Oblique Correction Parameters Research Articles

Symmetry and geometry in a generalized Higgs effective field theory: Finiteness of oblique corrections versus perturbative unitarity

We formulate a generalization of Higgs effective field theory (HEFT) including arbitrary number of extra neutral and charged Higgs bosons (generalized HEFT, GHEFT) to describe non-minimal electroweak symmetry breaking models. Using the geometrical form of the GHEFT Lagrangian, which can be regarded as a nonlinear sigma model on a scalar manifold, it is shown that the scalar boson scattering amplitudes are described in terms of the Riemann curvature tensor (geometry) of the scalar manifold and the covariant derivatives of the potential. The coefficients of the one-loop divergent terms in the oblique correction parameters S and U can also be written in terms of the Killing vectors (symmetry) and the Riemann curvature tensor (geometry). It is found that perturbative unitarity of the scattering amplitudes involving the Higgs bosons and the longitudinal gauge bosons demands the flatness of the scalar manifold. The relationship between the finiteness of the electroweak oblique corrections and perturbative unitarity of the scattering amplitudes is also clarified in this language: we verify that once the tree-level unitarity is ensured, then the one-loop finiteness of the oblique correction parameters S and U is automatically guaranteed.

Holographic construction of technicolor theory

We construct a dual description of technicolor theory based on the D4/D8 brane configuration. A strongly-coupled technicolor theory is identified as the effective theory on D-branes, and from the gauge/gravity correspondence, we explore the weakly-coupled holographic description of dynamical electroweak symmetry breaking. It is found from the D-brane probe action that the masses of W and Z bosons are given by the decay constant of technipion, and the technimesons become hierarchically heavy. Moreover, the couplings of heavier modes to standard model fermions are rather suppressed. The oblique correction parameters are also evaluated and found to be small except for the S parameter, which can be reduced by modifying the model. The fermion fields are introduced at the intersections of D-branes and their masses are generated via massive gauge bosons from open strings stretching between D-branes.

Constraints on the electroweak chiral Lagrangian from the precision data

In the framework of the effective field theory method, we use the experimental data and the perturbative unitarity bounds to determine the values and uncertainty of all the 11 chiral coefficients ( α i , i = 0 , … , 10 ) of the standard electroweak chiral Lagrangian. Up to linear terms in α i , we provide the one-loop renormalization group equations of all the chiral coefficients, which are calculated in the Feynman–'t Hooft gauge using the modified minimal subtraction scheme. With the improved renormalization group equations to sum over the logarithmic corrections, we analyze the current experimental uncertainty of oblique correction parameters, S ( Λ ) and T ( Λ ) . We find that, due to the large uncertainty in the triple gauge-boson coupling measurements, the parameter space of positive S ( Λ ) for Λ > 1 TeV is still allowed by the current experimental data. T ( Λ ) tends to increase with Λ even in the presence of the operators that contribute to the triple and quartic gauge-boson couplings.

Warped fermions and precision tests

We analyze the behavior of standard model matter propagating in a slice of ${\mathrm{A}\mathrm{d}\mathrm{S}}_{5}$ in the presence of infrared-brane kinetic terms. Brane kinetic terms are naturally generated through radiative corrections and can also be present at tree level. The effect of the brane kinetic terms is to expel the heavy Kaluza-Klein (KK) modes from the infrared brane, and hence to reduce their coupling to the localized Higgs field. In a previous work we showed that sizable gauge kinetic terms can allow KK mode masses as low as a few TeV, compatible with present precision measurements. We study here the effect of fermion brane kinetic terms and show that they ameliorate the behavior of the theory for third generation fermions localized away from the infrared brane, reduce the contribution of the third generation quarks to the oblique correction parameters and maintain a good fit to the precision electroweak data for values of the KK masses of the order of the weak scale.

NuTeV anomaly, neutrino mixing, and a heavy Higgs boson

Recent results from the NuTeV experiment at Fermilab and the deviation of the Z invisible width, measured at LEP/SLC, from its Standard Model (SM) prediction suggest the suppression of neutrino-Z couplings. Such suppressions occur naturally in models which mix the neutrinos with heavy gauge singlet states. We postulate a universal suppression of the Z�� couplings by a factor of (1 − ) and perform a fit to the Z-pole and NuTeV observables with and the oblique correction parameters S and T. Compared to a fit with S and T only, inclusion of leads to a dramatic improvement in the quality of the fit. The values of S and T preferred by the fit can be obtained within the SM by a simple increase in the Higgs boson mass. However, if the W mass is also included in the fit, a non-zero U parameter becomes necessary which cannot be supplied within the SM. The preferred value of suggests that the seesaw mechanism may not be the reason why neutrinos are so light.

The oblique corrections from the diagonal ETC interaction

We study the effect of the diagonal extended technicolor(ETC) gauge boson on the oblique correction parameters. It is shown that the T parameter is unacceptably large when the Zbb vertex correction and S parameter are consistent with the experiments in the ETC model.

CONSTRAINTS ON THE NUMBER OF HEAVY GENERATIONS FROM THE S AND T PARAMETERS

We show that precision measurements of the oblique correction parameters S and T can be used to derive an upper bound on the number of extra generations of heavy fermions Neg. The bound depends on the type of heavy neutrino masses. We have obtained Neg≤2 in the case of Dirac neutrinos and Neg≤5 in the case of neutrinos with Majorana masses, under some assumptions on the flavor and color contents of extra generations.

Electroweak oblique corrections from quark-lepton substructure.

We consider the electroweak oblique corrections from quark-lepton substructure through its form factor. We derive general formulas for the oblique correction parameters T, U, and S for a general class of the form factors. Then we precisely calculate them for the case of the single-pole form factor. It turns out that the effects on T become sizable when the new physics scales break global isospin symmetry at the order of the weak interaction scale.

Effective Lagrangian for a Technicolor Model without Exact Custodial Symmetry

Effective Lagrangian including technimesons is constructed for a realistic one-family Technicolor model without exact custodial symmetry. Tree level contribution to oblique correction parameters $S$ and $U$ due to spin 1 technimesons are computed with the effective Lagrangian. An isospin breaking term which is associated with technilepton vector mesons gives a negative contribution to the electroweak radiative correction parameter $S$ due to mixing between $I=0$ and $I=1$ vector mesons. $U$ receives non-zero contribution due to exotic left-handed charged vector mesons and its sign can be both negative and positive.

Estimation of oblique electroweak corrections.

I will first review the experimental limits placed on the oblique correction parameters S and T. Then, I will discuss how the value of S can be estimated for running and walking technicolor theories.