GBAR Gravitational Behavior of Antihydrogen at Rest

The GBAR experiment aims to test the Equivalence Principle with antimatter by measuring the time of flight of ultra-cold antihydrogen atoms H in free fall. Antihydrogen atoms at ¯ ∼20 µK are provided by sympathetic cooling of antihydrogen ions $\bar{H}^+$ with laser cooled $Be^+$ ions. $\bar{H}^+$ ions are produced via two successive reactions using antiprotons and positroniums. The synthesis of $\bar{H}^+$ is obtained by the injection of a pulse of $10^7$ slow antiprotons from the AD at CERN in a dense cloud of positronium. This target of positronium is created with a positron-to-positronium converter and requires an intense source of slow positrons, a few $10^8$ per second. Such a source based on a small electron accelerator is under construction at Saclay. A few $10^{10}$ positrons are accumulated in a Penning-Malmberg trap from which they are ejected towards the $e^+/Ps$ converter to produce the target. The overall scheme of the experiment is described along with the estimated efficiency of each step.