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.