Precision comparison of the g-factor of the proton and anti-proton

We report the first measurement of the antiproton magnetic moment using a single antiproton. The magnetic moment in nuclear magnetons is $\mu_{\bar{p}}/\mu_N = - 2.792 845 \pm 0.000 012$, a 4.4 parts per million (ppm) measurement. This represents a factor of 680 improvement in precision over previous work using exotic atom spectroscopy, which has achieved a 3000 ppm precision and remained essentially unchanged in the past 20 years.\footnote{A. Kreissl, et al. Z. Phys. C: Part. Fields \textbf{37}, 557 (1988).}$^{,}$\footnote{T. Pask, et al. Phys. Lett. B \textbf{678}, 55 (2009).} Our measurement allows for an improved comparison of the proton and antiproton magnetic moments, yielding a result consistent with the prediction of charge, parity and time reversal symmetry. Following a proof of principle, 2.5 ppm measurement of the proton magnetic moment,\footnote{J. DiSciacca and G. Gabrielse. Phys. Rev. Lett. \textbf{108}, 153001 (2012)} the experiment was moved to CERN for the antiproton experiment. Initial work focused on catching, cooling and trapping a single antiproton from the 5 MeV beam at CERN's Antiproton Decelerator. Following this work, we undertook a magnetic moment measurement. The spin and cyclotron frequency are measured to determine the g-factor, $g/2 = f_s/f_c$. Prospects for further improvement should be possible with single spin flip detection, similar to what was used to measure the electron magnetic moment - currently the most precisely measured property of a fundamental particle.\footnote{D. Hanneke, S. Fogwell, and G. Gabrielse, Phys. Rev. Lett. \textbf{100}, 120801 (2008).} The new antiproton magnetic moment measurement is likely a first step towards improved precision by an additional factor of $10^3$ or $10^4$ improvement, with a precision at the part per billion level.\footnote{N. Guise, J. DiSciacca, and G. Gabrielse. Phys. Rev. Lett. \textbf{104}, 143001 (2009).}$^{,}$\footnote{S. Ulmer, et al. Phys. Rev. Lett. \textbf{106}, 253001 (2011).}$^{,}$\footnote{C. C. Rodegheri, et a.l New J. Phys. \textbf{14}, 063011 (2012).}