Constraints on extended neutral gauge structures

Abstract

Indirect precision data are used to constrain the masses of possible extra Z ′ bosons and their mixings with the ordinary Z. We study a variety of Z ′ bosons as they appear in E 6 and left-right unification models, the sequential Z boson, and the example of an additional U (1) in a concrete model from heterotic string theory. In all cases the mixings are severely constrained (sin θ < 0.01). The lower mass limits are generally of the order of several hundred GeV and competitive with collider bounds. The exception is the Z ψ boson, whose vector couplings vanish and whose limits are weaker. The results change little when the ρ parameter is allowed, which corresponds to a completely arbitrary Higgs sector. On the other hand, in specific models with minimal Higgs structures the limits are generally pushed into the TeV region.The possibility of additional neutral gauge bosons, Z ′ s, is among the best motivated types of physics beyond the Standard Model (SM). They are predicted by most unifying theories, such as Grand Unified Theories (GUTs), left-right unification, superstring theories and their strong coupling generalizations. In many cases their masses remain unpredicted and may or may not be of the electroweak scale. In the context of superstring models, however, which are much more constrained than purely field theoretical models, they are often predicted to arise at the electroweak scale, as we will discuss below. In this paper we consider six different types of Z ′ bosons:1. The Z χ boson is defined by SO(10) → SU(5) × U(1) χ . This boson is also the unique solution to the conditions of (i) family universality, (ii) no extra matter other than the right-handed neutrino, (iii) absence of gauge and mixed gauge/gravitational anomalies, and (iv) orthogonality to the hypercharge generator. In the context of a minimal SO(10) GUT, conditions (i) and (ii) are satisfied by assumption, while (iii) and (iv) are automatic. Relaxing condition (iv) allows other solutions (including the Z LR below) which differ from the Z χ by a shift proportional to the third component of the right-handed isospin generator.2. The Z ψ boson is defined by E 6 → SO(10)×U(1) ψ . It possesses only axial-vector couplings to the ordinary fermions. As a consequence it is the least constrained of our examples.

Sensitivity of low energy neutrino experiments to physics beyond the standard model

“…Another class of Z ′ models is coming from left-right symmetric models that have the premise that the fundamental weak interaction Lagrangian is invariant under parity symmetry at energies about 100 GeV. The gauge group of this type of models is given by SU(2)) L ⊗ SU(2) R ⊗ U(1) B−L , which gives an additional neutral gauge boson plus a charge gauge boson [29,30]. We will concentrate in this work on the neutral currents.…”

Electroweak Baryogenesis in a SupersymmetricU(1)′Model

“…Higgs masses lighter than those allowed by LEP in the MSSM are also possible, with the limits relaxed by mixings between Higgs doublets and singlets. There are stringent limits on an extra Z ′ from direct searches during Run I at the Tevatron [12] and from indirect precision tests at the Z-pole, at LEP 2, and from weak neutral current experiments [13]. In typical models M Z ′ > (500 − 800) GeV and the Z − Z ′ mixing angle α Z−Z ′ is smaller than a few ×10 −3 .…”

Electroweak model and constraints on new physics

“…For minimal Higgs sectors it can be found in reference [152]. Table 10.7 shows the 95% CL lower mass limits obtained from a somewhat earlier data set [181] for ρ 0 free and ρ 0 = 1, respectively. In cases of specific minimal Higgs sectors where C is known, the Z ′ mass limits are generally pushed into the TeV region.…”

“…The limits on |θ| are typically < few ×10 −3 . For more details see [181,182] and the Section on "The Z ′ Searches" in this Review. Also listed in Table 10.7 are the direct lower limits on Z ′ production from CDF [183] and LEP 2 bounds [41,184].…”