Antiprotonic helium andCPTinvariance

Abstract

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.

Precise laser spectroscopy of antiprotonic helium at CERN’s Antiproton Decelerator

“…We next determined the antiproton-to-electron mass ratio M p /m e = 1836.152674(5) which results in theoretical values of the pHe + transition frequencies that agree best with experiments. More details can be found in [4] and [1].…”

Experimental Apparatus for Antiproton-Nucleus Annihilation Cross Section Measurements at Low Energy

“…Since the beginning of the AD, the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Anti-protons) Collaboration has studied different physics subjects using antiprotons [1]. The experiments performed at the AD by the ASACUSA Collaboration are laser spectroscopy 1 of antiprotonic helium atoms [2,3], microwave spectroscopy of antihydrogen ground-state hyper-fine structure [4,5], and nuclear collision cross section measurements ofp at low energies (∼0.1-5 MeV) [6][7][8]. The existing experimental data on the antinucleon-nuclei annihilation show more than one discrepancy in the low energy region [9]: firstly, the same optical models which well describe the behavior of antineutron and antiprotons and higher energies, predict values which are smaller than the measured ones; secondly, the experimental data for the antineutron seem to be well fitted by functions with a 1/p 2 term which is expected for low energy antiprotons, or more generally for charged particles (a focusing effect is expected for the antiproton due to the Coulomb attraction of the charged projectile by the target nucleus).…”

Antiproton–to–electron mass ratio determined by two-photon laser spectroscopy of antiprotonic helium atoms

“…Antiprotonic helium (pHe + ) is a three-body atom [1][2][3][4] consisting of a helium nucleus, an electron in the 1s state, and an antiproton occupying a Rydberg state with high principal and angular momentum quantum numbers n ∼ + 1 ∼ 38. The transition frequencies of pHe + have been calculated by QED calculations to fractional precisions of 1 × 10 −9 [5].…”