title: Positronium hyperfine interval measured via saturated absorption spectroscopy.
abstract: We report Doppler-free measurements of the positronium (Ps) Lyman-α transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength λ(L) = 243.0218 ± 0.0005 nm, we also observed a crossover resonance at λ(C) = 243.0035 ± 0.0005 nm, arising from the excitation of 1(3)S(1) atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1(1)S(0) ground state. Since (λ(L)-λ(C)) is related to the Ps hyperfine interval E(hfs), this observation constitutes the first optical measurement of this quantity and yields E(hfs) = 198.4 ± 4.2 GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states.
authors.affiliation: Department of Physics and Astronomy, University of California, Riverside, 92521-0413, USA.
authors.forename: D B
authors.initials: DB
authors.lastname: Cassidy
authors.forename: T H
authors.initials: TH
authors.lastname: Hisakado
authors.forename: H W K
authors.initials: HW
authors.lastname: Tom
authors.forename: A P
authors.initials: AP
authors.lastname: Mills