Precision measurement of the orthopositronium decay rate using the vacuum technique

A focused beam of 700-eV positrons is used to form orthopositronium (o-Ps) in evacuated, MgO-lined cavities of different sizes. The o-Ps decay rate ${\lambda }_{\mathit{T}}$ is determined to be 7.0482±0.0016 μ${\mathrm{s}}^{\mathrm{-}1}$ after extrapolation to infinite cavity size. This 230-ppm result is 4 times more accurate than previous measurements utilizing the vacuum technique. Subject to radically different systematic effects, it corroborates at the 6.2σ level the discrepancy between theory and a recent 200-ppm measurement of ${\lambda }_{\mathit{T}}$ in gases.