| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.HL.TZ2.25 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | State | Question number | 25 | Adapted from | N/A |
Question
This question is about the standard model and the Pauli exclusion principle.
State one conservation law that would be violated, if the following reactions were to occur.
(i) \({\pi ^0} \to {e^ + } + {\mu ^ - }\)
(ii) \({p^ + } + \bar n \to {e^ + } + {e^ - } + {\bar v_e} + {v_e}\)
The reaction \({\bar v_\mu } + {e^ - } \to {\bar v_\mu } + {e^ - }\) is an example of a neutral current reaction. Draw a Feynman diagram for this reaction labelling all the particles involved. The arrow provided indicates the direction of time.
Markscheme
(i) electron lepton number/muon lepton number/family lepton number;
Do not accept just “lepton number” as this is conserved.
(ii) electric charge;
arrows for e– point forward in time and arrows for \({\bar v_\mu }\) point backwards in time;
vertices \({\bar v_\mu }Z{\bar v_\mu }\) and e–Ze– ; (inspect carefully, many draw \({e^ - }Z{\bar v_\mu }\) vertices)
Z particle;
Award each marking point independently.
Award [2 max] if the diagram is rotated 90º.
Allow particles to be expressed in words.
Examiners report
Part (a)(i) was poorly answered as many just stated lepton number is not conserved. They needed to be specific. In (ii) a common mistake was to think that baryon number is not conserved when in fact it is charge that is not conserved.
The Feynman diagram in (b) was rarely correct. Far too many candidates drew vertices that did not conserve charge. However partial marks were obtained for identifying the Z boson or for correct arrow directions.
