Nonexotic Neutral Gauge Bosons

TL;DR

The paper analyzes neutral gauge bosons (Z') arising from an extra U(1) symmetry in a minimal extension of the Standard Model, demonstrating that anomaly cancellation with generation-independent charges and at least three right-handed neutrinos yields a two-parameter family of $U(1)_z$ charges, expressible as a linear combination of $Y$ and $B-L$. It shows that the Z' mass and couplings are governed by the breaking scale $v_{\varphi}$, the coupling $g_z$, and the charges $z_q$, $z_u$, with kinetic mixing diagonalized away at a chosen scale. Experimental constraints come from direct production (LEP II, Tevatron) and precision electroweak data at the Z-pole, yielding a typical lower bound of $M_{Z'}\sim \mathcal{O}(1~\text{TeV})$ for electromagnetic-strength couplings, while a $U(1)_{B-L}$-type charge assignment can suppress tree-level $Z$–$Z'$ mixing and shift sensitivity toward direct searches. The framework accommodates viable neutrino masses via Majorana and Dirac mass operators depending on the RH neutrino charges, and highlights several well-known and novel model realizations within the two-parameter charge space. Overall, the work clarifies how Z' phenomenology depends on the symmetry-breaking pattern, anomaly constraints, and collider data, offering a roadmap for future explorations in light of upcoming experiments.

Abstract

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 order 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 LEPII and the 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.

Figures (2)

• Figure 1: Bounds at the $95\%$ confidence level in the $(z_Ht_z,M_{Z'})$ plane, where $z_H t_z= z_H g_z/g$ is the ratio of the $U(1)_z$-coupling of the Higgs doublet to the weak coupling. Electromagnetic strength corresponds to $z_H t_z\simeq 0.48$. The limit $z_H t_z \rightarrow 0$ represents either an extremely weakly coupled new boson, or a model with $z\propto B-L$. The disallowed region due to the bound on the $T$ parameter for $M_H = 115$ GeV is shaded light gray. The dotted line shows the shift in the bound for $M_H = 300$ GeV. Here we have used $T = -0.02\pm 0.13 (+0.09)$, where the central value corresponds to $M_H = 115$ GeV and the shift (in parentheses) to $M_H = 300$ GeV Hagiwara:pw.
• Figure 2: Bounds from precision measurements of the total $Z$ width at the $95\%$ confidence level plotted as contours in the $(z_Ht_z,M_{Z'})$ plane, for $z_qt_z = \pm\frac{1}{10},\pm\frac{1}{2}$. In each case the allowed region is to the left of the line. Included in gray are bounds from $T$ for $M_H=115$ GeV.