Shaping the distribution of vertical velocities of antihydrogen in GBAR

Abstract

GBAR is a project aiming at measuring the freefall acceleration of gravity for antimatter, namely antihydrogen atoms (H). The 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 the vertical velocities of H, which improves these factors simultaneously. The method is based on quantum reflection of elastically and specularly bouncing 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 we show that the accuracy for measuring the free fall acceleration g of H could be pushed below 10 −3 under realistic experimental conditions.

The GBAR experiment

“…This procces depends strongly on the energy, for instance, an antihydrogen atom with a kinetic energy equivalent to a potential gravitational energy of about 100 µm (ifḡ = g) will be reflected form a silica surface 96% of the time. For these, an energy shaping device, 19 like shown in Fig. 6, has been envisaged to select only H with an vertical velocity low enough to bounce between the two discs.…”

The GBAR antimatter gravity experiment

“…In a recent publication [22], some of our collaborators proposed to reduce the initial vertical velocity dispersion by adding two small disks above and below the launch point. The high velocity atoms annihilate on such disks whilst those at lower energies bounce off on them taking advantage of the effect of quantum reflection.…”

Improved effective range expansion for Casimir–Polder potential

“…Quantum re ection should play a key role in the GBAR experiment that will test the weak equivalence principle on antihydrogen atoms [20][21][22][23] as it prevents the detection of antihydrogen atoms through annihilation on the detector [24][25][26]. It can also be turned into a tool for reaching better experimental accuracy [27,28]. In a new quantum measurement methods recently proposed to improve the accuracy of GBAR experiment [29], a precise knowledge of the Casimir-Polder shifts of quantum gravitational states [30,31] is required, and this requirement can be met by mastering the e ective range expansion for scattering amplitudes at low energy [32].…”