Measurement of the charged pion mass using X-ray spectroscopy of exotic atoms

The 5g−4f5g−4f transitions in pionic nitrogen and muonic oxygen were measured simultaneously by using a gaseous nitrogen–oxygen mixture at 1.4 bar. Due to the precise knowledge of the muon mass the muonic line provides the energy calibration for the pionic transition. A value of (139.57077 ± 0.00018) MeV/c2 (± 1.3 ppm) is derived for the mass of the negatively charged pion, which is 4.2 ppm larger than the present world average.