Shaping the distribution of vertical velocities of antihydrogen in GBAR

GBAR is a project aiming at measuring the free fall acceleration of gravity for antimatter, namely antihydrogen atoms $overline{H}$. Precision of this timing experiment depends crucially on the dispersion of initial vertical velocities of the atoms as well as on the reliable control of their distribution. We propose to use a new method for shaping the distribution of vertical velocities of overline{H}, which improves these factors simultaneously. The method is based on quantum reflection of elastically and specularly bouncing overline{H} with small initial vertical velocity on a bottom mirror disk, and absorption of atoms with large initial vertical velocities on a top rough disk. We estimate statistical and systematic uncertainties, and show that the accuracy for measuring the free fall acceleration overline{g} of overline{H} could be pushed below 10^-3 under realistic experimental conditions.