Physics final exam question: incorrect Feynman diagram

Physics 205B final exam, spring semester 2013
Cuesta College, San Luis Obispo, CA

The Feynman diagram (A) correctly shows an electron capture decay process, where a proton and an electron interact to convert into a neutron and a neutrino. Discuss why the other Feynman diagram (B) showing an antineutrino interacting with a proton to convert into a neutron and positron is incorrect. Explain your reasoning using the properties of Feynman diagrams, particles and antiparticles, and interactions.

Solution and grading rubric:

• p:
  Correct. Discusses (1) how the second Feynman diagram violates the in-out path vertex rule, and/or (2) draws a corrected Feynman diagram for an antineutrino and proton respectively turning into a positron and neutron after the interaction.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882
Exam code: finalpL3x
p: 16 students
r: 1 student
t: 5 students
v: 3 students
x: 2 students
y: 1 student
z: 2 students

A sample "p" response (from student 1408):
A sample "x" response (from student 8325), with at least some demonstration of understanding charge flow conservation: