In reading a theoretical paper on electric dipole moments (well, OK, skimming through it looking for numbers), I ran across several Feynman diagrams with an “X” on one of the particle arrows. The caption contains the presumably-intended-to-be-helpful note “The cross denotes a mass insertion.”
I have no idea what that means, and neither does the local person I ask to help me interpret such things. None of the undergrad-level nuclear/ particle texts we looked at contained an explanation, either.
This almost certainly means that it is some subtle technical point that is vastly beyond the level of knowledge that I need for the thing I was working on. Not knowing things bugs me, though, so anybody who does know what it means, and can provide or point to an upper-level-undergrad level explanation, please leave me a comment.