Publications

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 antiproton beam was delivered by the AD/ELENA facility. The positronium target was produced from a positron beam itself obtained from an electron linear accelerator. We observe an excess over background indicating antihydrogen production with a significance of 3-4 standard deviations.

P. Adrich et al., arXiv.2306.15801 [hep-ex] or cern preprint CERN-EP-2023-120

We present a description of the GBAR positron (e+) trapping apparatus, which consists of a three stage Buffer Gas Trap (BGT) followed by a High Field Penning Trap (HFT), and discuss its performance. The overall goal of the GBAR experiment is to measure the acceleration of the neutral antihydrogen (H) atom in the terrestrial gravitational field by neutralising a positive antihydrogen ion (H⁺), which has been cooled to a low temperature, and observing the subsequent H annihilation following free fall. To produce one H⁺ ion, about 10¹⁰ positrons, efficiently converted into positronium (Ps), together with about 10⁷ antiprotons (p), are required. The positrons, produced from an electron linac-based system, are accumulated first in the BGT whereafter they are stacked in the ultra-high vacuum HFT, where we have been able to trap 1.4(2) x 10⁹ positrons in 1100 seconds.

P. Blumer et al., Nuclear Inst. and Methods in Physics Research, A 1040 (2022) 167263

The GBAR (Gravitational Behavior of Antihydrogen at Rest) experiment at CERN requires efficient deceleration of 100 keV antiprotons provided by the new ELENA synchrotron ring to synthesize antihydrogen. This is accomplished using electrostatic deceleration optics and a drift tube that is designed to switch from -99 kV to ground when the antiproton bunch is inside – essentially a charged particle “elevator” – producing a 1 keV pulse. We describe the simulation, design, construction and successful testing of the decelerator device at -92 kV on-line with antiprotons from ELENA.

A. Husson et al., Nuclear Inst. and Methods in Physics Research, A 1002 (2021) 165245 A (2021)

For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN’s Antiproton Decelerator (AD) facility we have constructed a source of slow positrons, which uses a low-energy electron linear accelerator (linac). The driver linac produces electrons of 9 MeV kinetic energy that create positrons from bremsstrahlung-induced pair production. Staying below 10 MeV ensures no persistent radioactive activation in the target zone and that the radiation level outside the biological shield is safe for public access. An annealed tungsten-mesh assembly placed directly behind the target acts as a positron moderator. The system produces slow positrons per second, a performance demonstrating that a low-energy electron linac is a superior choice over positron-emitting radioactive sources for high positron flux.

M. Charlton et al., Nuclear Inst. and Methods in Physics Research, A 985 (2021) 164657 A (2020)

The GBAR experiment aims to measure the gravitational acceleration of antihydrogen H. It will use H⁺ ions formed by the interaction of antiprotons with a dense positronium cloud, which will require about 10¹⁰ positrons collected to produce one H⁺. We present the first results on the positron accumulation, reaching 3.8 ± 0.4 × 10⁸ e⁺ in 560 s

S. Niang et al., Acta Physica Polonica A 137, 164-166 (2020)

We have developed a PbWO4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross-section for the (anti)hydrogen formation by (anti)proton–positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment.

B.H. Kim et al., Acta Physica Polonica A 137, 122-125 (2020)

The GBAR project (Gravitational Behaviour of Anti hydrogen at Rest) at CERN, aims to measure the free fall acceleration of ultracold neutral anti hydrogen atoms in the terrestrial gravitational field. The experiment consists preparing anti hydrogen ions (one antiproton and two positrons) and sympathetically cooling them with Be⁺ ions to less than 10 μK. The ultracold ions will then be photo-ionized just above threshold, and the free fall time over a known distance measured. We will describe t he project, the accuracy that can be reached by standard techniques, and discuss a possible improvement to reduce the vertical velocity spread.

P. Pérez et al., Hyperfine Interactions 233, 21-27 (2015)

In the framework of the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment, cross sections for antihydrogen ion (H⁺) production in collisions between antiprotons (p) and excited positronium atoms (Ps), with intermediate production of antihydrogen (H), have been computed using a perturbative theory, namely Continuum Distorted Wave - Final State (CDW-FS). The results suggest to use antiprotons at 1, 2 or 6 keV with, respectively, Ps(3p,3d), Ps(2p) or no Ps excitation. A simulation using these cross sections is under development to investigate the reaction chamber geometry and the parameters of the different beams (positrons, antiprotons and laser). This simulation, focusing on Ps(3d), predicts at least one H⁺ ion per pulse of 3·10⁶ pbar at 1 and 6 keV, and highlights both the interest of positronium excitation and the need for short pulses of particles.

P. Comini, P. -A. Hervieux and F. Biraben, Hyperfine Interactions 228, 159 (2014)

The Einstein classical Weak Equivalence Principle states that the trajectory of a particle is independent of its composition and internal structure when it is only submitted to gravitational forces. This fundamental principle has never been directly tested with antimatter. However, theoretical models such as supergravity may contain components inducing repulsive gravity, thus violating this principle. The GBAR project (Gravitational Behaviour of Antihydrogen at Rest) proposes to measure the free fall acceleration of ultracold neutral antihydrogen atoms in the terrestrial gravitational field. The experiment consists in preparing antihydrogen ions (one antiproton and two positrons) and sympathetically cool them with Be⁺ ions to a few 10 μ K. The ultracold ions will then be photoionized just above threshold, and the free-fall time over a known distance measured. In this work, the GBAR project is described as well as possible improvements that use quantum reflection of antihydrogen on surfaces to use quantum methods of measurements.

P. Indelicato et al., Hyperfine Interactions 228, 141 (2014)

We study a method to induce resonant transitions between antihydrogen (H) quantum states above a material surface in the gravitational field of the Earth. The method consists of applying a gradient of magnetic field, which is temporally oscillating with the frequency equal to a frequency of transition between gravitational states of antihydrogen. A corresponding resonant change in the spatial density of antihydrogen atoms could be measured as a function of the frequency of applied field. We estimate an accuracy of measuring antihydrogen gravitational states spacing and show how a value of the gravitational mass of the H atom could be deduced from such a measurement. We also demonstrate that a method of induced transitions could be combined with a free-fall-time measurement in order to further improve the precision.

A.Yu. Voronin, V.V. Nesvizhevsky, G. Dufour, P. Debu, A. Lambrecht, S. Reynaud, O.D. Dalkarov, E.A. Kupriyanova, P.Froelich
arXiv:1403.4783 [physics.atom-ph] (2014)

We discuss an experimental approach allowing to prepare antihydrogen atoms for the GBAR experiment. We study the feasibility of all necessary experimental steps: The capture of incoming H⁺ ions at keV energies in a deep linear RF trap, sympathetic cooling by laser cooled Be⁺ ions, transfer to a miniaturized trap and Raman sideband cooling of an ion pair to the motional ground state, and further reducing the momentum of the wavepacket by adiabatic opening of the trap. For each step, we point out the experimental challenges and discuss the efficiency and characteristic times, showing that capture and cooling are possible within a few seconds.

L. Hilico, J-P. Karr, A. Douillet, P. Indelicato, S. Wolf, F. Schmidt Kaler
arXiv:1402.1695 [physics.atom-ph] (2014)

GBAR is a project aiming at measuring the free-fall acceleration of gravity for antimatter, namely antihydrogen atoms (H). The precision of this timing experiment depends crucially on the dispersion of initial vertical velocities of the atoms as well as on the reliable control of their distribution. We propose to use a new method for shaping the distribution of the vertical velocities of H, which improves these factors simultaneously. The method is based on quantum reflection of elastically and specularly bouncing H with small initial vertical velocity on a bottom mirror disk, and absorption of atoms with large initial vertical velocities on a top rough disk. We estimate statistical and systematic uncertainties, and we show that the accuracy for measuring the free fall acceleration g of H could be pushed below 10⁻³ under realistic experimental conditions.

G. Dufour, P. Debu, A. Lambrecht, V.V. Nesvizhevsky, S. Reynaud, A.Yu. Voronin
Eur. Phys. J. C 74 (2014) 2731

In the GBAR experiment, cold antihydrogen atoms will be left to fall on an annihilation plate with the aim of measuring the gravitational acceleration of antimatter. Here, we study the quantum reflection of these antiatoms due to the Casimir-Polder potential above the plate. We give realistic estimates of the potential and quantum reflection amplitudes, taking into account the specificities of antihydrogen and the optical properties of the plate. We find that quantum reflection is enhanced for weaker potentials, for example above thin slabs, graphene and nanoporous media.

G. Dufour, R. Guérout, A. Lambrecht, V.V. Nesvizhevsky, S. Reynaud, A.Yu. Voronin
arXiv:1312.6018 [quant-ph] (2013)

In the framework of the gravitational behaviour of antihydrogen at rest (GBAR) experiment, cross sections for the successive formation of H and H⁺ from collisions between positronium (Ps) and antiprotons (p) have been computed in the range 0–30 keV energy, using the continuum distorted wave-final state theoretical model in its three-body and four-body formulations. The effect of the electronic correlations in H⁺ on the total cross sections of H⁺ production has been studied using three different wave functions for H⁻ (the matter equivalent of H⁺). Ps excited states up to np = 3, as well as H excited states up to nh = 4, have been investigated. The results suggest that the production of H⁺ can be efficiently enhanced by using either a fraction of Ps(2p) and a 2 keV (p) beam or a fraction Ps(3d) and antiprotons with kinetic energy below 1 keV.

P. Comini and P.-A. Hervieux
New J. Phys. 15 (2013) 095022

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.

L. Liszkay, P. Comini, C. Corbel, P. Debu, P. Dupré, P. Grandemange, P. Pérez, J-M. Rey, Y. Sacquin
J. Phys.: Conf. Ser. 443 (2013) 012006

The production and cooling of the 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 g, the acceleration of antimatter on Earth. 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 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 H excited states have been systematically investigated for different states of Ps. The results exhibit an increase of the H production toward low kinetic energies, in agreement with experimental data and previous calculations, whereas the largest H⁺ production is obtained with low energy ground-state antihydrogen atoms. These theoretical predictions suggest that the overall production of H⁺ could be optimal for 2 keV antiproton impact energy, using positronium atoms prepared in the 2p state.

Pauline Comini and Paul-Antoine Hervieux
J. Phys.: Conf. Ser. 443 (2013) 012007

Low energy positron beams are of major interest for fundamental science and materials science. IRFU has developed and built a slow positron source based on a compact, low energy (4.3 MeV) electron linac. The linac-based source will provide positrons for a magnetic storage trap and represents the first step of the GBAR experiment (Gravitational Behavior of Antimatter in Rest) recently approved by CERN for an installation in the Antiproton Decelerator hall. The installation built in Saclay will be described with its main characteristics. The ultimate target of the GBAR experiment will be briefly presented as well as the foreseen development of an industrial positron source dedicated for materials science laboratories.

J-M. Rey, G. Coulloux, P. Debu, H. Dzitko, P. Hardy, L. Liszkay, P. Lotrus, T. Muranaka, C. Noel, P. Pérez, O. Pierret, N. Ruiz
J. Phys.: Conf. Ser. 443 (2013) 012077

We study quantum reflection of antihydrogen atoms from nanoporous media due to the Casimir-Polder potential. Using a simple effective medium model, we show a dramatic increase of the probability of quantum reflection of antihydrogen atoms if the porosity of the medium increases. We discuss the limiting case of reflections at small energies, which have interesting applications for trapping and guiding antihydrogen using material walls.

G. Dufour, R. Guérout, A. Lambrecht, V.V. Nesvizhevsky, S. Reynaud, A.Yu. Voronin
Phys. Rev. A 87 (2013) 022506

We study quantum reflection of antihydrogen atoms from matter slabs due to the van der Waals–Casimir-Polder potential. By taking into account the specificities of antihydrogen and the optical properties and width of the slabs, we calculate realistic estimates for the potential and quantum reflection amplitudes. Next we discuss the paradoxical result of larger reflection coefficients estimated for weaker potentials in terms of the Schwarzian derivative. We analyze the limiting case of reflections at small energies, which are characterized by a scattering length and have interesting applications for trapping and guiding antihydrogen using material walls.

G. Dufour, A. Gérardin, R. Guérout, A. Lambrecht, V.V. Nesvizhevsky, S. Reynaud, A.Yu. Voronin
Phys. Rev. A 87 (2013) 012901

The recently recommended experiment GBAR is foreseen to run at CERN at the AD/ELENA antiproton source . It aims at performing the first measurement of the Earth's gravitational acceleration on antimatter by observing the free-fall of antihydrogen atoms. This requires creating anti-atoms at an unprecedented low energy. The different steps of the experiment and their present status are reviewed.

P. Pérez, Y. Sacquin
Quantum Grav. 29 (2012) 184008

The GBAR project aims to perform the first test of the Equivalence Principle with antimatter by measuring the free fall of ultra-cold antihydrogen atoms. The objective is to measure the gravitational acceleration to better than a percent in a first stage, with a long term perspective to reach a much higher precision using gravitational quantum states of antihydrogen. The production of ~20 μK atoms proceeds via sympathetic cooling of H⁺ ions by Be⁺  ions. H⁺ ions are produced via a two-step process, involving the interaction of bursts of 10⁷ slow antiprotons from the AD (or ELENA upgrade) at CERN with a dense positronium cloud. In order to produce enough positronium, it is necessary to realize an intense source of slow positrons, a few 10⁸ per second. This is done with a small electron linear accelerator. A few 10¹⁰ positrons are accumulated every cycle in a Penning–Malmberg trap before they are ejected onto a positron-to-positronium converter. The overall scheme of the experiment is described and the status of the installation of the prototype positron source at Saclay is shown. The accumulation scheme of positrons is given, and positronium formation results are presented. The estimated performance and efficiency of the various steps of the experiment are given.

P. Debu
New Journal of Physics 14 (2011) 065009

We propose to use a 13 keV antiproton beam passing through a dense cloud of positronium (Ps) atoms to produce an H⁺ ‘‘beam’’. These ions can be slowed down and captured by a trap. The process involves two reactions with large cross-sections under the same experimental conditions. These reactions are the interaction of p with Ps to produce H and the e⁺ capture by H reacting on Ps to produce H⁺. Once decelerated with an electrostatic field and captured in a trap, the H⁺ ions could be cooled and the e⁺ removed with a laser to perform a measurement of the gravitational acceleration of neutral antimatter in the gravity field of the Earth.

P. Pérez and A. Rosowsky
Nucl. Instr. Meth. A 545, 20-30 (2005)

We describe a novel design for an intense source of slow positrons based on pair production with a beam of electrons from a 10 MeV accelerator hitting a thin target at a low incidence angle. The positrons are collected with a set of coils adapted to the large production angle. The collection system is designed to inject the positrons into a Greaves–Surko trap (Phys. Rev. A 46 (1992) 5696). Such a source could be the basis for a series of experiments in fundamental and applied research and would also be a prototype source for industrial applications, which concern the field of defect characterization in the nanometer scale.

P. Pérez and A. Rosowsky
Nucl. Instr. Meth. A 532, 523-532 (2004)