(i)     elementary particle.

(ii)     antiparticle of a lepton.

The electron is a lepton and its antiparticle is the positron. The following reaction can take place between an electron and positron.

$${e^ - } + {e^ + } \to \gamma  + \gamma$$

Sketch the Feynman diagram for this reaction and identify on your diagram any virtual particles.

(i)     quark structure of the ${\pi ^ + }$ meson.

(ii)     reason why the following reaction does not occur.

$${p^ + } + {p^ + } \to {p^ + } + {\pi ^ + }$$

(i)     a particle that cannot be made from any smaller constituents/particles;

(ii) has the same rest mass (and spin) as the lepton but opposite charge (and opposite lepton number);

Award [1] for each correct section of the diagram.

${e^ - }$ correct direction $\gamma$;

${e^ + }$ correct direction and $\gamma$;

virtual electron/positron;

Accept all three time orderings.

(i)     ${\rm{u\bar d}}$ / up and anti-down;

(ii)     baryon number is not conserved / quarks are not conserved;

Part (a) was often correct.

The Feynman diagrams rarely showed the virtual particle.

A significant number of candidates had a good understanding of quark structure.