$\mathcal{C}\mathcal{P}\mathcal{T}$-invariant two-fermion Dirac equation with extended hyperfine operator

Viktor Hund, Hartmut Pilkuhn

For the S-states of muonium and positronium, the hyperfine shifts to the order ${\alpha }^6$ of a recently derived two-fermion equation with explicit $\mathcal{C}\mathcal{P}\mathcal{T}$-invariance are checked agai…