A fundamental experiment that has the potential to show whether motion of free electrons is involved in Mu formation is to measure the muonium polarization function in a sample to which an external electric field has been applied. The idea is very simple - if an external electric field does have an effect, then the muonium must be formed from a free electron and muon that are initially well separated. When muonium is formed directly by collisions of energetic muons with neutral atoms, the muon and electron are never separated by a distance sufficient to allow a modest external field to have much effect against the overwhelming coulomb force between charged particles in close proximity.
Previously, only a very few experiments have been performed in which samples were subjected to electric fields. Ito et al. investigated the effect of an external electric field on the muonium signal in liquid hydrocarbons with fields up to 20 kV/cm and in fused quartz up to 60 kV/cm, observing no effect on either the asymmetry or relaxation rate. [14,15]