Constraint on additional neutral gauge bosons from electroweak radiative corrections

Quantum loop corrections to the quark mixing matrix resulting from additional neutral gauge bosons are computed. Agreement between the finding {}Vud{{{}}}²+{V}ᵤₛ²+{}Vub²=0.9984±{}0.0021 and the unitarity value of 1 is used to provide a generic bound on masses and couplings of such bosons. For grand unified models of the type SO(100{→}SU(3)C×{}SU(2)L×{}U(1){}U(1)X, {}266 GeV at 90% confidence limit. That bound also applies to the extra boson in some specific E₆ superstring-motivated models. In the general case of E₆{→}SU(3)C×{}SU(2)L×{}U(1) ×{}U(1)_η×{}U(1)_η' with , we obtain the bound {≥}254 GeV at 90% C.L.