1. The problem statement, all variables and given/known data Griffiths Particle Physics, problem 7.24. Evaluate the amplitude for electron-muon scattering in the center-of-momentum system, assuming the e and mu approach one another along the z-axis, repel, and return back along the z-axis. Assume initial and final particles to have helicity +1. [Answer is given: M=-2(g^2)] 2. Relevant equations The solutions to the Dirac equation. Also, the form of the amplitude derived for two-particle scattering from Feynman rules. I'll have to scan the equation because it's complicated and I don't know Latex. Sorry. I will upload a scan of the eq's and my work later tonight. I just am hoping someone will be be familiar enough with the subject to help anyway. 3. The attempt at a solution The equation for this process obtained from the lowest-level Feynman diagram contains, u's, v's, p's (4-vectors of the 4 particles, or 2 before and after), and gamma matricex (gamma_mu). The u's and v's are the spinors that satisfy the Dirac equation, and u-bar is (u*)(gamma_0) where the * denotes the adjoint or Hermitian conjugate. The particular u's and v's to use depend on the spins of the particle, so taking the assumption given regarding their helicity, I assume they all spin counter-clockwise around their direction of motion - i.e., the right-hand rule. Therefore, taking the direction of motion to be along the z-axis, have particles 1 and 4 with spin up, 2 and 3 spin down. Is this properly applied? Then I simply plug in the values for the u,v spinors and their transpose conjugates from Griffiths section on the Dirac equation (section 7.2). The gamma_0 's just switch the sign of the last two entries in the transposed spinors, and the "N_i" coefficients of the spinors can just come out of the expression and group together. So basically I am not sure now how to multiply something of the form (u*)(gamma_mu)(u) Do I just add (u*)gamma_0 + (u*)gamma_1 + ... (u*)gamma_3 ? Actually here it will be (u3*)(gamma_mu)(u1)(u4*)(gamma_mu)(u2) where the numbers just indicate the particle's label. I'd really appreciate any help!