The vacuum decay rate, λT, of orthopositronium (o-Ps), formed in a gas, has been measured to be λT=7.0516±{}0.0013 {μ}s^-1. This result, which is 4 times more accurate than any previous measurement, is ten standard deviations above the theoretical value. Extensive systematic tests included measurement of λT in 3 different gases, each to an accuracy of at least ±{}0.0025 {μ}s^-1 (350 ppm), and the use of two entirely different digital timing systems.
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Westbrook et al. (1987) studied this question.
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