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 (p¯He+≡p¯+He2++e‑) atoms are reviewed. The p¯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 Mp¯/me=1836.1526734(15) by comparisons with the results of three-body quantum electrodynamics calculations. This agreed with the known proton-to-electron mass ratio.