Description
We recently carried out laser spectroscopy of three-body metastable pionic helium atoms (${\pi}{\rm ^4He}^+\equiv\pi^-+{\rm ^4He}^{2+}+e^-$) at the Paul Scherrer Institute's (PSI) 590 MeV ring cyclotron facility. This constituted the first laser excitation of an atom containing a meson. Of the many known varieties of pionic atom, ${\pi}{\rm ^4He}^+$ retains an anomalously long, nanosecond-scale lifetime even in liquid helium. Its structure is unique among the atoms that can be studied by laser spectroscopy due to the absence of hyperfine interactions between the spin-0 $\pi^-$ and the $^4$He nucleus. By comparing the experimental transition frequencies with the results of quantum electrodynamics (QED) calculations, the mass of $\pi^-$ and other properties may be determined with a higher precision than before. The proposed High Intensity Muon Beamline (HIMB) upgrade of the PSI facility would benefit this experiment by the increased availability of the high-intensity pion and muon beamlines. These studies will complement the ongoing laser spectroscopy of antiprotonic helium atoms by the ASACUSA collaboration at CERN's Antiproton Decelerator (AD) and ELENA.