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.

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 s.

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 5×107 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.

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.

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.

The GBAR experiment aims to measure the gravitational acceleration of antihydrogen (H) over bar. It will use (H) over bar (+) ions formed by the interaction of antiprotons with a dense positronium ...

We report experiments in which positronium (Ps) atoms were created in thin, porous silica films containing isolated voidswith diameters ranging from 5 to 75 nm. Ps lifetimes in the pore structures weremeasured directly via time-delayed laser excitation of 1(3)S(1) -> 2(3)P(J) transitions. In a film containing 5-nm pores Ps was predominantly emitted into vacuum, with a small component of confined Ps with a lifetime of 75 ns also observed. In films with larger pores Ps atoms were not emitted into vacuum except from the film surface, and confined Ps lifetimes of approximate to 90 ns were measured with no dependence on the pore size. However, for such large pores, extended Tao-Eldrup (ETE)-type models predict Ps lifetimes close to the 142-ns vacuum value. Moreover, 1(3)S(1) -> 2(3)P(J) excitation of Ps atoms inside the pores was found to result in annihilation and exhibited an extremely broad (approximate to 10 THz) linewidth. We attribute these observations to a process in which nonthermal Ps atoms in the isolated voids become temporarily trapped in a series of surface states that dissociate following excitation. The occurrence of this mechanism is not necessarily apparent from ground-state Ps decay rates without some prior knowledge of the sample structure, and it precludes the application of ETE-type models as they do not take into account surface interactions other than pickoff annihilation.

B. S. Cooper,1 J.-P. Boilot,2 C. Corbel,3 F. Guillemot,2,* L. Gurung,1 L. Liszkay,3 and D. B. Cassidy1 1Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom 2Groupe de Chimie du Solide, Laboratoire de Physique de la Matière Condensée, UMR CNRS 7643, Ecole Polytechnique, 91128 Palaiseau, France 3DRF, IRFU and IRAMIS, CEA, University Paris-Saclay F-91191 Gif-sur-Yvette Cedex, France

The experimental setup and results of the first search for invisible decays of orthopositronium (o-Ps) confined in a vacuum cavity are reported. No evidence of invisible decays at a level Br(o-Ps -> invisible) < 5.9 x 10(-1) (90% C.L.) was found. This decay channel is predicted in hidden sector models such as the mirror matter (MM), which could be a candidate for dark matter. Analyzed within the MM context, this result provides an upper limit on the kinetic mixing strength between ordinary and mirror photons of epsilon < 3.1 x 10(-7) (90% C.L.). This limit was obtained for the first time in vacuum free of systematic effects due to collisions with matter.

We have studied the properties of a commercially available 4H-SiC epitaxial layer and evaluated its potential application as an efficient positron remoderator. A remoderation efficiency of more than 65% has been measured for incident positrons with 1 keV energy. We have determined the work function and the energy distribution of the emitted slow positrons, a property which is essential for practical applications. Comparison of the positron moderation properties of the epitaxial layer with results from a n-type 4H-SiC single crystal, indicate that the epitaxially grown layer is a superior secondary moderator than its substrate counterpart.

We report the achievement of spatial confinement of muonium atoms (the bound state of a positive muon and an electron). Muonium emitted into a vacuum from mesoporous silica reflects between two ${\mathrm{SiO}}_{2}$ confining surfaces separated by 1 mm. From the data, one can extract that the reflection probability on the confining surfaces kept at 100 K is about 90% and the reflection process is well described by a cosine law. This technique enables new experiments with this exotic atomic system and is a very important step towards a measurement of the $1S\ensuremath{-}2S$ transition frequency using continuous-wave laser spectroscopy.

The aim of the GBAR (Gravitational Behavior of Antimatter at Rest) experiment is to measure the free fall acceleration of an antihydrogen atom, in the terrestrial gravitational field at CERN and therefore test the Weak Equivalence Principle with antimatter. The aim is to measure the local gravity with a 1% uncertainty which can be reduced to few parts of 10 -3 .

We report measurements of positronium (Ps) formation following positron irradiation of mesoporous SiO2 films and Ge(100) single crystals at temperatures ranging from 12-700 K. As both of these materials generate Ps atoms via nonthermal processes, they are able to function as positron-positronium converters at cryogenic temperatures. Our data show that such Ps formation is possibly provided the targets are not compromised by adsorption of residual gas. In the case of SiO2 films, we observe a strong reduction in the Ps formation efficiency following irradiation with UV laser light (lambda = 243.01 nm) below 250 K, in accordance with previous observations of radiation-induced surface paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser light (lambda = 532 nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous SiO2 films and Ge crystals were found to produce Ps efficiently in cryogenic environments. Accordingly, these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold Ps atoms.

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 the project, the accuracy that can be reached by standard techniques, and discuss a possible improvement to reduce the vertical velocity spread.

The specific antiproton- and positron-beam requirements of the CERN AD-7 experiment, GBAR (Gravitational Behavior of Antimatter at Rest) are presented. GBAR will synthesize antihydrogen ions which will be sympathetically cooled before performing a free-fall experiment on the atom. Antiprotons delivered by CERN’s ELENA facility in 100-keV, 300-ns pulses will be electrostatically decelerated and transformed to keV energies using a pulsed drift tube. Positrons are created using a linear electron accelerator and collected into a Penning-Malmberg trap. Descriptions of these ion optical systems are given along with the status.

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.

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).

P. Dupré , P. Grandemange, D. Lunney, V. Manea, A. Badertscher, D. Banerjee, P. Crivelli, A. Curioni, A. Marchionni, B. Rossi, A. Rubbia, V. Nesvizhevsky, P-A. Hervieux, G. Manfredi, D. Brook-Roberge, P. Comini, P. Debu, L. Liszkay, B. Mansoulié, T. Mortensen, P. Pérez, J-M. Rey, J-M. Reymond, Y. Sacquin, B. Vallage, O. Dalkarov, N. Kolachevskiy, K. Khabarova, A. Voronin, F. Biraben, P. Cladé, A. Douillet, G. Dufour, S. Guellati, L. Hilico, P. Indelicato, A. Lambrecht, J-P. Karr, F. Nez, S. Reynaud, V-Q. Tran, F. Schmidt-Kaler, S. Wolf , J. Walz, M. Staszczak, S. Wronka, A. Mohri, Y. Yamazaki, M. Charlton, S. Eriksson, N. Madsen, D.P. van der Werf , N. Kuroda, Y. Matsuda, H. Torii, Y. Nagashima, P. Froelich, S. Jonsell

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 ((g) over bar) of antihydrogen atoms in free fall. Its originality is to produce (H) over bar (+) 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.