Hossein Aghai-Khozani

Buffer-gas cooling of antiprotonic helium to 1.5 to 1.7 K, and antiproton-to–electron mass ratio

Exotic molecule tests fundamental symmetry Spectroscopy of exotic molecules can offer insight into fundamental physics. Hori et al. studied the transition frequencies of an unusual helium atom in which one of the two electrons was substituted by an antiproton, the negatively charged antiparticle partner of the proton (see the Perspective by Ubachs). The antiprotonic helium was cooled down to low temperatures to allow the frequencies to be measured with high precision. The extracted mass of the antiproton (relative to the electron mass) was in good agreement with previous measurements of the proton mass. This finding is in keeping with the implications of the combined charge, parity, and time-reversal symmetry of physical laws. Science, this issue p. 610; see also p. 546 Spectroscopy of a cold exotic molecule yields a precise value of the antiproton mass relative to the mass of the electron. Charge, parity, and time reversal (CPT) symmetry implies that a particle and its antiparticle have the same mass. The antiproton-to-electron mass ratio Mp¯/me can be precisely determined from the single-photon transition frequencies of antiprotonic helium. We measured 13 such frequencies with laser spectroscopy to a fractional precision of 2.5 × 10−9 to 16 × 10−9. About 2 × 109 antiprotonic helium atoms were cooled to temperatures between 1.5 and 1.7 kelvin by using buffer-gas cooling in cryogenic low-pressure helium gas; the narrow thermal distribution led to the observation of sharp spectral lines of small thermal Doppler width. The deviation between the experimental frequencies and the results of three-body quantum electrodynamics calculations was reduced by a factor of 1.4 to 10 compared with previous single-photon experiments. From this, Mp¯/me was determined as 1836.1526734(15), which agrees with a recent proton-to-electron experimental value within 8 × 10−10.

Observation of the 1154.9 nm transition of antiprotonic helium

The resonance transition (n, l) = (40, 36) ? (41, 35) of the antiprotonic helium (He+) isotope at a wavelength of 1154.9?nm was detected by laser spectroscopy. The population of He+ occupying the resonance parent state (40, 36) was found to decay at a rate of 0.45 ? 0.04 ?s?1, which agreed with the theoretical radiative rate of this state. This implied that very few long-lived He+ are formed in the higher-lying states with principal quantum number n ? 41, in agreement with the results of previous experiments.