Three-photon-annihilation contributions to positronium energies at order $m{\alpha }^7$

Gregory S. Adkins, Minji Kim, Christian Parsons +1

Positronium spectroscopy (n=1 hyperfine splitting, n=2 fine structure, and the 1S-2S interval) has reached a precision of order 1 MHz. Vigorous ongoing efforts to improve the exper…