Structure of theZ0resonance and the physical properties of theZ0boson

The proximity of multiparticle production thresholds means that the ${\mathit{Z}}^0$ resonance is formed, not by a single particle, but by a ‘‘multiplet’’ of distinct ${\mathit{Z}}^0$ bosons differing slightly in mass. The effects that these thresholds can have on the gauge-invariant pole expansion near the ${\mathit{Z}}^0$ resonance are studied. It is shown how this expansion naturally yields physical quantities from which the mass and partial widths of the unstable ${\mathit{Z}}^0$ bosons can be defined. The recent discussion on the definition of the ${\mathit{Z}}^0$ boson mass is reviewed and commented upon, stressing the distinction between the renormalized and physical masses.