Precision measurement of the orthopositronium decay rate using the vacuum technique

Abstract

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 ${\ensuremath{\lambda}}_{\mathit{T}}$ is determined to be 7.0482\ifmmode\pm\else\textpm\fi{}0.0016 \ensuremath{\mu}${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ 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 6.2\ensuremath{\sigma} level the discrepancy between theory and a recent 200-ppm measurement of ${\ensuremath{\lambda}}_{\mathit{T}}$ in gases.