Precision measurement of the orthopositronium decay rate using the vacuum technique

A focused beam of 700-eV positrons is used to form orthopositronium ({ital o}-Ps) in evacuated, MgO-lined cavities of different sizes. The {ital o}-Ps decay rate {lambda}{sub {ital T}} is determined to be 7.0482{plus minus}0.0016 {mu}s{sup {minus}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{sigma} level the discrepancy between theory and a recent 200-ppm measurement of {lambda}{sub {ital T}} in gases.