Abstract
We show that the mass and width of an unstable particle are precisely defined
by the pole in the complex energy plane, $= m - (i/2)\Gamma$, by using the
defining relationship between the width and the lifetime, $\Gamma = 1/\tau$. 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.
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