Physics final exam question: valid or invalid Feynman diagram?

Physics 205B Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA

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

Solution and grading rubric:

• p:
  Correct. Argues why this Feynman diagram is invalid by either one of two arguments:
  1. this process is not a proper permutation of a known weak interaction (as the nucleon changes into a lepton in the top path); or
  2. charge conservation is violated (zero charge of neutron does not equal the net positive charge of neutrino, positron, 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. May argue why this Feynman diagram is invalid as does not correspond to a known weak decay process (but discussion of the known weak decay process is garbled); or argues that this Feynman diagram is valid as the "one line in, one line out" vertex rule is obeyed by both vertices (although charge conservation is violated, and the nucleon and lepton paths are mixed).
• 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, particles and antiparticles, and interactions.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of Feynman diagrams, particles and antiparticles, and interactions.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: finalmR3x
p: 3 students
r: 3 students
t: 18 students
v: 2 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7117):