Precision Spectroscopy of the Pionic Helium-4

The transition frequency of (n, l) = (17, 16) -> (16, 15) in pionic helium-4 is calculated to an accuracy of 4 ppb (parts per billion), including relativistic and quantum electrodynamic corrections up to O(R-infinity alpha(5)). Our calculations significantly improve the recent theoretical values [Hori et al., Phys. Rev. A 89, 042515 (2014)]. In addition, collisional effects between pionic helium and target helium on transition frequencies are estimated. Once measurements reach the ppb level, our Letter will improve the value of the pi(-) mass by 2-3 orders of magnitude.

Automated summary

The transition frequency of (n, l) = (17, 16) -> (16, 15) in pionic helium-4 is calculated with high accuracy, including relativistic and quantum electrodynamic corrections. The calculations significantly improve the previous theoretical values and estimate the collisional effects between pionic helium and target helium on the transition frequencies. This study has the potential to greatly improve the value of the pi(-) mass once measurements reach the ppb level.