Status report on the GBAR CERN experiment

GBAR is a recently approved experiment at CERN/AD. Its aim is to perform the first test of the Weak Equivalence Principle with Antimatter. The objective is to measure the gravitational acceleration of antihydrogen atoms on Earth with 1% precision in a first phase, and better than a per mil in a second phase. The method is to detect the free fall of ultra-cold atoms. By sympathetic cooling of antihydrogen positive ions H-bar + with beryllium ions, and after photodetachment of the excess positron, antiatoms are formed at m/s velocities and their free fall can be directly measured. Antihydrogen ions are produced by the interaction of keV antiprotons with a dense positronium cloud, the latter being formed by dumping positrons onto a porous material. We describe here briefly the experimental techniques and report on the most recent results. In particular, a flux of around 4 × 106 e+s−1 slow positrons is now produced by the source installed at Saclay, cross sections for the production of the H-bar + ions have been computed, and a dedicated beam line for the study of positronium formation and for applications in Materials Science has been realized. The tentative schedule of the GBAR developments is given