Hi, I am self-teaching Quantum Elctrodynamics, and have come across something which I do not understand. I would appreciate feedback from anyone on this specific issue from Atchison & Hey, "Guage Theories in Particle Physics" pg 238-239: In calculating the u-channel electron-muon scattering amplitude at the one-photon exchange, one can simplify the calculation by introducing the electron and muon tensors: LμγMμγ, where Lμγ = 2[k'μkγ+k'γkμ+(q2/2)gμγ] (electron tensor) and Mμγ = 2[p'μpγ+p'γpμ+(q2/2)gμγ] (muon tensor) Now qμ = (k-k')μ = (p-p')μ is the 4-momentum of the exchanged photon; p, p' are the intial and final momenta of the muon; k, k' the initial and final 4-momenta of the electron. It is claimed that qμLμγ = qγLμγ = 0, which is fine because L is the product of 2 4-currents and qμjμe- = 0. However, according to the text that I am reading, the qμLμγ = qγLμγ condition implies that we can replace p' in the muon tensor with (p+q); ie, Meffective = 2[2pμpγ + (q2/2)gμγ. Does anyone know how to go from the condition qμLμγ = 0 to the constraint condition p' = (p+q)? Thank you in advance for your assistance.