Zero-shift tuning in geonium by variation of trapped charge.

Measuring the g factor, or gyromagnetic ratio of an individual electron or positron permanently confined in ultrahigh vacuum at liquid helium temperature, provides one of the few avenues for testing the currently accepted standard model that views these elementary particles, on the same level as the quarks, as point-like objects without internal structure. Our results, even though their error limits are the smallest ever attained, would still benefit by possibly two orders of magnitude if a shift, estimated at 4 parts in 10(12), caused by interaction of the cyclotron motion with standing electromagnetic waves in the trap cavity confining the electron could be eliminated. Reexamination of experimental data obtained in another connection suggests that it is practical to identify certain critical cyclotron frequency values for that the shift disappears by testing if the cyclotron frequency measured on a cloud of electrons does not vary with the number of electrons it contains. Clouds here must be kept very much smaller than the wavelength of the above standing waves.