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 P. Pérez et al.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.

International audienceThe 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 mu 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

In the framework of the gravitational behaviour of antihydrogen at rest (GBAR) experiment, cross sections for the successive formation ofH and H + from collisions between positronium (Ps) and antiprotons (p) have been computed in the range 0-30 keVp energy, using the continuum distorted wavefinal state theoretical model in its three-body and four-body formulations. The effect of the electronic correlations inH + on the total cross sections ofH + production has been studied using three different wave functions for H − (the matter equivalent ofH + ). Ps excited states up to n p = 3, as well asH excited states up to n h = 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.