Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium (pHe(+)) is a three-body atom consisting of a normal helium nucleus, an electron in its ground state and an antiproton (p) occupying a Rydberg state with high principal and angular momentum quantum numbers, respectively n and l, such that n ≈ l + 1 ≈ 38. These atoms are amenable to precision laser spectroscopy, the results of which can in principle be used to determine the antiproton-to-electron mass ratio and to constrain the equality between the antiproton and proton charges and masses. Here we report two-photon spectroscopy of antiprotonic helium, in which p(3)He(+) and p(4)He(+) isotopes are irradiated by two counter-propagating laser beams. This excites nonlinear, two-photon transitions of the antiproton of the type (n, l) → (n - 2, l - 2) at deep-ultraviolet wavelengths (λ = 139.8, 193.0 and 197.0 nm), which partly cancel the Doppler broadening of the laser resonance caused by the thermal motion of the atoms. The resulting narrow spectral lines allowed us to measure three transition frequencies with fractional precisions of 2.3-5 parts in 10(9). By comparing the results with three-body quantum electrodynamics calculations, we derived an antiproton-to-electron mass ratio of 1,836.1526736(23), where the parenthetical error represents one standard deviation. This agrees with the proton-to-electron value known to a similar precision.

We review recent progress in the laser and microwave spectroscopy of antiprotonic helium atoms (pHe + ≡ e − − p − He ++ ) carried out at CERN's Antiproton Decelerator facility (AD). Laser transitions were here induced between Rydberg states (n, ) and (n ± 1, − 1) of pHe + (n ∼ 40 and n − 1 being the principal and orbital angular momentum quantum numbers of the antiproton orbit). Successive refinements in the experimental techniques improved the fractional precision on the pHe + frequencies from 3 parts in 10 6 to ∼1 part in 10 8 . These included a radiofrequency quadrupole decelerator, which reduced the energy of the antiprotons from 5.3 MeV (the energy of the beam emerging from AD) to ∼100 keV. This enabled the production of pHe + in ultra-low density targets, where collisional effects with other helium atoms are negligible. A continuous wave pulse-amplified dye laser, stabilized against a femtosecond optical frequency comb, was then used to measure the pHe + frequencies with ppb-scale precision. This progress in the experimental field was matched by similar advances in computing methods for evaluating the expected transition frequencies in three-body QED calculations. The comparison of experimental (ν exp ) and theoretical (ν th ) frequencies for seven transitions in p4 He + and five in p3 He + yielded an antiproton-to-electron mass ratio of m p/m e = 1836.152 674(5). This agrees with the known proton-to-electron mass ratio at the level of ∼2× 10 −9 . The experiment also set a limit on any CPT-violating difference between the antiproton and proton charges and masses, (Q p − |Q p|)/Q p ∼ (m p − m p)/m p < 2 × 10 −9 to a 90% confidence level. If on the other hand we assume the validity of the CPT invariance, the m p/m e result can be taken to be equal to m p /m e . This can be used as an input to future adjustments of fundamental constants. The hyperfine structure of a state in p4 He + has also been measured by microwave spectroscopy to a precision of 3 × 10 −5 . This corresponds to the accuracy of the most precise three-body QED calculations. Further increases in the experimental precision may soon yield an improvement in the value of the antiproton magnetic moment.

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 10 ÿ9 . 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 p =m e 1836:152 6745. DOI: 10.1103/PhysRevLett.96.243401 PACS numbers: 36.10.ÿk, 06.20.Dk, 14.20.Dh, 32.70.Jz We report here new measurements on the transition frequencies of antiprotonic helium atoms ( pHe e ÿ ÿ p ÿ 4 He 2 ) [1] using a femtosecond optical frequency comb [2,3] in conjunction with a continuous-wave (cw) pulse-amplified laser (Fig. 1). Their experimental precision is a factor 6 -20 better than our previous best ones [4], and now approaches those of, e.g., the 1 1 s-2 1 s [5] and 1 1 s-2 1 p [6] transitions in ordinary helium. From the frequencies of 12 transitions measured to the Doppler-broadened limit at a cryogenic temperature of 10 K, we have deduced the mass and charge of the antiproton relative to both the proton and the electron with a precision of the order of the known proton-to-electron mass ratio [7].Reference [4] describes how a radio-frequency quadrupole decelerator was used to slow down the antiprotons emerging from the CERN Antiproton Decelerator to 100-keV energies. They were then stopped in a helium target of low atomic density 10 18 cm ÿ3 to produce pHe atoms which filled a volume V 100 cm 3 . Antiprotons in pHe states with high principal (n 38) and angular momentum (') quantum numbers reach the helium nucleus over a period of several microseconds. The resulting delayed annihilation time spectra (DATS), i.e., the annihilation rate versus time elapsed since pHe formation, was measured by Cherenkov counters [ Fig. 2(a)]. In all but one of the present experiments, linearly polarized laser pulses of energy density " 0:04-1 mJ=cm 2 (e.g., applied here at t 1 s) stimulated transitions with dipole moments 0.02 -0.3 D from these pHe states, to states with nanosecond-scale lifetimes against Auger emission [1] and annihilation. The resulting peak in the DATS signaled the resonant frequency.Only pulsed lasers can provide the megawatt-scale intensities needed here to induce the pHe transitions. However, fluctuations in their frequency and linewidth and the difficulty of calibrating the wide range of pHe wavelengths 264:7-726:1 nm have so far limited our experimental precision [4]. We have now circumvented these problems by basing our experiments on a cw laser whose frequency cw could be stabilized with a precision <4 10 ÿ10 against an optical comb. Its intensity was then amplified [6,8,9] by a factor 10 6 to produce a pulsed laser beam of frequency pl cw with an accuracy and resolution 1-2 orders of magnitude higher than before [4].This was done as follows: First, a Nd:YVO 4 laser (Coherent Verdi, B in Fig. 1