Precision measurement of the orthopositronium vacuum decay rate using the gas technique

The vacuum decay rate λT of orthopositronium (1³{S}₁$) formed in a gas has been measured to be ${{λ}}T=7.0514±0.0014μ{s}^{{{-}}1}$. Measurements of ${{λ}}T$ in four different gases, all in agreement, are averaged to obtain this result. As systematic tests, two entirely separate digital timing systems are simultaneously used throughout the experiment; the cross section for collisional quenching of the long-lived 2 ³S₁ excited state is determined; λT is redetermined in two of the gases (N₂ and Ne) using only high-gas-density measurements; and the collisional quenching rate of water vapor, the major residual gas contaminant, is directly measured. The final value of λT from this gas experiment, which represents a factor of 4 improvement in accuracy over previous measurements, is 9.4 experimental standard deviations above the theoretical value.