Proposed method for laser spectroscopy of pionic helium atoms to determine the charged-pion mass

الأصل · EN

Metastable pionic helium (π He+) is a three-body atom composed of a helium nucleus, an electron occupying the 1s ground state, and a negatively charged pion π- in a Rydberg state with principal- and orbital angular momentum quantum numbers of n ℓ+1 16. We calculate the spin-independent energies of the π ³He+ and π ⁴He+ isotopes in the region n=15--19. These include relativistic and quantum electrodynamics corrections of orders R∞α² and R∞α³ in atomic units, where R∞ and α denote the Rydberg and fine structure constants. The fine-structure splitting due to the coupling between the electron spin and the orbital angular momentum of the π-, and the radiative and Auger decay rates of the states are also calculated. Some states (n,ℓ)=(16,15) and (17,16) retain nanosecond-scale lifetimes against π- absorption into the helium nucleus. We propose to use laser pulses to induce π- transitions from these metastable states, to states with large (10¹¹ s⁻¹) Auger rates. The π He²⁺ ion that remains after Auger emission of the 1s electron undergoes Stark mixing with the s, p, and d states during collisions with the helium atoms in the experimental target. This leads to immediate nuclear absorption of the π-. The resonance condition between the laser beam and the atom is thus revealed as a sharp spike in the rates of neutrons, protons, deuterons, and tritons that emerge....(continued)