Resolution of the Orthopositronium-Lifetime Puzzle

The long-standing discrepancy [G. S. Adkins, R. N. Fell, and J. Sapirstein, Ann. Phys. (N.Y.) 295, 136 (2002)] between the theoretical calculations of the orthopositronium ($o\mathrm{\text{−}}\mathrm{P}\mathrm{s}$) annihilation decay rate (${\lambda }_{\mathrm{T}}=1/\mathrm{\text{lifetime}}$) and some of the experimental measurements has been resolved. A focused beam of positrons incident on a special nanoporous silica film produces near-thermal energy $o\mathrm{\text{−}}\mathrm{P}\mathrm{s}$ in vacuum that is slow enough to be virtually free of perturbing interactions. The fitted decay rate requires only a 500 ppm correction for nonthermal $o\mathrm{\text{−}}\mathrm{P}\mathrm{s}$ effects. The new value of ${\lambda }_{\mathrm{T}}=7.0404(10)(8)\mu {\mathrm{s}}^{-1}$ is in excellent agreement with theory.