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

Abstract

The vacuum decay rate ${\ensuremath{\lambda}}_{T}$ of orthopositronium (${1}^{3}$${S}_{1}$) formed in a gas has been measured to be ${\ensuremath{\lambda}}_{\mathit{T}}$=7.0514\ifmmode\pm\else\textpm\fi{}0.0014\ensuremath{\mu}${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. Measurements of ${\ensuremath{\lambda}}_{T}$ 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 $^{3}S_{1}$ excited state is determined; ${\ensuremath{\lambda}}_{T}$ is redetermined in two of the gases (${\mathrm{N}}_{2}$ 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 ${\ensuremath{\lambda}}_{T}$ 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.