20140520

Physics quiz question: Feynman diagram vertex interpretation

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

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

Correct answer (highlight to unhide): (A)

This Feynman diagram vertex is valid, as it obeys the four rules of drawing Feynman diagrams, especially the first two:
  1. You can draw two kinds of lines, a straight line with an arrow or a wiggly line. You can draw these pointing in any direction.
  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
Since the diagram is valid, tracing the straight line with arrows from left-to-right (the convention used in this class) shows a neutron converting into a proton (as it emits a negatively-charged π meson).

Sections 30882, 30883
Exam code: quiz07bC4n
(A) : 34 students
(B) : 0 students
(C) : 1 student
(D) : 1 student
(E) : 2 students

Success level: 89%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.20

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