Positronium decay rate

Fig. 7.2. The left-hand boxes show the fitted ortho-positronium decay rate at two values of isobutane pressure, for various start times of the fit to the component. The right-hand plot shows the observed decay rates, and their extrapolation to zero density, of Westbrook et al. (1989). The error bars on the individual points are approximately equal to the thickness of the line.

Fig. 7.3. Schematic illustration of the timed and gated slow positron beam used by Nico et al. (1990) to measure the vacuum decay rate of ortho-positronium. Reprinted from Physical Review Letters 65, Nico et at, Precision measurement of the ortho-positronium decay rate using the vacuum technique, 1344-1347, copyright 1990 by the American Physical Society.

Miller, B.N., Reese, T.L. and Worrell, G.A. (1996). Virial expansion of the positron and positronium decay rate in a dilute gas the linear contribution. 

Temperature dependence of positronium decay rates in gases. J. Phys. B At. Mol. Opt. Phys. 33 1047-1055. 

The ubiquitous problem encountered in all positronium decay rate measurements to date is isolating the positronium from the formation medium in order to determine the vacuum decay rate. In both gas and powder experiments the interactions (Ps quenching and Ps polarization) with these media need to be accounted for. This can involve extrapolations of A to zero density in the formation medium, as in the Michigan experiment, and/or spectroscopic corrections for 2q decays as in the Tokyo experiment. At Michigan we decided a decade ago to abandon powder media in future precision experiments because we could... 

Table 2. Positronium Decay Rates (Units -pis 1, Ex=Experiment, Th=Theory)...

The annihilation constant A is given as the number of annihilation events per unit density and unit time. It is obtained from the life time of positronium, which for the singlet state (para-positronium) is T2y = 1.25 X 10 ° s [13], and for the triplet state (ort/io-positronium) is = 1.42 X 10 s [14]. The positronium decay rate is related to the decay constant via... 

Experiments on these two gases, reported by Griffith and Heyland (1978), showed that a fast component, with a density-dependent decay rate, was present in the lifetime spectra, and this was tentatively linked to the dearth of long-lived ortho-positronium. Furthermore, it was found for mixtures of krypton with helium that the maximum value of F, which was observed at a concentration of around 0.01% of krypton, was in excess of the sum of the individual F-values for the two gases when pure. 

Once the background is subtracted, the component of the spectrum due to the annihilation of ortho-positronium is usually visible (see Figure 6.5(a), curve (ii) and the fitted line (iv)). The analysis of the spectrum can now proceed, and a number of different methods have been applied to derive annihilation rates and the amplitudes of the various components. One method, introduced by Orth, Falk and Jones (1968), applies a maximum-likelihood technique to fit a double exponential function to the free-positron and ortho-positronium components (where applicable). Alternatively, the fits to the components can be made individually, if their decay rates are sufficiently well separated, by fitting to the longest component (usually ortho-positronium) first and then subtracting this from the... 

Since the earliest work with positronium by Deutsch and coworkers (e.g. Deutsch, 1951 Deutsch and Brown, 1952) its annihilation lifetimes, or decay rates, have been studied both theoretically and experimentally. 

Fig. 7.5. Plot of the measured A(p), as defined by equation (7.7), in N2-isobutane mixtures. Data were taken at magnetic fields of 0.375 T (x) and 0.425 T (o). Reprinted from Physical Review Letters 72, Al-Ramadhan and Gidley, New precision measurement of the decay rate of singlet positronium, 1632-1635, copyright 1994 by the American Physical Society.

Fig. 8.2. Apparatus used by Mills for studies of the positronium negative ion Gi is a pile-up-reducing grid, G2 is the Ps -forming carbon film and G3 is the acceleration grid. See the text for further details. Reprinted from Physical Review Letters 50, Mills, Measurement of the decay rate of the positronium negative ion, 671-674, copyright 1983 by the American Physical Society.

Al-Ramadhan, A.H. and Gidley, D.W. (1994). New precision measurement of the decay rate of singlet positronium. Phys. Rev. Lett. 72 1632-1635. 

Griffith, T.C., Heyland, G.R., Lines, K.S. and Twomey, T.R. (1978b). The decay rate of ortho-positronium in vacuum. J. Phys. B At. Mol. Phys. 11 L743-L748. 

Mills Jr., A.P., Friedman, P.G. and Zuckerman, D.M. (1990). Decay rate and other properties of the positronium negative ion. In Annihilation in Gases and Galaxies, NASA Conference Publication 3058, ed. R.J. Drachman pp. 213-221. 

Precision redetermination of the fine-structure interval of the ground state of positronium and a direct measurement of the decay rate of parapositronium. Phys. Rev. A 2 707-721. 

In the case of the positronium spectrum the accuracy is on the MHz-level for most of the studied transitions (Is hyperfine splitting, Is — 2s interval, fine structure) [13] and the theory is slightly better than the experiment. The decay of positronium occurs as a result of the annihilation of the electron and the positron and its rate strongly depends on the properties of positronium as an atomic system and it also provides us with precise tests of bound state QED. Since the nuclear mass (of positronium) is the positron mass and me+ = me-, such tests with the positronium spectrum and decay rates allow one to check a specific sector of bound state QED which is not available with any other atomic systems. A few years ago the theoretical uncertainties were high with respect to the experimental ones, but after attempts of several groups [17,18,19,20] the theory became more accurate than the experiment. It seems that the challenge has been undertaken on the experimental side [13]. 

Abstract. The current experimental situation regarding tests of fundamental physics using positronium is reviewed. Five measurements are discussed and compared with theoretical predictions the singlet and triplet annihilation decay rates, the ground state and the n = 2 energy intervals, and the Doppler-free two-photon excitation of the IS to 2S transition. Previous results, recent progress (where appropriate), and the outlook for future improvements in these measurements are discussed. 

Abstract. We review our recent results on higher order corrections in positronium physics. We discuss a calculation of the recoil 0(ma6) corrections to the hyperfine splitting [1] and energy levels of a positronium atom [2], 0(ma7 In2 a) contributions to the positronium S-wave energy levels [3] and Ola2) radiative corrections to the parapositronium decay rate [4],... 

In this section we present the results of the calculation of the basic positronium properties using theoretical methods described above. We begin with positronium spectroscopy and then discuss the calculation of the second order corrections to the parapositronium decay rate. 

In this paper we briefly review all positronium (Ps) decay rate measurements (including those of the excited states and the Ps negative ion) that have been completed to date. The results are compared with theoretical values. The Ps system represents the most rigorous confrontation with theoretical decay rate calculations for any QED system. 

Table I Presently known values of energy differences and decay rates in positronium...

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