AMO
In the literature: open
Gravitational free fall of muonium or positronium
In plain words
Muonium (an antimuon with an electron) and positronium (an electron with its antiparticle) contain no protons or neutrons. Measuring how they fall tests gravity on matter made purely of leptons, partly of antimatter.
Precise statement
Measure the gravitational acceleration of muonium (2.2 microsecond lifetime) or long-lived Rydberg positronium with uncertainty below 10% of g, and determine whether it differs from g.
What would settle it
A deflection or interferometric measurement of muonium (LEMING) or Rydberg positronium free fall at 10% or better.