# Tag Info

Let's just focus on the top vertex. First, draw an arrow pointing down. Then, you can determine from charge conservation (or looking for the relevant term in the lagrangian) that the particle must be $W^+$. Because $(W^+)^\ast=W^-$, this vertex has precisely the same value if you reverse that arrow and replace the $W^+$ with a $W^-$. Here I am just using ...