| Date | May 2015 | Marks available | 3 | Reference code | 15M.3.HL.TZ2.23 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Discuss and State | Question number | 23 | Adapted from | N/A |
Question
This question is about particles and interactions.
(i) State what is meant by an antiparticle.
(ii) Some particles are identical to their antiparticles. Discuss whether the neutron and the antineutron are identical.
The Feynman diagram represents the decay \({K^{\rm{ - }}} \to {\pi ^{\rm{ + }}}{\rm{ + }}{\pi ^{\rm{ - }}}{\rm{ + }}{\pi ^{\rm{ - }}}\).
Particles X and Y are exchange particles.
(i) Explain what is meant by an exchange particle.
(ii) Identify X.
(iii) Determine the electric charge of Y.
(iv) Calculate the change in strangeness in the decay of the K –.
Markscheme
(i) a particle with the same mass but opposite quantum numbers/charge;
(ii) the neutron has baryon number +1, so the antineutron has baryon number -1
so they are different;
or
the neutron consists of three quarks (udd) and the antineutron consists of three antiquarks \(\left( {{\rm{\bar u\bar d\bar d}}} \right)\);
so they are different;
Award [0] for a bald correct answer.
(i) a short lived/virtual particle/(gauge) boson;
that transfers energy/momentum/force between interacting particles;
(ii) W–;
(iii) zero;
(iv) \(\Delta S = 0 - \left( { - 1} \right) = + 1\)
Examiners report
In (a)(i) the fact that antiparticles have equal mass was often not mentioned. Most realised that antineutrons and neutrons were not identical as they had different quark structure or opposite baryon numbers.
Part (b)(i) was usually partially correct. Few mentioned that exchange particles were bosons. Parts (ii), (iii) and (iv) were answered correctly only if candidates knew the charges on the various quarks in the Feynman diagram. About half did know.
