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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?