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We show that the mass and width of an unstable particle are precisely definedby the pole in the complex energy plane, $\mu = m - (i/2)\Gamma$, byidentifying the width, $\Gamma$, with the particle's decay rate and the mass,$m$, with the oscillatory frequency. We find that the physical $Z$ boson masslies 26 MeV below its quoted value, while the physical $W$ boson mass lies 20MeV below. We also clarify the various Breit-Wigner formulae that are used todescribe a resonance.

Mass and width of an unstable particle