Recent progress of laser spectroscopy experiments on antiprotonic helium

The Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration is currently carrying out laser spectroscopy experiments on antiprotonic helium $(\mathrm{\bar{p}He^+ \equiv \bar{p} + He^{2+} + e^-})$ atoms at CERN’s Antiproton Decelerator facility. Two-photon spectroscopic techniques have been employed to reduce the Doppler width of the measured $\mathrm{\bar{p} He^+}$ resonance lines, and determine the atomic transition frequencies to a fractional precision of 2.3–5 parts in $10^9$. More recently, single-photon spectroscopy of buffer-gas cooled $\mathrm{\bar{p}He^+}$ has reached a similar precision. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as $M_{\bar{p}} / m_e = 1836.15267334(15)$, which agrees with the known proton-to-electron mass ratio with a precision of $8 \times 10^{−10}$. The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should enable further improvements in the experimental precision.