Nonexotic neutral gauge bosons

We study theoretical and experimental constraints on electroweak theories including a new color-singlet and electrically neutral gauge boson. We first note that the electric charges of the observed fermions imply that any such Z^' boson may be described by a gauge theory in which the Abelian gauge groups are the usual hypercharge along with another U (1) component in a kinetic-diagonal basis. Assuming that the observed quarks and leptons have generation-independent U (1) charges, and that no new fermions couple to the standard model gauge bosons, we find that their U (1) charges form a two-parameter family consistent with anomaly cancellation and viable fermion masses, provided there are at least three right-handed neutrinos. We then derive bounds on the Z^' mass and couplings imposed by direct production and Z-pole measurements. For generic charge assignments and a gauge coupling of electromagnetic strength, the strongest lower bound on the Z^' mass comes from Z-pole measurements, and is of the order of 1 TeV. If the new U (1) charges are proportional to B-L, however, there is no tree-level mixing between the Z and Z^', and the best bounds come from the absence of direct production at CERN LEP II and the Fermilab Tevatron. If the U (1) gauge coupling is one or two orders of magnitude below the electromagnetic one, these bounds are satisfied for most values of the Z^' mass.