The [Formula presented] 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.
Trassinelli, M., Anagnostopoulos, D. F., Borchert, G., Dax, A., Egger, J. P., Gotta, D., … Wells, A. (2016). Measurement of the charged pion mass using X-ray spectroscopy of exotic atoms. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 759, 583–588. https://doi.org/10.1016/j.physletb.2016.06.025