Additional Neutral Gauge Boson Research Articles

Neutral current and LEP constraints on an extra E 6 neutral gauge boson. A global fit to electroweak parameters

We perform an analysis of the laboratory constraints on the parameters of an additional neutral gauge boson from the E 6 gauge group. We combine the recent constraints on the mass of the Z as well as its leptonic, hadronic and total decay widths obtained by the four LEP collaborations, with those that follow from deep-inelastic ν μ N scattering and other low energy neutral current experiments, in a global fit. The constraints on the mass and mixing of the Z′ are given for the general unconstrained E 6 model as well as for the case of constrained string models.

ELECTROWEAK INTERACTIONS WITH CHIRAL HYPERCHARGES

We consider an extension of the electroweak gauge interactions from SU (2) L × U (1)Y to SU (2) L × U (1) L × U (1) R . This extension is motivated by the observed structure of the quarks and leptons, i.e., they form left-handed doublets and right-handed singlets eigenstates of electroweak interactions. In this new extension, the U (1) R serves only the right-handed singlet fermions. Consistent with the measurements of the mass of the standard massive neutral boson Z0 at the SLAC and CERN colliders and the neutral current couplings involving neutrinos and electrons, the additional massive neutral gauge boson can be as light as a few hundred GeV. The model employs the generalized see-saw mechanism to give masses to all the fermions of the theory.

Chiral electroweak gauge interactions.

The hypercharge ${\mathrm{U}(1)}_{Y}$ of the standard electroweak model is split into chiral hypercharges ${\mathrm{U}(1)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{R}$. Under the new gauge symmetry ${\mathrm{SU}(2)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{R}$, quarks and leptons are left-handed doublets transforming only under ${\mathrm{SU}(2)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{U}(1)}_{L}$ and right-handed singlets transforming only under ${\mathrm{U}(1)}_{R}$. Consistent with the measurements of the mass of the standard massive neutral boson ${Z}^{0}$ at the SLAC and CERN colliders and the neutral-current couplings involving neutrino beams and electron beams, the additional massive neutral gauge boson can be as light as a few hundred GeV. The model utilizes the generalized see saw mechanism of Gell-Mann, Ramond, and Slansky to give masses to all the fermions of the theory.

Cosmological constraints on additional light neutrinos and neutral gauge bosons

We analyse the cosmological constraints that follow from primordial nucleosynthesis on E 6 models with additional light neutrinos and a light neutral gauge boson Z′. These constraints are often tighter than existing laboratory limits.

Tests of extended gauge structures with high statistics at the Z peak

We study the implications of the SU(2) L×U(1) Y fermionic quantum numbers on nonpolarized forward-backward asymmetries into μ +μ −, cc̄, bb̄ and on the τ polarization. Two relations among these observable, satisfied in the context of the standard model, are singled out, and proven to hold at one loop order. We show that deviations from the predicted values can arise only in the presence of a low energy extended gauge symmetry and we present an analysis of these effects for a class of models containing one additional neutral gauge boson.

Upper limits on new gauge-boson masses from Z mass shifts.

An observed downward shift ($\ensuremath{\Delta}M$) in the mass of the standard model $Z$ can be used to place a model-dependent upper limit on the mass of new ${Z}^{\ensuremath{'}}$ gauge bosons for a given value of the top-($t$-)quark mass. Model-dependent lower bounds on the absolute value of the $Z\ensuremath{-}{Z}^{\ensuremath{'}}$ mixing angle ($\ensuremath{\varphi}$) are also obtained from this analysis. For definiteness, this situation is analyzed within the context of two classes of ${\mathrm{E}}_{6}$ superstring-inspired models as well as the left-right-symmetric model all of which contain additional neutral gauge bosons.

Extra neutral gauge boson and the processes e+e--->ZiHa in a superstring-inspired model.

We discuss the processes ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${Z}_{i}$${H}_{a}$ (i=1,2; a=1,2,3) in a superstring-inspired model involving two Higgs-boson doublets and one singlet, and also an additional neutral gauge boson (${Z}_{E}$), at CERN LEP II and higher energies. Our main result is that, for ${Z}_{2}$ sitting on resonance at LEP II energy, the total cross section for the process ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${Z}_{1}$${H}_{2}$ is significantly large as compared with those for ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${\ensuremath{\mu}}^{+}$${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$, ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${W}^{+}$${W}^{\mathrm{\ensuremath{-}}}$. For ${Z}_{2}$ at off resonance, the cross sections are almost the same as that for the standard model.

On the particle spectrum of E(6) superstrings

The allowed parameter space of a superstring-inspired supergravity model with one additional neutral gauge boson Z′ is investigated in detail, under the assumption that no intermediate scale exists and that the supersymmetry breaking in the visible sector can be parameterized by m 0, m 1 2 and A as in usual supergravity models. It is shown that the model allows for large Yukawa couplings and thus a relatively small supersymmetry breaking scale. Additional superpotential couplings which do not involve Higgs superfields allow to reduce slepton and squark masses even further. Therefore, sparticle masses can lie well within the range of the SLC, Tevatron, and LEP even if the Z′-boson is very heavy. Bounds on the masses of exotic fermions and the total decay width of the Z′ are also given.

Neutral currents within beyond the standard model

We present new, model-independent analyses of the observables in low-energy neutral-current experiments in the neutrino-hadron, neutrino-electron, electron/muon-hadron and electron- muon/tau sectors. We combine them with the latest experimental measurements of the W and Z masses to make global fits in the context of the standard model and of superstring-inspired models. We find in the standard model that sin 2θ w = 0.228 ± 0.004 if ϱ is fixed to be 1, whilst a two parameter fit yields sin 2θ w = 0.229 ± 0.006 and ϱ = 1.001 ± 0.007. This new value of sin 2θ w for ϱ = 1 lies uncomfortably outside the predicted region of the minimal SU(5) model, but is consistent with supersymmetric SU(5) models. In the minimal standard model with ϱ = 1 and equal Higgs and Z masses we find that mt<168 GeV at the 90% confidence level. We establish lower bounds on the masses of additional neutral gauge bosons Z′ in three superstring-inspired models and upper limits on their mixing with the standard model Z 0. In particular, we find that mZ′129 GeV at the 90% confidence level in a minimal rank-5 superstring-inspired model with canonical Higgs structure.

O(5)�U(1) electroweak gauge theory and the neutrino oscillations in matter

A study is made of the groupO(5)×U(1). The group is economical in the number of gauge bosons, which we associate with each of its generators, and is anomaly-free. The left-handed leptonsL ≡(v e ,e,μ,v μ ) L are assigned to the four-dimensional spinorial representations ofO(5). The right-handed particles are taken to be the singlets of the group. The theory has three sets of gauge bosons: (1) analogues of the GWS model, (2) additional charged gauge bosons, and (3) a set of three additional neutral gauge bosons as compared to the GWS model. We introduce neutrino mixing by mixing the additional charged gauge bosons. We develope a theory of neutrino oscillations in matter in such a way that in the absence of matter the scattering length reduces to the usual scattering length in vacuum. Even if the neutrino masses are equal or the neutrinos are massless, we still have neutrino oscillations in matter, a result already noted by Wolfenstein.

E+e--->W+W- in E

We examine the influence of additional neutral gauge bosons (Z'), which can exist in E/sub 6/ models, on the total (sigma) and differential (dsigma/dcostheta) sections for e/sup +/e/sup -/..-->..W/sup +/W/sup -/. Four different models of this kind with additional Z's are examined. We find that this process is quite sensitive to the existence of these new gauge bosons even if they are very heavy (M/sub Z//sub '/>1/2--1 TeV) and the Z-Z' mixing is small (sinphiapprox. =0.025). The four models are distinguishable from each other and the standard model using data from either sigma or dsigma/d costheta measurements.

Constraint on additional neutral gauge bosons from electroweak radiative corrections.

Quantum loop corrections to the quark mixing matrix resulting from additional neutral gauge bosons are computed. Agreement between the finding \ensuremath{\Vert}${\mathit{V}}_{\mathrm{ud}}$${\mathrm{\ensuremath{\Vert}}}^{2}$+\ensuremath{\Vert}${\mathit{V}}_{\mathrm{us}}$${\mathrm{\ensuremath{\Vert}}}^{2}$+\ensuremath{\Vert}${\mathit{V}}_{\mathrm{ub}}^{2}$=0.9984\ifmmode\pm\else\textpm\fi{}0.0021 and the unitarity value of 1 is used to provide a generic bound on masses and couplings of such bosons. For grand unified models of the type SO(100\ensuremath{\rightarrow}SU(3${)}_{\mathit{C}}$\ifmmode\times\else\texttimes\fi{}SU(2${)}_{\mathit{L}}$\ifmmode\times\else\texttimes\fi{}U(1)\ensuremath{\approxeq}U(1${)}_{\mathit{X}}$, \ensuremath{\gtrsim}266 GeV at 90% confidence limit. That bound also applies to the extra boson in some specific ${\mathrm{E}}_{6}$ superstring-motivated models. In the general case of ${\mathrm{E}}_{6}$\ensuremath{\rightarrow}SU(3${)}_{\mathrm{C}}$\ifmmode\times\else\texttimes\fi{}SU(2${)}_{\mathrm{L}}$\ifmmode\times\else\texttimes\fi{}U(1) \ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathrm{\ensuremath{\eta}}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathrm{\ensuremath{\eta}}\mathcal{'}}$ with , we obtain the bound \ensuremath{\ge}254 GeV at 90% C.L.

Gauge structures beyond the standard model and 100-GeV mass region.

We study various forward-backward and polarization asymmetries evaluated near the ${Z}^{0}$ resonance for theories with SU(2${)}_{\mathrm{L}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathrm{Y}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathrm{Y}\mathcal{'}}$ and SU(2${)}_{\mathrm{L}}$\ifmmode\times\else\texttimes\fi{}SU(2${)}_{\mathrm{R}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathrm{B}\mathrm{\ensuremath{-}}\mathrm{L}}$ gauge structures. Extension to other gauge structures is very simple in our formalism. We construct a linear combination of polarized forward-backward asymmetry and polarization asymmetry with initial-state-electron longitudinal polarization whose deviation from the value of the standard model can measure the effects of new currents directly. The analysis is exact at the tree level of the theory and enables one to study any model with any Higgs sector in terms of a fixed number of parameters. The results show that for a typical class of models the measurement of different asymmetries to 1% will impose a lower bound on ${M}_{Z\mathcal{'}}$, the mass of an additional neutral gauge boson, to be of order 10 ${M}_{Z}$. Even much less accurate measurements will yield interesting information about new gauge structures. We also examine the implications of extended gauge structures for the precise value of the ${W}^{\ifmmode\pm\else\textpm\fi{}}$ mass.

Modifications in neutrino counting due to additional gauge bosons.

We examine the possible influence of additional heavy neutral gauge bosons (Z/sub 1//sup 0/) on attempts at neutrino counting in e/sup +/e/sup -/ annihilation below the Z/sub 1//sup 0/ (approx. =93 GeV) resonance. A large class of two-Z/sup 0/-boson models is examined. We find that the apparent shift in the number of neutrinos, N/sub ..nu../, due to the extra Z/sup 0/ boson in these models is never larger than approx. =0.4 if N/sub ..nu../ = 3 and somewhat smaller if N/sub ..nu../ = 4. Thus we should not expect a large ''error'' in neutrino counting to result in these extensions of the standard model.

OBSERVABLES IN LOW-ENERGY SUPERSTRING MODELS

We compile phenomenological constraints on the minimal low-energy effective theory which can be obtained from the superstring by Calabi-Yau compactification. Mixing with the single additional neutral gauge boson in this model reduces the mass of the conventional Z0, Field vacuum expectation values are constrained by the experimental upper bound on this shift. Then, requiring the sneutrino mass squared to be positive constrains the scale of supersymmetry breaking more than do lower bounds on the masses of new charged particles and of sparticles. More model-dependent constraints follow from the “naturalness” requirement that observables do not depend sensitively on input parameters. We find a preference for the second neutral gauge boson to weigh ≲320 GeV, [Formula: see text] GeV and [Formula: see text] GeV. Dynamical generation of the gauge hierarchy is possible if mt≲70 GeV, with lower values of mt being favoured.

Low-energy neutral current phenomenology and grand unified theories

We derive necessary and sufficient conditions to be satisfied by any expanded electroweak gauge model in order to reproduce the standard model low-energy neutral current predictions. These conditions imply several constraints on the neutral gauge boson masses and quantum number assignments for the ordinary fermions. Using these conditions, we prove that the popular grand unified theories based on the gauge groups SO(10) and E 6 can only accomodate trivial extensions of the standard model. As a consequences, if any of these grand unified models works at some energy scale, present low-energy neutral current phenomenology implies that the Z-boson must be produced with the expected mass and couplings to the ordinary fermions. Any additional neutral gauge boson (with the possible exception of very heavy ones) could only be produced in hadronic collisions and it would not decay in e +e −.

B−Las the fourth color within anSU(2)L×U(1)R×U(1)model

Quark-lepton correspondence and the corresponding interpretation of B - L as the fourth color are combined in an SU(2)/sub L/ x U(1)/sub R/ x U' (1) electroweak model. The right-handed neutrino interactions are automatically suppressed and the standard model is recovered up to a desirable first-order fine structure. A fundamental distinction is established between the real Weinberg angle and its effective (measured) values. For neutrino interactions, our predictions are in general of the Weinberg-Salam (WS) type, only with the effective replacement sin/sup 2/theta/sub W/ ..-->.. sin/sup 2/theta/sup eff//sub W/ such that (a) sin/sup 2/theta/sup eff//sub W/ (purely leptonic processes) >sin/sup 2/theta/sup eff//sub W/ (..nu..q scattering), and (b) for sin/sup 2/theta/sub W/ = 1/4 one can still have a lower value for the weighted average of sin/sup 2/theta/sup eff//sub W/. Our expressions for the asymmetry parameter in electron-induced reactions agree with those of the WS model up to a second-order term. The additional neutral gauge boson is expected to be only 1 order of magnitude heavier than W/sup + -/. We argue that the present neutral-current experimental data are not capable of distinguishing between the standard and the present models. Finally we discuss the possibility of embedding our model in amore » unified gauge theory, showing that only spinorial SO(4k + 2) theories are available for this purpose.« less

Sakata-like electro/weak model

A left-handed SU (2) L × U(1) × U′(1) electro/weak model (with Q = T 3 + 1 2 Y − 2 3 L ) is proposed. For neutrino interactions, our predictions are à la WS only with sin 2 Θ W → sin 2 Θ eff W < sin 2 W for processes which involve quarks, and (b) sin 2Θ eff W (purely leptonic processes) > sin 2Θ eff W (νq-scattering). For the polarized process eD → eX, the asymmetry parameter agrees with the WS one up to a second-order term. The additional neutral gauge boson is expected to be only one order of magnitude heavier than W ±. We argue that the present neutral-current experimental results are not capable of distinguishing between the standard and the present models.

SU(2) ⊗ U(1) ⊗ U(1)′ weak-interaction model andΔI=12enhancement

The ..delta..I = 1/2 enhancement in nonleptonic decays is achieved in an SU(2) x U(1) x U(1)' model by the role of an additional neutral gauge boson, X/sub ..mu../. The new vector boson couples to the hadronic neutral current that has predominantly isoscalar ..delta..S = 0 term, plus a small ..delta..S not equal to 0 term, but it does not interact with leptons. The associated ..delta..S = 2 processes can be made small under due conditions on parameters. Several manifestations of X/sub ..mu../ as a gauge boson or gluon bound state are speculated. (AIP)