We study single collisions between orthopositronium beams and targets with unpaired electrons, in which the conversion (quenching) of the long-lived orthopositronium into the short-lived parapositronium occurs as a result of electron exchange. We consider unpolarized and polarized targets with one and two unpaired electrons. Angular-momentum coupling and density-matrix techniques are used to calculate the probabilities of obtaining either para- or orthopositronium after the collisions. The probability of the initial orthopositronium being converted to parapositronium (quenching probability) is given in terms of complex scattering amplitudes labeled with total electron spin. Quenching probabilities are calculated for polarized and unpolarized targets, with and without detection of the spins of the targets after the collisions. Ratios of the probabilities of producing parapositronium to that of obtaining orthopositronium are given in terms of electron-spin amplitudes.
When orthopositronium collides with a closed-shell atom or ion the ``pick-off'' process is the dominant process for positron annihilation. In pick-off annihilation the positron annihilates with electrons in the target but not with the electron that is part of the positronium atom. The rate for pick-off annihilation, expressed in terms of the dimensionless parameter
1Zeff, for orthopositronium scattering from
Li+,Na+, and
K+ was computed with an
L2 variant of the fixed-core stochastic variational method. The threshold
1Zeff for
Li+ and
Na+, which can form a stable bound state with Ps were found to be 0.067 and 0.73, respectively, while that for
K+, which does not bind Ps, was 0.064.
The scattering of orthopositronium from Kr and Xe is investigated with the fixed core stochastic variational method. The scattering lengths for Ps-Kr and Ps-Xe scattering in the fixed core approximation are
3.18a0 and
3.82a0, respectively. Inclusion of core-polarization potentials leads to a significant reduction in the scattering lengths. The Ps-Kr and Ps-Xe scattering lengths were
1.98a0 and
2.29a0 for polarization potentials that were tuned to an average of the model electron-atom and positron-atom scattering lengths for the respective atoms. Other choices of the polarization potential resulted in scattering lengths that could be larger by
0.3a0 or smaller by
0.8a0. The pick-off annihilation parameter
1Zeff has also been computed as part of the analysis. The present values of
1Zeff are about 3 to 10 times smaller than the accepted experimental values since short-range electron-positron correlations were ignored when the annihilation matrix element was evaluated.
The scattering of orthopositronium from He, Ne, Ar,
Li+, Na+, and
K+ is investigated with the fixed-core stochastic variational method. The scattering length for $\mathrm{Ps}\ensuremath{-}\mathrm{He}$ scattering was
1.57a0, a value consistent with the threshold cross sections derived from three positron lifetime experiments. The scattering lengths for the $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{N}\mathrm{e}$ and $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{r}$ systems were
1.55a0 and
1.79a0, respectively. That there was only a 15% variation among the scattering lengths is compatible with the experiment of Coleman et al. [J. Phys. B 27, 981 (1994)], who found the low-energy cross sections for He, Ne, and Ar to be about the same size. The scattering lengths for Ps scattering from
Li+, Na+, and
K+ were
12.9a0, 28.5a0, and $\ensuremath{-}{1.9a}_{0}.$ The relatively small magnitude of the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ scattering length strongly supports previous suggestions that the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ system does not support a bound state. The annihilation parameter
1Zeff has also been computed as part of the analysis. The present values of
1Zeff are about 2.5--3 times smaller than the accepted experimental values since short-range electron-positron correlations were not taken into consideration when the annihilation matrix element was evaluated.
The positronium ion (
Ps−), a coulombic three-body bound state of two electrons and a positron, predominantly decays via electron-positron annihilation into electromagnetic final states. While its radiative decay channels have been extensively studied, much less attention has been given to weak processes in this system. In this work, we investigate the rare decay
Ps−→e−νμνˉμ, obtained by replacing the photon in
Ps−→e−γ with a virtual
Z boson. Treating the three-body process as an effective two-body transition,
Ps−→e−Z∗(→νμνˉμ), we compute the decay rate by explicitly evaluating all spin configurations of the initial bound state and final particles. The result agrees with that obtained using the standard spin-summation formalism of quantum field theory. We find that the branching ratio is comparable to that of the weak decay of ortho-positronium,
o-Ps→γννˉ.
Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.