The goal of this project is to conduct the first force-sensitive inertial experiments using metastable Positronium (Ps), the short-lived bound state of an electron and a positron. Such experiments have the potential of probing the gravitational interaction on both a matter/antimatter and purely leptonic system. In the first phase of the project, inertial sensitivity will be obtained combining a source of metastable 23S Ps with a classical moiré deflectometer, i.e. a set of regular gratings and a position-sensitive detector, detecting the deflection of the atoms’ trajectories during their free-fall. A suitable long-lived velocity-tunable beam of metastable Ps can be obtained by laser-exciting cold ground-state 13S Ps to its 33P level, then letting the atoms spontaneously decay to the 23S metastable level. Adopting already-demonstrated techniques, a force sensitivity of ~10-25 N will be reached, yielding a first proof-of-principle test of the weak equivalence principle on leptonic matter. Furthermore, this sensitivity level is sufficient – in the second phase of the experiment – for observing Ps atoms’ deflection by a laser dipole force. Introducing a laser standing-wave, Bragg scattering experiments can be first performed on antimatter, demonstrating the feasibility of Bragg optics (mirror/beam splitters). Atom optics based on Bragg scattering will allow the construction, in the third phase of the experiment, of a three-grating Mach-Zehnder interferometer using light gratings. Such a device has - potentially - gravity sensitivity even with 23S Ps sources demonstrated so far. Several technical advances in producing cold beams of 23S metastable Ps can also be achieved within this project. Among the many: efficient Raman 13S-33P-23S excitation schemes to increase the source efficiency; laser cooling of the Ps 13S-23P transition to provide beam collimation; optimization of positron/positronium converters for efficient forward emission.

Ruggero Caravita – TIFPA