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Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio


Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry1). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium (He+) is a three-body atom2 consisting of a normal helium nucleus, an electron in its ground state and an antiproton () 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 3He+ and 4He+ 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 109. 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.

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Figure 1: Energy levels, Cherenkov detector signals and experimental layout for two-photon spectroscopy of He+.
Figure 2: Profiles of sub-Doppler two-photon resonances.
Figure 3: Two-photon transition frequencies.
Figure 4: Antiproton-to-electron and proton-to-electron mass ratios.


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This work was supported by the European Science Foundation (EURYI), Monbukagakusho (grant no. 20002003), the Munich Advanced Photonics cluster of the Deutsche Forschungsgemeinschaft, the Hungarian Research Foundation (K72172) and the Austrian Federal Ministry of Science and Research. We thank the CERN Antiproton Decelerator and Proton Synchrotron operational staff, the CERN cryogenics laboratory, J. Alnis, D. Bakalov, J. Eades, R. Holzwarth, V. I. Korobov, M. Mitani, W. Pirkl and T. Udem.

Author information




M.H. designed the two-photon experiment. M.H. and A.D. developed the laser systems and carried out the caesium and rubidium measurements. M.H. and D.B. constructed the cryogenic target. M.H. developed the antiproton beam profile monitors, Cherenkov counters, cryogenic optics and data acquisition system. D.B. and M.H. wrote the analysis software. All authors contributed to the beam-time data taking and analysis. M.H. wrote the manuscript and all authors discussed the results and contributed to the editing.

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Correspondence to Masaki Hori.

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The authors declare no competing financial interests.

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Hori, M., Sótér, A., Barna, D. et al. Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio. Nature 475, 484–488 (2011).

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