Measuring antimatter gravity with muonium

¹ Illinois Institute of Technology, Chicago, Illinois 60616, USA
² Paul Scherrer Institute, Villigen, Switzerland
³ ETH, Zürich, Switzerland
⁴ Argonne National Laboratory, Argonne, Illinois 60439, USA
⁵ Harvard Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA
⁶ also at Muons, Inc., Batavia, Illinois 60510, USA

^a e-mail: kaplan@iit.edu

Published online: 29 May 2015

Abstract

The gravitational acceleration of antimatter, ḡ, has never been directly measured and could bear importantly on our understanding of gravity, the possible existence of a fifth force, and the nature and early history of the universe. Only two avenues for such a measurement appear to be feasible: antihydrogen and muonium. The muonium measurement requires a novel, monoenergetic, low-velocity, horizontal muonium beam directed at an atom interferometer. The precision three-grating interferometer can be produced in silicon nitride or ultrananocrystalline diamond using state-of-the-art nanofabrication. The required precision alignment and calibration at the picometer level also appear to be feasible. With 100 nm grating pitch, a 10% measurement of ḡ can be made using some months of surface-muon beam time, and a 1% or better measurement with a correspondingly larger exposure. This could constitute the first gravitational measurement of leptonic matter, of 2nd-generation matter and, possibly, the first measurement of the gravitational acceleration of antimatter.