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Helium is unique in the sense that about 3% of low-energy antiprotons stopped in it survive with an average lifetime of a few microseconds, forming metastable states of the exotic antiprotonic helium atom (\(\overline{p}\)-He + + -e −). This lifetime is sufficient to carry out laser spectroscopy measurements of atomic transitions of this exotic atom. The antiproton-to-electron mass ratio \(M_{\overline{p}}/m_e\) can be deduced from comparisons with three-body QED calculations. A systematic study of the energy levels of this exotic atom started soon after its discovery, continuously aiming for higher precision (for a review see Yamazaki et al., Phys Rep 366:183, (2002) and references therein). Recently, at the Antiproton Decelerator of CERN, a femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies to fractional precisions of (9 − 16)×10 − 9, yielding an antiproton-to-electron mass ratio of 1836.152674(5).

Determination of the antiproton-to-electron mass ratio by laser spectroscopy of $\overline{\boldsymbol{p}}\boldsymbol{\mathrm{He}}^{\boldsymbol +}$