Charged Pion Mass Determination and Energy — Calibration Standards Based on Pionic X-ray Transitions

Recent experiments are aiming at an accuracy of 1 ppm for the mass of the charged pion using the characteristic X-rays from exotic atoms. Once the pion mass is established with that precision, the narrow lines from medium Z pionic atoms can be used as a calibration standard in the few keV range. The precision of this new standard is not limited by the large natural line width of fluorescence X-rays and their complex structure due to multi-hole excitations.