New Gauge Bosons from String Models

TL;DR

The paper analyzes mass ranges and constraints for new neutral gauge bosons ($Z'$) predicted by a class of string-inspired models with an extra $U(1)'$. It shows that radiative breaking, driven by large Yukawa couplings to exotic states, can yield a $Z'$ mass $M_{Z'}$ in the electroweak range or at an intermediate scale, with mixing to the SM $Z$ ($ heta_{Z-Z'}$) constrained by precision data. Two main scenarios are explored: (i) breaking via a single SM-singlet $S$ allowing a potentially TeV-scale $M_{Z'}$ with suppressed mixing, and (ii) breaking via mirror-like singlets $S_1,S_2$ driving $M_{Z'}$ to an intermediate scale and leaving exotic masses tied to the same Yukawa structure. The work provides calculable, model-dependent predictions for $M_{Z'}$, its mixing, and the associated exotic matter, highlighting their testability at colliders and their relevance to precision electroweak constraints.

Abstract

We address the mass ranges of new neutral gauge bosons and constraints on the accompanying exotic particles as predicted by a class of superstring models. Under certain assumptions about the supersymmetry breaking parameters we show that breaking of an additional U(1)' symmetry is radiative when the appropriate Yukawa couplings of exotic particles are of order one, analogous to the radiative breaking of the electro-weak symmetry in the supersymmetric standard model due to the large top-quark Yukawa coupling. Such large Yukawa couplings occur for a large class of string models. The Z' and exotic masses are either of O(M_Z), or of a scale intermediate between the string and electro-weak scales. In the former case, M_Z'=O(1 TeV) may be achievable without excessive fine tuning, and is within future experimental reach. (This paper is a summary, with a more phenomenological emphasis and additional discussion, of the results in our earlier article "Implications of Abelian Extended Gauge Structures from String Models", hep-ph/9511378.)

Figures (3)

• Figure 1: Loop corrections which lead to the running of the mass-square term for the scalar $S$. $f$ and $\tilde{f}$ denote the fermion(s) and their supersymmetric partner(s), respectively.
• Figure 2: The running of the scalar mass-square terms (in units of $m_{3/2}^2$) for a singlet field $S$ (dashes) coupled to a pair of $SU(3)$ triplets $D$ (solid line) (example 2 from the appendix to CLI). $t$, defined as $\log (\mu/M_{string})/16 \pi^2$, varies from $-0.23$ at the electro-weak scale $\mu = M_Z$ to 0 at the string scale $\sim 5 \times 10^{17}$ GeV.
• Figure 3: Theoretical expectation for $\theta_{Z-Z'}$ as a function of $M_{Z'}/M_Z$ from (\ref{['thetath']}) for $2 g' |Q_H'|/G = 0.5$ and 0.1. Also shown are the upper limits on $\theta$ implied by the shift in the $Z$ from its standard model expectation (eqn (\ref{['thetamz']})) for $\sqrt{\rho_1-1}= 0.03$ (experimental $\sin^2 \theta_W$ not affected by mixing), and 0.06 ($\sin^2 \theta_W$ affected).