Single-photon laser spectroscopy of cold antiprotonic helium

Some laser spectroscopy experiments carried out by the Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration to measure the single-photon transition frequencies of antiprotonic helium ((overline {p}text {He}^{+}equiv overline {p}+text {He}^{2+}+e^{-})) atoms are reviewed. The (overline {p}text {He}^{+}) were cooled to temperature T =?1.5–1.7 K by buffer-gas cooling in a cryogenic gas target, thus reducing the thermal Doppler width in the single-photon resonance lines. The antiproton-to-electron mass ratio was determined as (M_{overline {p}}/m_{e}=?1836.1526734(15)) by comparisons with the results of three-body quantum electrodynamics calculations. This agreed with the known proton-to-electron mass ratio.