Ah. Hoang et al., SINGLE-PHOTON ANNIHILATION CONTRIBUTIONS TO THE POSITRONIUM HYPERFINESPLITTING TO ORDER M(E)ALPHA(6), Physical review letters, 79(18), 1997, pp. 3387-3390
The single photon annihilation contributions for the positronium groun
d state hyperfine splitting are calculated analytically to order m(e)
alpha(6) using nonrelativistic QED. Based on intuitive physical argume
nts the same result can also be determined using results from previous
calculations. Our result completes the hyperfine splitting calculatio
n to order m(e) alpha(6). We compare the theoretical prediction with t
he most recent experimental measurement.