title: Direct observation of the hyperfine transition of ground-state positronium.
abstract: We report the first direct measurement of the hyperfine transition of the ground state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 10 kW power using a gyrotron, a mode converter, and a Fabry-Pérot cavity. The hyperfine transition has been observed with a significance of 5.4 standard deviations. The transition probability is measured to be A = 3.1(-1.2)(+1.6) × 10(-8) s(-1) for the first time, which is in good agreement with the theoretical value of 3.37 × 10(-8) s(-1).
authors.affiliation: Department of Physics, Graduate School of Science, and International Center for Elementary Particle Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Yamazaki
authors.forename: A
authors.initials: A
authors.lastname: Miyazaki
authors.forename: T
authors.initials: T
authors.lastname: Suehara
authors.forename: T
authors.initials: T
authors.lastname: Namba
authors.forename: S
authors.initials: S
authors.lastname: Asai
authors.forename: T
authors.initials: T
authors.lastname: Kobayashi
authors.forename: H
authors.initials: H
authors.lastname: Saito
authors.forename: I
authors.initials: I
authors.lastname: Ogawa
authors.forename: T
authors.initials: T
authors.lastname: Idehara
authors.forename: S
authors.initials: S
authors.lastname: Sabchevski