One-Particle Measurement of the Antiproton Magnetic Moment

For the first time a single trapped antiproton ($\overline{p}$) is used to measure the $\overline{p}$ magnetic moment ${\mathit{\mu }}_{\overline{p}}$. The moment ${\mathit{\mu }}_{\overline{p}}={\mu }_{\overline{p}}\mathit{S}/(\hslash /2)$ is given in terms of its spin $\mathit{S}$ and the nuclear magneton (${\mu }_N$) by ${\mu }_{\overline{p}}/{\mu }_N=-2.792845\pm 0.000012$. The 4.4 parts per million (ppm) uncertainty is 680 times smaller than previously realized. Comparing to the proton moment measured using the same method and trap electrodes gives ${\mu }_{\overline{p}}/{\mu }_p=-1.000000\pm 0.000005$ to 5 ppm, for a proton moment ${\mathit{\mu }}_p={\mu }_p\mathit{S}/(\hslash /2)$, consistent with the prediction of the $CPT$ theorem.

Figure 1

Uncertainties in measurements of the $\overline{p}$ magnetic moment measured in nuclear magnetons, ${\mu }_{\overline{p}}/{\mu }_N$.

Figure 2

(a) Electrodes of the analysis trap (cutaway side view) are copper with an iron ring. (b) The iron ring significantly alters $B$ on axis. (c) Top view of the paths of the oscillating current for the spin flip drive. (d) An oscillating electric field (top view) drives $\overline{p}$ cyclotron motion.

Figure 3

(a) Spin measurement cycle. (b) The power shift in $f_z$ due to the spin flip drive (points) is lower than was observed for the proton measurement with no transmission line transformer (dashes).

Figure 4

(a) The spin line. (b) The cyclotron line.