Laser Spectroscopy of Exotic Helium Atoms Containing Antiprotons and Pions

Metastable antiprotonic helium is an exotic atom composed of a helium nucleus, electron, and an antiproton. It is among the hadron-antihadron systems with the longest known lifetimes. Laser light can be used to excite atomic transitions involving the antiproton orbital. Comparisons with the results of QED calculations allowed the antiproton-to-electron mass ratio to be determined as 1836.1526734(15). Efforts are currently underway to improve the experimental precision using CERN’s ELENA facility.

Metastable pionic helium (πHe+) contains a negative pion occupying a state of n≈l-1≈17, and retains a 7 ns average lifetime. We recently used the 590 MeV ring cyclotron facility of Paul Scherrer Institute to synthesize the atoms, and irradiated them with infrared laser pulses. This induced a pionic transition within the atom and the π- being absorbed into the helium nucleus. This constitutes the first laser excitation and spectroscopy of an atom containing a meson. By improving the experimental precision, the pion mass may be determined to a high precision as in the antiproton case. 

[1] M. Hori et al. Science 354, 610 (2016).
[2] M. Hori, H. Aghai-Khozani, A. Soter, A. Dax, D. Barna, Nature 581, 37 (2020).
[3] A. Soter, H. Aghai-Khozani, D. Barna, A. Dax, L. Venturelli, M. Hori, Nature 603, 411 (2022).

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