Effective chiral Lagrangians for new vector bosons to O(p^4) order

TL;DR

This paper develops an effective chiral Lagrangian including O(p^4) terms to describe a hypothetical new vector resonance in a Higgsless electroweak symmetry breaking scenario, analyzing its unitarity and experimental constraints.

Contribution

It introduces the complete O(p^4) invariant terms in the chiral Lagrangian for a single vector resonance, extending previous models and assessing their impact on unitarity and phenomenology.

Findings

O(p^4) operators significantly affect scattering amplitudes.

Constraints on model parameters from unitarity and precision tests.

Comparison shows O(p^4) terms alter the cutoff energy scale.

Abstract

If the SM Higgs boson does not exist, electroweak symmetry breaking may be realized via a strong interaction with a typical scale Lambda > 1 TeV. Resonances from the strong sector may help to unitarize WW scattering, which becomes strong in the absence of an Higgs field, and could be detected at the LHC. In this paper we describe such a scenario, in the minimal case in which only one new vector resonance is present, via a chiral SU(2) x SU(2)/SU(2) lagrangian also including all possible invariant terms up to O(p^4) order (assuming a parity symmetry in the strong sector). The O(p^4) invariants are not usually taken into account in similar studies in the literature; however, they have been shown to be potentially important, at least, to reconcile this kind of scheme with electroweak precision tests. Here we use the O(p^4) lagrangian to study the scattering amplitudes for the pi pi sector, investigating the partial wave unitarity properties of the model and its cut-off energy scale. We obtain constraints on the parameter space and compare our result to the one obtained with just the O(p^2) lagrangian, finding that the contribution of the new operators is indeed significant.