Determination of the Antiproton-to-Electron Mass Ratio by Precision Laser Spectroscopy ofp¯He+

A femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies of antiprotonic helium to fractional precisions of $(9–16)\times {10}^{-9}$. One of these is between two states having microsecond-scale lifetimes hitherto unaccessible to our precision laser spectroscopy method. Comparisons with three-body QED calculations yielded an antiproton-to-electron mass ratio of $M_{\overline{p}}/m_e=1836.152674(5)$.