Past exam of the particle and nuclear physics course of the University of Cambridge / 2015 / Paper 1 / 3 / a

At leading order the Higgs decay to a fermion pair hasEach quark contributes three distinguishable colour states, whereas a lepton has one. Neglecting the small phase-space factors for these masses, radiative corrections and the scale dependence of quark masses, the requested ratio isThe numerical masses here are those printed with question 3; the tau and electron approximations deliberately differ from those in question 1. Omitting the quark colour multiplicity would give the wrong relative rates.