Physics quiz question: Feynman diagram vertex validity?

Physics 205B Quiz 7, spring semester 2014
Cuesta College, San Luis Obispo, CA

This Feynman diagram vertex depicts an electron:
(A) remaining an electron.
(B) converting into a positron.
(C) annihilating with an electron.
(D) annihilating with a positron.
(E) (this vertex is not possible, as it is invalid.)

Correct answer (highlight to unhide): (E)

This Feynman diagram vertex appears to two electrons annihilating, but only if time runs from bottom-to-top instead of from left-to-right as explicitly noted for the convention used in this class.

If read from left-to-right, the vertex may be misinterpreted as an electron converting into a positron (a "time-reversed" electron path), but this would violate the second of the four rules of drawing Feynman diagrams:

2. You may only connect these lines if you have two lines with arrows meeting a single wiggly line. Note that the orientation of the arrows is important! You must have exactly one arrow going into the vertex and exactly one arrow coming out.

   --Phillip "Flip" Tanedo, "Let's Draw Feynman Diagrams," Cornell University/USLHC Collaboration

Thus this diagram is invalid.

Sections 30882, 30883
Exam code: quiz07bC4n
(A) : 0 students
(B) : 9 students
(C) : 4 students
(D) : 8 students
(E) : 17 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.34