antiprotons delivered by the AD/ELENA facility at CERN, with intermediate formation of antihydrogen atoms. In order to optimise the experimental production of bar H+ ions, we computed the total cross sections of the two corresponding reactions, within the same theoretical framework of the Continuum Distorted Wave – Final State (CDW-FS) model. The different contributions of the bar H excited states have been systematically investigated for different states of Ps. The results exhibit an increase of the bar H production toward low kinetic energies, in agreement with experimental data and previous calculations, whereas the largest bar H+ production is obtained with low energy ground-state antihydrogen atoms. These theoretical predictions suggest that the overall production of bar H+ could be optimal for 2 keV antiproton impact energy, using positronium atoms prepared in the 2p state.

A new slow positron beamline featuring a large acceptance positronium lifetime spectrometer has been constructed and tested at the linac-based slow positron source at IRFU CEA Saclay, France. The new instrument will be used in the development of a dense positronium target cloud for the GBAR experiment. The GBAR project aims at precise measurement of the gravitational acceleration of antihydrogen in the gravitational field of the Earth. Beyond application in fundamental science, the positron spectrometer will be used in materials research, for testing thin porous films and layers by means of positronium annihilation. The slow positron beamline is being used as a test bench to develop further instrumentation for positron annihilation spectroscopy (Ps time-of-flight, pulsed positron beam). The positron source is built on a low energy linear electron accelerator (linac). The 4.3 MeV electron energy used is well below the photoneutron threshold, making the source a genuine on-off device, without remaining radioactivity. The spectrometer features large BGO (Bismuth Germanate) scintillator detectors, with sufficiently large acceptance to detect all ortho-positronium annihilation lifetime components (annihilation in vacuum and in nanopores).

The aim of the recently approved GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment is to measure the acceleration of neutral antihydrogen atoms in the gravitational field of the Earth. The experimental scheme requires a high density positronium cloud as a target for antiprotons, provided by the Antiproton Decelerator (AD) – Extra Low Energy Antiproton Ring (ELENA) facility at CERN. We introduce briefly the experimental scheme and present the ongoing efforts at IRFU CEA Saclay to develop the positron source and the positron-positronium converter, which are key parts of the experiment. We have constructed a slow positron source in Saclay, based on a low energy (4.3 MeV) linear electron accelerator (linac). By using an electron target made of tungsten and a stack of thin W meshes as positron moderator, we reached a slow positron intensity that is comparable with that of 22Na-based sources using a solid neon moderator. The source feeds positrons into a high field (5 T) Penning-Malmberg trap. Intense positron pulses from the trap will be converted to slow ortho-positronium (o-Ps) by a converter structure. Mesoporous silica films appear to date to be the best candidates as converter material. We discuss our studies to find the optimal pore configuration for the positron-positronium converter.

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 ( ) of antihydrogen atoms in free fall. Its originality is to produce + ions and use sympathetic cooling to minimize the initial velocity. These ions are produced using charge exchange reactions with a dense positronium cloud, created by an intense pulse of electron-linac-produced positrons that are accumulated in a Penning-Malmberg trap.

The aim of the GBAR experiment is to measure the gravitational acceleration of antihydrogen by observing the free fall of ultracold anti-atoms. The experiment is installed at CERN’s Antiproton Decelerator/ELENA facility. Positrons are produced by a low energy (9 MeV) linear electron accelerator and captured in a modified Surko (buffer gas) trap. We have recently implemented a silicon carbide-based trapping scheme that replaces the routinely used nitrogen gas with a high quality silicon carbide single crystal in the first phase of the trap. The new setup has been providing stable and efficient positron trapping for more than a year. After a short accumulation in the buffer gas trap, the particles are transported to a high-field (5 T) Penning-Malmberg trap, where a high number of pulses can be collected in a deep potential well. We discuss the performance of the improved positron line and the present status of the experiment.