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The vacuum decay rate A. T of orthopositronium (1'S&) formed in a gas has been measured to be A, T=7.0514+0.0014 ps '. Measurements of kT in four di6'erent 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 S& excited state is determined; A, T 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 A, 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.

The 2 3 5'i-* 2 3 / > i and 2 Z S\-+ 2 3 Po transition frequencies (vi and vo, respectively) in positronium have been measured for the first time and an improved measurement of the 2 3 S'i-• 2 3 P2 transition frequency (v 2 ) has been made. The results v 0 = 18 504.1 ± 10.0 ± 1.7 MHz, vi = 13 001.3 ± 3.9 ± 0.9 MHz, and V2 =8619.6 ± 2.7 ± 0.9 MHz are in reasonable agreement with the predictions of quantum electrodynamics which are, to order a 3 ft (ft is the frequency equivalent of the rydberg); vo(a 3 ) = 18496.1 MHz, v\(a 3 ) = 13 010.9 MHz, and V2(a 3 ) =8625.2 MHz. More precise measurements are planned to test the, as yet uncalculated, order a 4 ft corrections.

The vacuum decay rate XT of orthopositronium (o-Ps), formed in a gas, has been measured to be XT =7.0516 ±0.0013 ^s _1 . This result, which is 4 times more accurate than any previous measurement, is 10 standard deviations above the theoretical value. Extensive systematic tests included measurement of XT in three different gases, each to an accuracy of at least ±0.0025 /is -1 (350 ppm), and the use of two entirely different digital timing systems.PACS numbers: 36.10. Dr, ll.10.St, 12.20.Fv Measurement of the properties of the electron-positron bound state (positronium, Ps) provides a rigorous testing ground for bound-state quantum electrodynamics (QED). The decay rates of the l S 0 (p-Ps) and the 3 5i(<9-Ps) positronium ground states as well as the energy splitting between them (Av) have been calculated 1 ' 2 including radiative corrections up to order a 2 \na.Calculations of these quantities to order a 2 are considered to be straightforward, though technically difficult, and the Av calculation is nearing completion. 3 The importance of experimentally checking these theoretical calculations goes beyond bound-state QED because the relativistic bound-state formalism used has analogies in calculations of quark-antiquark bound states in QCD. For example,