Proposal to measure the Gravitational Behaviour of Antihydrogen at Rest

We propose an experiment to measure the free fall acceleration of neutral antihydrogen atoms in order to test the Weak Equivalence Principle. The originality of this path is to first produce the antihydrogen ion Hbar+ (or anti H-). The ion is formed through two charge exchange processes involving the interaction of an antiproton with positronium to produce antihydrogen, followed by the interaction of this atom with positronium. The ion is then sympathetically cooled with laser cooled Be+ ions down to μK temperatures (i.e. m/s velocities). The excess positron can then be laser detached in order to recover the neutral antihydrogen atom. The laser pulse will give the start time for the antihydrogen free fall measurement. The stop will be determined by the detection of the charged pions coming from the annihilation of the antiprotons on a plate placed at a known distance from the initial position of the atoms. From the free fall time and distance one will can extract the value of g. Our goal is a measurement at the level of 1%.