Positronium—the bound state of an electron and positron—appears to decay more rapidly than calculations based on quantum electrodynamics predict. Four experiments over the last ten years have measured decay rates for positronium's triplet ground state that were higher than current theoretical estimates, but never by more than a few standard deviations. In a recent experiment with four times greater precision, Chris Westbrook, David Gidley, Ralph Conti and Arthur Rich at the University of Michigan determined a decay rate that is ten standard deviations higher than the theory predicts. They reported a value of per second, compared with the theoretical prediction of per second. Numerical integration contributes an uncertainty in the fifth decimal place of the prediction.
Barbara G. Levi; Experiment Challenges Theory in Positronium Measurements. Physics Today 1 September 1987; 40 (9): 22–24. https://doi.org/10.1063/1.2820177
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