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## Main Question or Discussion Point

1. Are there possible (4th order)process of "decay of photon" [tex]\gamma\rightarrow 3 \gamma[/tex] with one-photon initial state and 3-photon final state? Intuition tells me there are no such processes. But because of crossing symmetry of scattering amplitude one can argue that such processes should be possible.

2. In one book I found statement that 4-momentum transfer squared [tex]q^2[/tex] (through vitual photon) in electron-muon scattering is less than zero. But even in ultra-relativistic limit when electron mass can be neglected one can find that:

[tex]q^2=-2k\cdot k^{'}[/tex] where [tex]k,k^{'}[/tex] are initial and final 4-momentum vectors of electron.

But [tex]k\cdot k^{'}[/tex] depends on scattering angle and so the sign of this term....or maybe I missed something? Can someone give me a hint?

2. In one book I found statement that 4-momentum transfer squared [tex]q^2[/tex] (through vitual photon) in electron-muon scattering is less than zero. But even in ultra-relativistic limit when electron mass can be neglected one can find that:

[tex]q^2=-2k\cdot k^{'}[/tex] where [tex]k,k^{'}[/tex] are initial and final 4-momentum vectors of electron.

But [tex]k\cdot k^{'}[/tex] depends on scattering angle and so the sign of this term....or maybe I missed something? Can someone give me a hint?