Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 305 / 4 / d / Solution

The strange quark is light enough compared with typical hadronic scales to permit an approximatechiral expansion. It adds kaons and the eta to the pseudo-Goldstone multiplet, although convergence is poorer because $m_s$ is substantially larger than $m_u,m_d$.

The charm quark is too heavy for the same light-flavour chiral expansion. At low energies it is integrated out; processes containing charm are instead treated using heavy-quark expansions or other effective theories. Thus the analysis extends approximately to $s$, but not to $c$ as another pseudo-Goldstone-producing light flavour.