Mass and width of an unstable particle

We show that the mass and width of an unstable particle are precisely defined by the pole in the complex energy plane, μ = m - (i/2)Γ μ = m - ( i / 2 ) Γ , by identifying the width, Γ Γ , with the particle’s decay rate and the mass, m , with the oscillatory frequency. We find that the physical Z boson mass lies 26 MeV below its quoted value, while the physical W boson mass lies 20 MeV below. We also clarify the various Breit–Wigner formulae that are used to describe a resonance.