Electroweak Precision Measurements Research Articles

Monte Carlo Exploration of Warped Higgsless Models

We have performed a detailed Monte Carlo exploration of the parameter space for a warped Higgsless model of electroweak symmetry breaking in 5 dimensions. This model is based on the SU(2)L × SU(2)R × U(1)B−L gauge group in an AdS5 bulk with arbitrary gauge kinetic terms on both the Planck and TeV branes. Constraints arising from precision electroweak measurements and collider data are found to be relatively easy to satisfy. We show, however, that the additional requirement of perturbative unitarity up to the cut-off, 10 TeV, in WL+WL− elastic scattering in the absence of dangerous tachyons eliminates all models. If successful models of this class exist, they must be highly fine-tuned.

The Vertex Tracker at future e+e− linear colliders

The physics program of high-energy e + e − linear colliders relies on the accurate identification of fermions to study in details the profile of the Higgs boson, search for new particles and later probe the multi-TeV mass region by direct searches and precision electro-weak measurements. This paper reviews the requirements, conceptual design and sensor R&D for the Vertex Tracker.

Electroweak symmetry breaking and radion stabilization in universal extra dimensions

We discuss the stabilization of the scalar sector, including the radion, in the gauge model with one universal extra dimension, within Higgs and Higgsless scenarios. The stabilization occurs at the one-loop level, through the fermionic contribution to the effective potential; in the Higgs case, for stabilization to take place the bosonic contribution must be balanced by the fermionic one, hence the scales of these two cannot differ too much. However, there is no need for (softly broken) supersymmetry to achieve the stabilization - it can be arranged for a reasonably wide range of couplings and mass scales. The primary instability in the model is the run-away of the radion vacuum expectation value. It turns out that the requirement of the radion stability, in the Higgs case, favours a Higgs boson mass below 0.26 TeV, which is consistent with the Standard Model upper bound that follows from the electroweak precision measurements. The typical radion mass is of the order of 10^{-6} eV. The radion mass can be made larger by rising the scale of fermion masses, as clearly seen in the Higgsless case. The cosmological constant may be cancelled by suitable counterterms, in such a way that the stabilization is not affected.

Cosmological gravitino problem confronts electroweak physics

A generic feature of gauge-mediated supersymmetry breaking models is that the gravitino is the lightest supersymmetric particle (LSP). In order not to overclose the universe, the gravitino LSP should be light enough (~ 1 keV), or appropriately heavy (~ 1 GeV). We study further constraints on the mass of the gravitino imposed by electroweak experiments, i.e., muon g-2 measurements, electroweak precision measurements, and direct searches for supersymmetric particles at LEP2. We find that the heavy gravitino is strongly disfavored from the lower mass bound on the next-to-LSP. The sufficiently light gravitino, on the other hand, has rather sizable allowed regions in the model parameter space.

Global fit to electroweak precision data

Electroweak precision measurements performed at LEP, SLC and at the Tevatron are sensitive to higher order corrections within the Standard Model of particle physics and can be used to test the consistency of the theory at the quantum level. The present status is reviewed here.

The little Higgs from a simple group

We present a model of electroweak symmetry breaking in which the Higgs boson is a pseudo-Nambu-Goldstone boson. By embedding the standard model SU(2) x U(1) into an SU(4) x U(1) gauge group, one-loop quadratic divergences to the Higgs mass from gauge and top loops are canceled automatically with the minimal particle content. The potential contains a Higgs quartic coupling which does not introduce one-loop quadratic divergences. Our theory is weakly coupled at the electroweak scale, it has new weakly coupled particles at the TeV scale and a cutoff above 10 TeV, all without fine tuning. We discuss the spectrum of the model and estimate the constraints from electroweak precision measurements.

TeV symmetry and the little hierarchy problem

Constraints from precision electroweak measurements reveal no evidence for new physics up to 5 - 7 TeV, whereas naturalness requires new particles at around 1 TeV to address the stability of the electroweak scale. We show that this "little hierarchy problem" can be cured by introducing a symmetry for new particles at the TeV scale. As an example, we construct a little Higgs model with this new symmetry, dubbed T-parity, which naturally solves the little hierarchy problem and, at the same time, stabilize the electroweak scale up to 10 TeV. The model has many important phenomenological consequences, including consistency with the precision data without any fine-tuning, a stable weakly-interacting particle as the dark matter candidate, as well as collider signals completely different from existing little Higgs models, but rather similar to the supersymmetric theories with conserved R-parity.

Can the two-Higgs-doublet model survive the constraint from the muon anomalous magnetic moment, as suggested?

Requiring the two-Higgs-doublet model II to accommodate the 3$\sigma$ deviation in the muon anomalous magnetic moment imposes specific constraints on the Higgs spectrum. We analyze the combination of all the relevant, available, constraints on the model parameter space. The use of constraints from $b\to s \gamma$, the precision electroweak measurements of $R_b$, and the $\rho$ parameter, together with exclusions from direct searches at LEP, give extremely severe restrictions on the model parameters. That is "almost enough" to kill the model altogether. The exclusion would be even stronger if the direct searches can be optimized to complement the other constraints, as will be discussed in details in this work.

Where is the Higgs boson?

Electroweak precision measurements indicate that the standard model Higgs boson is light and that it could have already been discovered at LEP 2, or might be found at the Tevatron run 2. In the context of a TeV −1 size extra-dimensional model, we argue that the Higgs boson production rates at LEP and the Tevatron are suppressed, while they might be enhanced at the LHC or at CLIC. This is due to the possible mixing between brane and bulk components of the Higgs boson, that is, the non-trivial brane-bulk ‘location’ of the lightest Higgs. To parametrize this mixing, we consider two Higgs doublets, one confined to the usual space dimensions and the other propagating in the bulk. Calculating the production and decay rates for the lightest Higgs boson, we find that compared to the standard model (SM), the cross section receives a suppression well below but an enhancement close to and above the compactification scale M c . This impacts the discovery of the lightest (SM like) Higgs boson at colliders. To find a Higgs signal in this model at the Tevatron run 2 or at the LC with s =1.5 TeV, a higher luminosity would be required than in the SM case. Meanwhile, at the LHC or at CLIC with s ∼3 –5 TeV one might find highly enhanced production rates. This will enable the latter experiments to distinguish between the extra-dimensional and the SM for M c up to about 6 TeV.

Supersymmetric effects in deep inelastic neutrino–nucleus scattering

We compute the supersymmetric (SUSY) contributions to ν ( ν ̄ )-nucleus deep inelastic scattering in the Minimal Supersymmetric Standard Model (MSSM). We consider the ratio of neutral current to charged current cross sections, R ν and R ν ̄ , and compare with the deviations of these quantities from the Standard Model predictions implied by the recent NuTeV measurement. After performing a model-independent analysis, we find that SUSY loop corrections generally have the opposite sign from the NuTeV anomaly. We also study for R-parity-violating (RPV) contributions. Although RPV effects could, in principle, reproduce the NuTeV anomaly, such a possibility is also ruled out by other precision electroweak measurements.

Precision electroweak measurements: a Theorist point of view

Present global fits to electroweak data are characterized by two results that differ from SM expectations by about 3σ, the NuTeV result and the b quark asymmetries measured at LEP. I review possible SM and new physics explanations of these anomalies and the implications for the indirect determination of the Higgs mass.

Electroweak physics

The status of precision electroweak measurements as of summer 2002 is reviewed. The recent results on the anomalous magnetic moment of the muon and on neutrino-nucleon scattering are discussed. Precision results on the electroweak interaction obtained by the experiments at the SLC, LEP and TEVATRON colliders are presented. The experimental results are compared with the predictions of the minimal Standard Model and are used to constrain its parameters, including the mass of the Higgs boson. The final LEP results on the direct search for the Higgs boson of the Standard Model are presented.

Electroweak prospects for tevatron RunII

The prospects of precision electroweak measurements from CDF and DO using RunII data is reviewed.

Electroweak physics

The status of precision electroweak measurements as of summer 2002 is reviewed. The recent results on the anomalous magnetic moment of the muon and on neutrino-nucleon scattering are discussed. Precision results on the electroweak interaction obtained by the experiments at the SLC, LEP and TEVATRON colliders are presented. The experimental results are compared with the predictions of the minimal Standard Model and are used to constrain its parameters, including the mass of the Higgs boson. The final LEP results on the direct search for the Higgs boson of the Standard Model are presented.

Resource Letter: SM-1: The standard model and beyond

This Resource Letter provides a guide to literature on the standard model of elementary particles and possible extensions. In the successful theory of quarks and leptons and their interactions, important questions remain, such as the mechanism of electroweak symmetry breaking, the origin of quark and lepton masses, the source of the baryon asymmetry of the Universe, and the makeup of its matter and energy density. References are cited for quarks and leptons, gauge theories, color and chromodynamics, weak interactions, electroweak unification, CP violation, dynamics of heavy quarks, Higgs bosons, precision electroweak measurements, supersymmetry, dynamical electroweak symmetry breaking, composite quarks and leptons, grand unification and extended gauge groups, string theories, large extra dimensions, neutrino masses, cosmic microwave background radiation, dark matter, dark energy, accelerator facilities, and non-accelerator experiments.

Electroweak precision measurements in e+e− annihilation into bosons

This contribution gives an overview of the most recent electroweak results concerning four-fermion physics at LEP2.

Electroweak precision measurements in e +e − annihilation into fermions

Results of precision electroweak measurements in fermion pair production are reviewed. The combination of results obtained by LEP and SLC are compared with Standard Model predictions and used to constrain its parameters. In particular, predictions for the mass of the Higgs boson are discussed. Fermion pair production at LEP2 is used to test the Standard Model at high energies and to derive limits on parameters of new physics models.

THE SEARCHES FOR HIGGS BOSONS AT LEP

▪ Abstract The legacy of the LEP program encompasses an extensive investigation of the electroweak interaction and the most comprehensive search to date for the origin of spontaneous symmetry breaking. The results comprise a large variety of theoretical models challenged by dedicated searches and a persistent search for the standard-model Higgs boson. The direct search for the standard-model Higgs boson confronted an excess of signal-like events in the final year. This observation reaches a significance of approximately two standard deviations for a Higgs boson mass of 115.6 GeV/c2, a value consistent with the mass range indicated by electroweak precision measurements. The definitive confirmation of the standard-model Higgs boson search and the continued investigation for a Higgs sector at higher masses await new data from the Tevatron and LHC colliders.

On the possibility of a very lightA0at lowtanβin the minimal supersymmetric standard model

The searches at LEP II for the processes ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{h}^{0}Z$ and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{h}^{0}{A}^{0}$ in the minimal supersymmetric standard model fail to exclude regions of the ${m}_{h}{,m}_{A}$ plane where $\mathrm{tan}\ensuremath{\beta}<1,$ thus allowing a very light ${A}^{0}$ ${(m}_{A}<20 \mathrm{GeV}).$ Such a parameter choice would predict a light ${H}^{\ifmmode\pm\else\textpm\fi{}}$ with ${m}_{{H}^{\ifmmode\pm\else\textpm\fi{}}}<{m}_{W}.$ Although the potentially large branching ratio for ${H}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{A}^{0}{W}^{*}$ would ensure that ${H}^{\ifmmode\pm\else\textpm\fi{}}$ also escaped detection in direct searches at LEP II and the Tevatron Run I, we show that this elusive parameter space is overwhelmingly disfavored by electroweak precision measurements through its large contribution to the $\mathrm{Zb}\overline{b}$ vertex.

Light Bottom Squark and Gluino Confront Electroweak Precision Measurements

We address the compatibility of a light sbottom (mass approximately 2-5.5 GeV) and a light gluino (mass approximately 12-16 GeV) with electroweak precision measurements. Such light particles have been suggested to explain the observed excess in the b quark production cross section at the Tevatron. The electroweak observables may be affected by the sbottom and gluino through the supersymmetric-QCD (SUSY-QCD) corrections to the Zbb vertex. We examine, in addition to the SUSY-QCD corrections, the gauge boson propagator corrections from the stop which are allowed to be light from the SU(2)(L) symmetry. We find that this scenario is strongly disfavored from electroweak precision measurements.