The rest mass energy of a proton (or antiproton) is approximately 938 MeV. Therefore, the total energy of the photon is $2.09 \times 10^3 \mathrm{MeV} = 2 \times 938 \mathrm{MeV} + K_{p} + K_{\bar{p}}$, where $K_{p}$ and $K_{\bar{p}}$ are the kinetic energies of
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