Main Question or Discussion Point
Hello! I read in several (more accessible) papers (e.g. https://www.sciencedirect.com/science/article/pii/S1049250X0860110X) that the EDM of the electron is so small, because one needs to go to 4 loops or higher to get a non-zero effect. It seems like at 1 and 2 loops there are some symmetry arguments as to why those terms cancel, but for 3 loops there were extensive calculations made and in the end it turned out that all the diagrams cancelled and one gets zero contributions at 3 loops, too. I was wondering if there is any physical reason for all these 3 loops diagrams to perfectly cancel i.e. are there some symmetry arguments from which one could have at least got a hint that these diagram would cancel, without explicitly doing the math (usually there are some deeper meanings when some terms are perfectly equal)?