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 [Formula: see text] atoms at CERN's Antiproton Decelerator facility. Two-photon spectroscopic techniques have been employed to reduce the Doppler width of the measured [Formula: see text] resonance lines, and determine the atomic transition frequencies to a fractional precision of 2.3-5 parts in 109 More recently, single-photon spectroscopy of buffer-gas cooled [Formula: see text] has reached a similar precision. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as [Formula: see text], which agrees with the known proton-to-electron mass ratio with a precision of 8×10-10 The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should enable further improvements in the experimental precision.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.