Past exam of the particle physics course of the University of Oxford / 2016 / 5 / 3 / c / Solution

Assuming that the two listed leptonic decay categories exhaust the leptonic modes, the inclusive hadronic branching fraction is $100-17.83-17.41=64.76\%$. ThusAt the simplest massless quark level, the W boson couples to $u\bar{d}$ and $u\bar{s}$ in three color charge states, so equal quark and lepton weak coupling constant values predictThis is a rough check of color multiplicity in a decay width and universal weak-current normalization. It is much less precise than the two-lepton ratio because hadronic Quantum chromodynamics and electroweak interaction corrections are sizeable: the data require a net correction factor near $3.632/3.0024\simeq 1.21$. They must be included before extracting a quark-to-lepton coupling constant ratio. It would be incorrect to interpret the excess over three directly as a failure of weak universality. The displayed exclusive hadronic modes alone would underestimate the inclusive sum.