Hadronic R ratio ($R(s)={\sigma }_h(s)/[4\pi {\alpha }^2/(3s)]$)

The hadronic R ratio normalizes the photon-channel hadronic relativistic cross-section to the leading massless muon-pair value. The free active-quark result is $R=N_c{\sum }_f{Q}_f^2$. It depends on flavor charges as well as the number of active flavors; it is not simply $N_c{N}_f$.