New Apparatus and Methods for the Measurement of the Proton and Antiproton Magnetic Moments

Abstract

The first direct measurement of the antiproton magnetic moment was performed using a single particle in a Penning trap. The result, μˉp/μN = 2.792 845 (12) [4.4 ppm], is 680 times more precise than the previous best measurement. Together with a prior measurement of the proton magnetic moment in the same apparatus, this was the first direct comparison of the proton and antiproton magnetic moments. This stringent test of CPT invariance gave μˉp/μp = −1.000 000 (5) [5.0 ppm]. The observation of individual spin flips of a single proton, also reported here, opened the possibility of further improving measurement precision by orders of magnitude. Improving this result by a factor of ~104 requires measuring μ outside the large magnetic field gradient needed to detect spin flips. Two methods are proposed to avoid the leading uncertainties in such a high-precision two-trap measurement. One of these is to measure single spin-flips of a single proton or antiproton. The other is to induce multiple spin flips in the presence of a spin-cyclotron coupling drive, and observe the resulting change in the cyclotron energy. The design, construction, and commissioning of an appropriate apparatus, with high-quality particle detection and newly designed Penning trap electrodes, is reported.