Physics final exam question: invalid Feynman diagram

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA

Discuss why this Feynman diagram is invalid. Explain your reasoning using the properties of Feynman diagrams, particles and antiparticles, and interactions.

Solution and grading rubric:

• p:
  Correct. Recognizes that the diagram is invalid by discussing:
  1. this was originally an electron capture process that has been rotated, but the proton path is reversed; or
  2. the "one line in, one line out" vertex rule is violated; or
  3. charge conservation is violated (net negative charge of electron and antineutrino in does not equal the net positive charge of neutron and proton out).
• 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. Some garbled attempt at applying properties of Feynman diagrams.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 26 students
r: 7 students
t: 2 students
v: 3 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2691):
Another sample "p" response (from student 3158):