Progress Towards a Muonium Gravity Experiment
  The gravitational acceleration of antimatter, $\bar g$, has yet to be directly measured but could change our understanding of gravity, the Universe, and the possibility of a fifth force. Three avenues are apparent for such a measurement: antihydrogen, positronium, and muonium, the last requiring a precision atom interferometer and benefiting from a novel muonium beam under development. The interferometer and its few-picometer alignment and calibration systems appear to be feasible. With 100 nm grating pitch, measurements of $\bar g$ to 10%, 1%, or better can be envisioned. This could constitute the first gravitational measurement of leptonic matter, of second-generation matter and, possibly, the first measurement of the gravitational acceleration of antimatter. 
Daniel M. Kaplan (1), Klaus Kirch (2), Derrick C. Mancini (1), James   D. Phillips, Thomas J. Phillips (1), Robert D. Reasen...
physics.ins-det hep-ph