Primordial nucleosynthesis corners the Z′

Abstract

We investigate the implications on the Z′ gauge boson parameters that come from enforcing the primordial nucleosynthesis constraints on the effective number of neutrino-like degrees of freedom in the presence of a Z′ that couples to three types of light right-handed neutrinos. We obtain stringent bounds on the Z-Z′ mixing angle, −0.004 ≲ φ ⩽ 0, and on the mass of the heavy eigenstate, M 2 ≳ 1 TeV. These bounds are stronger than the current experimental limits derived from precise measurements of the electroweak gauge boson masses. In this case we also find that the effect of Z-Z′ mixing on the Z width is completely negligible, and hence that it could not account for any small (positive or negative) shift on the Z width with respect to the standard model value.