New Precision Measurements of the Decay Rates of Singlet and Triplet Positronium

Both the singlet and triplet decay rates (${\lambda }_S, {\lambda }_T$) of positronium have been measured in a new direct timing experiment based on singlet-triplet state mixing for positronium formed in isobutane gas in a magnetic field. ${\lambda }_S=7.994\mathrm{}\pm 0.011$ ${\mathrm{nsec}}^{-1\mathrm{}}$ was found, a factor of 10 improvement in accuracy and the first measurement sensitive enough to test the radiative corrections to ${\lambda }_S$. ${\lambda }_T=7.051\mathrm{}\pm 0.005$ μ${\sec }^{-1\mathrm{}}$ was found, in agreement with previous measurements. The combined ${\lambda }_T$ data are not in satisfactory agreement with theory.