Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

hi

in "Introduction to Elementary Particles" ed. 2 / David Griffiths
the writer states that a bilinear vertex in two different fields is always impossible (my words).
or in other words: theoretically we can't have a fundamental vertex with one particle coming in and one going out.

and I feel like an idiot asking, but I have to ask since to the writer it seems obvious that I should know why, and unfortunately I don't. Which conservation laws does it violate?

I'll appreciate any answer.
Thank you
Ariel