159 research outputs found
The oblique S parameter in higgsless electroweak models
We present a one-loop calculation of the oblique S parameter within Higgsless
models of electroweak symmetry breaking. We have used a general effective
Lagrangian with at most two derivatives, implementing the chiral symmetry
breaking SU(2)_L x SU(2)_R -> SU(2)_{L+R} with Goldstones, gauge bosons and one
multiplet of vector and axial-vector resonances. The estimation is based on the
short-distance constraints and the dispersive approach proposed by Peskin and
Takeuchi.Comment: 5 pages, 2 figures. Talk given at QCD@work 2012, International
Workshop on Quantum Chromodynamics: Theory and Experiment, 18-21th June
(2012), Lecce (Italy
A few words about Resonances in the Electroweak Effective Lagrangian
Contrary to a widely spread believe, we have demonstrated that strongly
coupled electroweak models including both a light Higgs-like boson and massive
spin-1 resonances are not in conflict with experimental constraints on the
oblique S and T parameters. We use an effective Lagrangian implementing the
chiral symmetry breaking SU(2)_L x SU(2)_R -> SU(2)_{L+R} that contains the
Standard Model gauge bosons coupled to the electroweak Goldstones, one
Higgs-like scalar state h with mass m_h=126 GeV and the lightest vector and
axial-vector resonance multiplets V and A. We have considered the one-loop
calculation of S and T in order to study the viability of these
strongly-coupled scenarios, being short-distance constraints and dispersive
relations the main ingredients of the calculation. Once we have constrained the
resonance parameters, we do a first approach to the determination of the low
energy constants of the electroweak effective theory at low energies (without
resonances). We show this determination in the case of the purely Higgsless
bosonic Lagrangian.Comment: 6 pages, 2 figures. Talk given at XIth Quark Confinement and Hadron
Spectrum, 8-12th September (2014), Saint Petersburg (Russia
Fingerprints of heavy scales in electroweak effective Lagrangians
The couplings of the electroweak effective theory contain information on the
heavy-mass scales which are no-longer present in the low-energy Lagrangian. We
build a general effective Lagrangian, implementing the electroweak chiral
symmetry breaking , which couples the
known particle fields to heavier states with bosonic quantum numbers
and . We consider colour-singlet heavy fields that are in
singlet or triplet representations of the electroweak group. Integrating out
these heavy scales, we analyze the pattern of low-energy couplings among the
light fields which are generated by the massive states. We adopt a generic
non-linear realization of the electroweak symmetry breaking with a singlet
Higgs, without making any assumption about its possible doublet structure.
Special attention is given to the different possible descriptions of massive
spin-1 fields and the differences arising from naive implementations of these
formalisms, showing their full equivalence once a proper short-distance
behaviour is required.Comment: 57 pages, 1 pdf figure. Version published at JHE
Resonance saturation of the chiral couplings at NLO in 1/Nc
The precision obtainable in phenomenological applications of Chiral
Perturbation Theory is currently limited by our lack of knowledge on the
low-energy constants (LECs). The assumption that the most important
contributions to the LECs come from the dynamics of the low-lying resonances,
often referred to as the resonance saturation hypothesis, has stimulated the
use of large-Nc resonance lagrangians in order to obtain explicit values for
the LECs. We study the validity of the resonance saturation assumption at the
next-to-leading order in the 1/Nc expansion within the framework of Resonance
Chiral Theory (RChT). We find that, by imposing QCD short-distance constraints,
the chiral couplings can be written in terms of the resonance masses and
couplings and do not depend explicitly on the coefficients of the chiral
operators in the Goldstone boson sector of RChT. As we argue, this is the
counterpart formulation of the resonance saturation statement in the context of
the resonance lagrangian. Going beyond leading order in the 1/Nc counting allow
us to keep full control of the renormalization scale dependence of the LEC
estimates.Comment: 9 pages, 3 eps figures. Added references, version published in Phys.
Rev.
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