= Solution
A leading short-distance <Standard Model> contribution to <neutral D-meson mixing> is a <box Feynman diagram> with two <W bosons> and two internal down-type <quark> lines:
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-305-q3a.png]
{title=Short-distance box contribution to neutral D-meson mixing}
The incoming $c\bar u$ pair becomes $u\bar c$; the internal labels run over the <down quark>, <strange quark> and <bottom quark>. Each corner is a <weak charged current> vertex. Four such vertices make the contribution of order $g^4$. Summing the internal flavours produces <Cabibbo-Kobayashi-Maskawa matrix> factors and the <Glashow-Iliopoulos-Maiani mechanism>: the flavour-independent term cancels by $\sum_i V_{ci}^*V_{ui}=0$. This <box Feynman diagram> is one contribution; long-distance intermediate <hadron> states can also contribute to <neutral D-meson mixing>.
Back to article page
