New precision measurement of the decay rate of singlet positronium

Abstract

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 ${\ensuremath{\lambda}}_{\mathit{s}}$=7990.9\ifmmode\pm\else\textpm\fi{}1.7 \ensuremath{\mu}${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. 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 ${\mathrm{\ensuremath{\alpha}}}^{2}$ln\ensuremath{\alpha} term in the singlet decay rate calculation.