Laser spectroscopy of pionic helium atoms

Charged pions¹ are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons^2-9). Determining the mass of the π^- meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino^10-13. However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei¹. Metastable pionic helium (π⁴He⁺) is a hypothetical^14-16 three-body atom composed of a helium-4 nucleus, an electron and a π^- occupying a Rydberg state of large principal (n ≈ 16) and orbital angular momentum (l ≈ n - 1) quantum numbers. The π⁴He⁺ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out¹⁷. Its atomic structure is unique owing to the absence of hyperfine interactions^18,19 between the spin-0 π^- and the ⁴He nucleus. Here we synthesize π⁴He⁺ in a superfluid-helium target and excite the transition (n, l) = (17, 16) → (17, 15) of the π^--occupied π⁴He⁺ orbital at a near-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the π^- and undergoing fission^20,21. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π⁴He⁺. This work enables the use of the experimental techniques of quantum optics to study a meson.