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

Abstract

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.

Calculation ofα2corrections to parapositronium decay

“…1 Thus, the present experimental situation is rather unclear. If future experimental efforts confirm the Ann Arbor results (6), the orthopositronium lifetime puzzle could be solved if B o turns out to be unusually large, e.g.…”

“…In Sections III and IV we discuss, respectively, the so-called soft and hard O (α 2 ) contributions. The final result is presented in Section V. 1 Another measurement of the o-Ps lifetime was performed earlier in Mainz [14] with the result Γ exp o−Ps = 7.031(7) µs −1 . This result is consistent with the theoretical predictions but uncertainties are clearly larger.…”

“…However, there are a few observables where the agreement is not satisfactory. The best known example is the lifetime of the orthopositronium (o-Ps), where some experimental results [1,2] differ from the theory by several standard deviations. The theoretical prediction is not yet complete, because two-loop QED corrections to the o-Ps decay have not been evaluated yet.…”

Two-Loop Correction to the Orthopositronium Decay Rate

“…The two most precise results are the 200 ppm Michigan gas [32,33] and vacuum [34] results of 1989 and 1990, which are in the latter group. Improved measurements by both the Tokyo [36] and the Michigan [39] groups are in the works, and the inconclusive experimental situation might soon be improved.…”

“…Subsequent theoretical refinements shifted the result only slightly. Since 1980, all measured results have fallen into two disjoint and mutually inconsistent groups: those clustered around the theoretical prediction of 7.040 µs −1 [31,35,36] and those near 7.050 µs −1 [30,[32][33][34].…”