Solution of Orthopositronium lifetime Puzzle

The intrinsic decay rate of orthopositronium formed in SiO2 powder is measured using the direct 2γ correction method such that the time dependence of the pick-off annihilation rate is precisely determined. The decay rate of orthopositronium is found to be 7.0396 ± 0.0012(stat.) ± 0.0011(sys.)μs, which is consistent with our previous measurements with about twice the accuracy. Results agree well with the O(α) QED prediction, and also with a result reported very recently using nanoporous film. ICEPP International Center for Elementary Particle Physics, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Mailing address: CERN EP-Div, CH-1211, Geneva 23, Switzerland, E-mail address: [email protected] 1 History: Orthopositronium lifetime puzzle Positronium (Ps), the bound state of an electron and a positron, is a purely leptonic system, and the triplet (1S1) state of Ps, orthopositronium(o-Ps), decays slowly into three photons. Precise measurement of this decay rate gives us direct information about quantum electrodynamics(QED) in bound state. Three precision measurements[1, 2, 3] of the o-PS decay rate were performed at Ann Arbor, which reported decay rate values much larger, i.e., 5.2 – 9.1 experimental standard deviations, than a QED prediction[4] (7.039934(10) μs) corrected up to O(α). This discrepancy has been referred as ‘orthopositronium lifetime puzzle’, and was long-standing problem. To elucidate discrepancies, a variety of experiments have since been carried out to search for the exotic decay mode of o-Ps, resulting in no evidence so far [5, 6, 7, 8, 9, 10]. As some fraction of o-Ps inevitably results in ‘pick-off’ annihilations due to collisions with atomic electrons of the target material, the observed o-Ps decay rate λobs is a sum of the intrinsic o-Ps decay rate λo-Ps and the pick-off annihilation rate into 2γ’s, λpick, i.e., λobs(t) = λ3γ + λpick(t). (1) λpick(t) is proportional to the rate of o-Ps collisions with the target materials, i.e.; λpick = nσav(t), where n is product of the density of the target, σa the annihilation cross-section, and v(t) the time dependent velocity of o-Ps. Due to the thermalization process of o-Ps, this necessitates expressing λpick as a function of time whose properties are dependent on the surrounding materials. Thermalization process should be carefully treated even in the cavity experiment[3]. Although pickoff correction is small in cavities, disappearance of o-Ps through the cavity entrance aperture has large contribution to λobs. This disappearance rate is also proportional to v(t), as the same reason. Since the rate of elastic collision is extremely small in cavities, it takes much time, longer than 1 μs, to thermalize well, and the disappearance rate still depends strongly on time. In previous measurements[1, 2, 3], λobs’s were measured by varying the densities of the target materials, size of the cavities and also the entrance aperture of the cavities. The extrapolation to zero density or aperture was expected to yield the decay rate in a vacuum, λ3γ , under the assumption of quick thermalization (shorter than 170-180 nsec) with constant o-Ps velocity. However, this assumption contains a serious systematic error as pointed out in reference[11, 12]. We have proposed the following entirely new method[12], which is free from above-mentioned systematic error. The energy distribution of photons from the 3-body decay is continuous below the steep edge at 511 keV, whereas the pick-off annihilation is 2-body which produces a 511 keV monochromatic peak. Energy and timing information are simultaneously measured with high-energy resolution germanium detectors such that λpick(t)/λ3γ can be determined from