Positronium Production and Scattering below Its Breakup Threshold

Recent findings on the similarity between electron and positronium scattering at the same velocity [Brawley et al., Science 330, 789 (2010)] have guided us towards the realization of a detectable flux of positronium atoms at beam energies five times lower than previously obtained, enabling total cross sections to be measured in the energy range $\sim (1–7)\mathrm{eV}$ for the first time. In collision with Ar and Xe, the total cross sections of positronium are found to be smallest at the lowest energy probed, approaching those of the Ramsauer-Townsend minima for electron projectiles. Additional structure has been observed in the case of positronium scattering at incident energies around 5 eV.

Figure 1

$Q_T^{\mathrm{Ps}}$ (blue) for Ar compared to available theories and a selection of $Q_T^{-}$ (red) and $Q_T^{+}$ (green) for the same target. Also shown in the inset are $Q_m^{\mathrm{Ps}}$ (cyan) for collisions at less than 0.5 a.u. Experimental $Q_T^{\mathrm{Ps}}$: circles, present results; hollow triangles, Ref. [14]; hollow squares, Refs. [6, 23]. Experimental $Q_m^{\mathrm{Ps}}$: crossed square, Ref. [24]; crossed diamond, Ref. [25]; short dash, Ref. [26]; crossed circle, Ref. [27]. Experimental $Q_T^{-}$: up triangles, Ref. [28]; squares, Ref. [29]; circles, Ref. [30]. Recommended experimental $Q_T^{+}$: down triangles, Ref. [31]. Theoretical $Q_T^{\mathrm{Ps}}$: (solid line) coupled-pseudostate plus first Born approximation [32]; (long dashed line) coupled-pseudostate plus first Born approximation without exchange [33]; (crosses) stochastic variational method [34]; (dash double dotted line) impulse approximation [35]; (dash dotted line) pseudopotential approach [36].

Figure 2

$Q_T^{\mathrm{Ps}}$ (blue) for Xe compared with available theories and a selection of $Q_T^{-}$ (red) and $Q_T^{+}$ (green) for the same target. Experimental $Q_T^{\mathrm{Ps}}$: circles,) present results; hollow up triangles, Ref. [14]. Experimental $Q_T^{-}$: circles, Ref. [37]; up triangles, Ref. [28]; squares, Ref. [29]; diamonds, Refs. [38, 39]. Recommended experimental $Q_T^{+}$: down triangles, Ref. [31]. Theoretical $Q_T^{\mathrm{Ps}}$: (dash dotted line) static exchange calculation [32]; (crosses) stochastic variational method [40]; (solid line) sum of static exchange and inelastic cross sections [32, 41].