Electron-positron annihilation produces two photons: e^{-}+e^{+} \rightarrow 2 \gamma The electron and positron both have mass me, and the photons are massless. If-in the center of mass frame the electron has 3-momentum p in the z-direction and one of the photons comes out with 3-momentum k in the y-direction, write all four components of the4-momentum of all four particles and find k as a function of p.

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