〈 Berger | Dillon 〉

Why is the particle process (e-) + (e+) → W+ not possible?

Change needs to be conserved at each vertex.. so vertex doesn’t exist

Not only that.. the process can’t even be induced at loop level since it’s at each vertex where charge is conserved

This comes down to QED being a U(1) gauge theory, which implies a globally conserved charge.. and since all of our operators are gauge invariant we must have that the "vertices" representing those operators obey that too