Low Energy Neutral Current Experiments Research Articles

Constraints on leptophobicZ′models from electroweak experiments

We study the constraints from updated electroweak data on the three leptophobic Z' models, the ${\eta}$ model with an appropriate $U(1)'-U(1)_Y$ kinetic mixing, a Z' model motivated by the flipped SU(5) \times U(1) unification, and the phenomenological Z' model of Agashe, Graesser, and Hinchliffe. The $Z-Z'$ mixing effects are parametrized in terms of a positive contribution to the $T$ parameter, $T_{new}$, and the effective mass mixing parameter, $\bar{\xi}$. All the theoretical predictions for the $Z$ boson parameters, the $W$ boson mass and the observables in low-energy neutral current experiments are presented together with the standard model radiative corrections. The allowed region in the ($\bar{\xi},T_{new}$) plane is shown for the three models. The 95% CL lower limit on the heavier mass eigenstate $Z_2$ is given as a function of the effective $Z-Z'$ mixing parameter $\zeta$.

Z′-bosons in supersymmetric E6 models confront electroweak data

We study constraints on additional Z′-bosons predicted in the supersymmetric (SUSY) E 6 models by using the updated results of electroweak experiments — Z-pole experiments, m w measurements and low-energy neutral current (LENC) experiments. We find that the effects of Z- Z′ mixing are parametrized by (i) a tree-level contribution to the T-parameter, (ii) the effective Z- Z′ mass mixing angle ξ . n addition, the effect of the exchange of the heavier mass eigenstate Z 2 in the LENC processes is parametrized by (iii) a contact term g E 2/ c χ 2 m Z 2 2. We give the theoretical predictions for the observables in the electroweak experiments together with the standard model radiative corrections. Constraints on T new and ξ from the Z-pole and m w experiments and those on g E 2/ c χ 2 m Z 2 2 from the LENC experiments are separately shown. Impacts of the kinetic mixing between the U(1) y and U(1)′ gauge bosons on the χ 2-analysis are studied. We show the 95% CL lower mass limit of Z 2 as a function of the effective Z- Z′ mixing parameter ζ, a combination of the mass and kinetic mixings. A theoretical prediction on ζ and g E is given for the χ-, ψ-, η- and ν-models by assuming the minimal particle content of the SUSY E 6 models. In a certain region of the parameter space, the Z 2-boson mass in the detectable range of LHC is still allowed.

Rb, Rc and New Physics: An Updated Model Independent Analysis

We analyze LEP and SLC data from the 1995 Summer Conferences as well as from low energy neutral current experiments for signals of new physics. The reasons for doing this are twofold: first to explain the deviations from the Standard Model observed in Rb and Rc and second to constrain nonstandard contributions to couplings of the Z0 boson to all fermions and to the oblique parameters. We do so by comparing the data with the Standard Model as well as with a number of test hypotheses concerning the nature of the new physics. These include nonstandard [Formula: see text]-, [Formula: see text]- and [Formula: see text]-couplings as well as the couplings of the Z0 to fermions of the entire first, second and third generations and universal corrections to all up- and down-type quark couplings (as can arise see for example in Z' mixing models). We find that nonstandard [Formula: see text] couplings are both necessary and sufficient to explain the data and in particular the Rb anomaly. It is not possible to explain Rb, Rc and a value of the strong coupling constant consistent with low energy determinations invoking only nonstandard [Formula: see text]- and [Formula: see text]-couplings. To do so one has to have also new physics contributions to the [Formula: see text] or universal corrections to all [Formula: see text] couplings.

New neutral gauge bosons and new heavy fermions in the light of the new LEP data

We derive limits on a class of new physics effects that are naturally present in grand unified theories based on extended gauge groups, and in particular in E 6 and SO(10) models. We concentrate on i) the effects of the mixing of new neutral gauge bosons with the standard Z 0; ii) the effects of a mixing of the known fermions with new heavy states. We perform a global analysis including all the LEP data on the Z decay widths and asymmetries collected until 1993, the SLC measurement of the left-right asymmetry, the measurement of the W boson mass, various charged current constraints, and the low energy neutral current experiments. We use a top mass value in the range announced by CDF. We derive limits on the Z 0- Z 1 mixing, which are always ≲ 0.01 and are at the level of a few per mille if some specific model is assumed. Model-dependent theoretical relations between the mixing and the mass of the new gauge boson in most cases require M Z′ > 1 TeV. Limits on light-heavy fermion mixings are also largely improved with respect to previous analyses, and are particularly relevant for a class of models that we discuss.

Simultaneous analysis of Z' and new fermion effects: Global constraints in E6 and SO(10) models.

In grand unified theories based on extended (rank >4) gauge groups, the new gauge interactions require new fermions to ensure anomaly cancellation. We analyze the two kinds of new physics effects that are naturally present in these models: (i) the effects of the new neutral gauge bosons; (ii) the effects of a mixing of the known fermions with the new ones. Concentrating in particular on ${\mathrm{E}}_{6}$ and SO(10) models, we perform a global analysis of the electroweak data to constrain simultaneously these two new physics effects, and we pay particular attention to their reciprocal interplay. Our set of experimental results includes the data at the CERN ${e}^{+}{e}^{\ensuremath{-}}$ collider LEP on the $Z$ decay widths and fermion asymmetries, low-energy neutral-current experiments (atomic parity violation, $\ensuremath{\nu}$ scattering), the $W$-boson mass ${M}_{W}$, as well as charged-current measurements such as the various tests of the universality of the $W$-lepton couplings and the constraints on unitarity of the Cabibbo-Kobayashi-Maskawa matrix. We derive stringent bounds on the ${Z}_{0}\ensuremath{-}{Z}_{1}$ mixing ($|\ensuremath{\varphi}|\ensuremath{\lesssim}0.02$), on the fermion mixing parameters (${sin}^{2}{\ensuremath{\xi}}_{i}\ensuremath{\lesssim}0.01$ in most cases), and on the mass of the new gauge boson (${M}_{{Z}^{\ensuremath{'}}}>170\ensuremath{-}350$ GeV, depending on the model). In many observables the different sources of new physics induce comparable effects that can compensate each other. We confront the results derived by considering only one effect at a time with the results of a joint analysis, and we point out which of the existing bounds are relaxed and which ones remain unaffected.

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.

Comparison of discovery limits for E6 neutral gauge bosons at future colliders.

We study and compare the phenomenology of extra ${\mathrm{E}}_{6}$ neutral gauge bosons at ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$, ep, and hadron colliders. Our purpose is to compare the discovery reach of the various colliders and see how they compare in studying the phenomenology of extra neutral gauge bosons. We find that of the future colliders, hadron colliders probe the highest mass scales for the existence of extra neutral gauge bosons although precision measurements at ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ colliders, the SLAC Linear Collider will put tight constraints on models predicting extra neutral gauge bosons. If deviations from the standard model are observed at ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ colliders at \ensuremath{\surd}As=${\mathrm{M}}_{\mathit{Z}}^{0}$, other measurements, especially those at the DESY ep collider HERA, will be necessary to unravel the physics. We also find that high-precision, low-energy neutral-current experiments will be competitive with ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ and ep colliders for detecting the presence of extra ${\mathrm{E}}_{6}$ neutral gauge bosons.

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.