Controlling Positronium Annihilation with Electric Fields

Precision Microwave Spectroscopy of the Positronium 2 ^{3}S_{1}→2 ^{3}P_{2} Interval

We report the results of a new measurement of the positronium 2 ^{3}S_{1}→2 ^{3}P_{2} (ν_{2}) interval. Using a modified experimental arrangement we have significantly reduced the effects of microwave reflections, which in previous experiments resulted in shifts and asymmetric line shapes. With the improved apparatus we obtain an experimental value of ν_{2}=8627.94±0.95  MHz, which is within 1.3σ of the theoretical value 8626.71±0.08  MHz.

Quantum suppression of antihydrogen formation in positronium-antiproton scattering

The interaction of antiprotons with low-energy positronium atoms is a fundamental three-body problem whose significance is its utility for formation of antihydrogen. Particular importance resides in understanding processes involving excited positronium states. Until recently such studies were performed using classical techniques. However, they become inapplicable in the low-energy domain. Here we report the results of comprehensive quantum calculations, which include initial excited positronium states with principal quantum numbers up to n i = 5. Contrary to expectation from earlier work, there are only muted increases in the cross-sections for antihydrogen formation for n i > 3. We interpret this in terms of quantum suppression of the reaction at higher angular momenta. Furthermore, the cross-sections for elastic scattering are around two orders of magnitude higher, which we attribute to the degeneracy of the positronium states. We outline some experimental consequences of our results.