New precision measurement of the decay rate of singlet positronium

The singlet decay rate has been measured using magnetic singlet-triplet state mixing for positronium formed in a ${\mathit{N}}_2$-isobutane gas mixture. We find ${\lambda }_{\mathit{s}}$=7990.9±1.7 μ${\mathrm{s}}^{\mathrm{-}1}$. At 215 ppm this result is 6.5 times more accurate than the previous measurement [D. W. Gidley, A. Rich, E. Sweetman, and D. West], and is the first measurement sensitive enough to test the relative order ${\mathrm{\alpha }}^2$lnα term in the singlet decay rate calculation.