i'm a bit confused about the currents in the expression for a matrix element for an interaction... e.g. you could have a current like (adjoint spinor)x(spinor) which is scalar, this makes sense to me. or you could have a current like (adjoint spinor)x(gamma matrix)x(spinor) which is vector according to all the books i've looked at. i don't get this - i would have thought that (gamma matrix)x(spinor) is either a vector or another spinor or something with 4 components, but then multiplying that by the adjoint spinor would just leave you with a scalar with 1 component. i'm guessing this is wrong but i can't see why. also, say the matrix element looks something like: (number)x[(adjoint)x(gamma-mu)x(spinor)]x[(adjoint)x(gamma-mu)x(spinor)] with one gamma-mu having the index up, and the other one index down; does this mean that i sum the above expression over mu; i.e. a sum of 4 terms each with a different gamma matrix? thanks in advance, and apologies for my lack of latex skills.