arXiv subjects

Ignasi Rosell

Publications and source records attributed to Ignasi Rosell.

At least 37 records · Page 2Linked to original sources

arXiv2016

Low-energy signals of strongly-coupled electroweak symmetry-breaking scenarios

The non-observation of new particles at the LHC suggests the existence of a mass gap above the electroweak scale. This situation is adequately described through a general electroweak effective theory with the established fields and Standard Model symmetries. Its couplings contain all information about the unknown short-distance dynamics which is accessible at low energies. We consider a generic strongly-coupled scenario of electroweak symmetry breaking, with heavy states above the gap, and analyze the imprints that its lightest bosonic excitations leave on the effective Lagrangian couplings. Different quantum numbers of the heavy states imply different patterns of low-energy couplings, with characteristic correlations which could be identified in future data samples. The predictions can be sharpened with mild assumptions about the ultraviolet behaviour of the underlying fundamental theory.

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arXiv2015

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.

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arXiv2014

Oblique S and T Constraints on Electroweak Strongly-Coupled Models with a Light Higgs

Using a general effective Lagrangian implementing the chiral symmetry breaking SU(2)L x SU(2)R -> SU(2){L+R}, we present a one-loop calculation of the oblique S and T parameters within electroweak strongly-coupled models with a light scalar. Imposing a proper ultraviolet behaviour, we determine S and T at next-to-leading order in terms of a few resonance parameters. The constraints from the global fit to electroweak precision data force the massive vector and axial-vector states to be heavy, with masses above the TeV scale, and suggest that the W+W- and ZZ couplings of the Higgs-like scalar should be close to the Standard Model value. Our findings are generic, since they only rely on soft requirements on the short-distance properties of the underlying strongly-coupled theory, which are widely satisfied in more specific scenarios.

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arXiv2013

Viability of light-Higgs strongly-coupled scenarios

Contrary to what is sometimes stated, the current electroweak precision data easily allow for massive composite resonance states at the natural EW scale, i.e., well over the TeV. The oblique parameters S and T are analyzed by means of an effective Lagrangian that implements the $SU(2)_L\otimes SU(2)_R\to SU(2)_{L+R}$ pattern of electroweak symmetry breaking. They are computed at the one-loop level and incorporating the newly discovered Higgs-like boson and possible spin-1 composite resonances. Imposing a proper ultraviolet behaviour is crucial and allows us to determine S and T at next-to-leading order in terms of a few resonance parameters. Electroweak precision data force the vector and axial-vector states to have masses above the TeV scale and suggest that the W+W- and ZZ couplings to the Higgs-like scalar should be close to the Standard Model value. Our findings are generic: they only rely on symmetry principles and soft requirements on the short-distance properties of the underlying strongly-coupled theory, which are widely satisfied in more specific scenarios.

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arXiv2013

Strongly Coupled Models with a Higgs-like Boson

Considering the one-loop calculation of the oblique S and T parameters, we have presented a study of the viability of strongly-coupled scenarios of electroweak symmetry breaking with a light Higgs-like boson. The calculation has been done by using an effective Lagrangian, being short-distance constraints and dispersive relations the main ingredients of the estimation. Contrary to a widely spread believe, we have demonstrated that strongly coupled electroweak models with massive resonances are not in conflict with experimental constraints on these parameters and the recently observed Higgs-like resonance. So there is room for these models, but they are stringently constrained. The vector and axial-vector states should be heavy enough (with masses above the TeV scale), the mass splitting between them is highly preferred to be small and the Higgs-like scalar should have a WW coupling close to the Standard Model one. It is important to stress that these conclusions do not depend critically on the inclusion of the second Weinberg sum rule.

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arXiv2013

One-loop calculation of the oblique S and T parameters within strongly-coupled scenarios with a light Higgs-like boson

We present a one-loop calculation of the oblique S and T parameters within strongly-coupled models of electroweak symmetry breaking with a light Higgs-like boson. We use a general effective Lagrangian, implementing the chiral symmetry breaking SU(2)_L x SU(2)_R -> SU(2)_{L+R} with Goldstones, gauge bosons, the Higgs-like scalar and one multiplet of vector and axial-vector massive resonance states. The estimation is based on the short-distance constraints and a dispersive approach. The experimentally allowed range forces the vector and axial-vector states to be heavy, with masses above the TeV scale, and suggests that the Higgs-like scalar should have a WW coupling close to the Standard Model one.

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arXiv2012

Viability of strongly-coupled scenarios with a light Higgs-like boson

We present a one-loop calculation of the oblique S and T parameters within strongly-coupled models of electroweak symmetry breaking with a light Higgs-like boson. We use a general effective Lagrangian, implementing the chiral symmetry breaking SU(2)_L x SU(2)_R -> SU(2)_{L+R} with Goldstones, gauge bosons, the Higgs-like scalar and one multiplet of vector and axial-vector massive resonance states. Using a dispersive representation and imposing a proper ultraviolet behaviour, we obtain S and T at the next-to-leading order in terms of a few resonance parameters. The experimentally allowed range forces the vector and axial-vector states to be heavy, with masses above the TeV scale, and suggests that the Higgs-like scalar should have a WW coupling close to the Standard Model one. Our conclusions are generic and apply to more specific scenarios such as the minimal SO(5)/SO(4) composite Higgs model.

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arXiv2012

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.

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arXiv2012

One-Loop Calculation of 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 and analyze the phenomenological implications of the available electroweak precision data. We use the most 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 massive resonance states. Using the dispersive representation of Peskin and Takeuchi and imposing the short-distance constraints dictated by the operator product expansion, we obtain S at the NLO in terms of a few resonance parameters. In asymptotically-free gauge theories, the final result only depends on the vector-resonance mass and requires M_V > 1.8 TeV (3.8 TeV) to satisfy the experimental limits at the 3 σ(1σ) level; the axial state is always heavier, we obtain M_A > 2.5 TeV (6.6 TeV) at 3σ(1σ). In strongly-coupled models, such as walking or conformal technicolour, where the second Weinberg sum rule does not apply, the vector and axial couplings are not determined by the short-distance constraints; but one can still derive a lower bound on S, provided the hierarchy M_V < M_A remains valid. Even in this less constrained situation, we find that in order to satisfy the experimental limits at 3σone needs M_{V,A} > 1.8 TeV.

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arXiv2011

The vector form factor at the next-to-leading order in 1/N(C): chiral couplings L9(mu) and C88(mu) - C90(mu)

Using the Resonance Chiral Theory Lagrangian, we perform a calculation of the vector form factor of the pion at the next-to-leading order (NLO) in the 1/N(C) expansion. Imposing the correct QCD short-distance constraints, one fixes the amplitude in terms of the pion decay constant F and resonance masses. Its low momentum expansion determines then the corresponding O(p4) and O(p6) low-energy chiral couplings at NLO, keeping control of their renormalization scale dependence. At mu0=0.77 GeV, we obtain L9(mu0) = (7.9 \pm 0.4) 10^{-3} and C88(mu0)-C90(mu0)=(-4.6 \pm 0.4) 10^{-5}.

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arXiv2010

Revisiting the vector form factor at next-to-leading order in 1/N(C)

Using the Resonance Chiral Theory lagrangian, we perform a calculation of the vector form factor of the pion at the next-to-leading order (NLO) in the $1/N_C$ expansion. Imposing the correct QCD short-distance constraints, one determines it in terms of $F$, $G_V$, $F_A$ and resonance masses. Its low momentum expansion fixes then the low-energy chiral couplings $L_{9}$ and $C_{88}-C_{90}$ at NLO, keeping full control of their renormalization scale dependence. At $μ_0=0.77$ GeV, we obtain $L_{9}^r(μ_0) = (7.6 \pm 0.6)\cdot 10^{-3}$ and $C_{88}^r(μ_0)-C_{90}^r(μ_0)=(-4.5 \pm 0.5)\cdot 10^{-5}$.

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arXiv2009

The Standard model prediction for K(e2)/K_(mu2) and pi(e2)/pi(mu2)

We have calculated the ratios $R_{e/μ}^{(P)} \equiv Γ(P \to e \barν_e [γ])/ Γ(P \to μ\barν_μ[γ])$ ($P=π,K$) in Chiral Perturbation Theory up to $\mathcal{O}(e^2p^4)$, finding $R_{e/μ}^{(π)} = (1.2352 \pm 0.0001) \times 10^{-4}$ and $R_{e/μ}^{(K)} = (2.477 \pm 0.001) \times 10^{-5}$. This observable is helicity suppressed in the Standard Model, so that it is a sensitive probe of all Standard Model extensions that induce pseudoscalar currents and nonuniversal corrections to the lepton couplings. Ongoing experimental searches plan to reach uncertainties that are comparable to these results. At the moment $R_{e/μ}^{(K)}$ is in agreement with the final result by the KLOE Collaboration at DAFNE and it is at $1.4 σ$ of the preliminary result by the NA62 Experiment at CERN. New measurements of $R_{e/μ}^{(π)}$ are under way by the PEN Collaboration at PSI and by the PIENU Collaboration at TRIUMF.

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arXiv2009

Resonance saturation at next-to-leading order

A proper estimation of the chiral low-energy constants of Chiral Perturbation Theory is a very important task. To this end resonance chiral Lagrangians have been used fruitfully. We have studied the determination of chiral couplings at next-to-leading (NLO) order in the 1/N(C) expansion, keeping full control of the renormalization scale dependence. We find that, by imposing short-distance constraints coming from QCD, resonance saturation at NLO in 1/N(C) is satisfied. In other words, 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 Resonance Chiral Theory.

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arXiv2009

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.

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arXiv2008

Current correlators and form factors in the resonance region

Within Resonance Chiral Theory and in the context of QCD current correlators at next-to-leading order in 1/N(C), we have analyzed the two-body form factors which include resonances as a final state . The short-distance constraints have been studied. One of the main motivations is the estimation of the chiral low-energy constants at subleading order, that is, keeping full control of the renormalization scale dependence. As an application we show the resonance estimation of some coupling, L(10)(mu_0)=(-4.4 \pm 0.9)10^{-3} and C(87)(mu_0)=(3.1 \pm 1.1)10^{-5}.

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arXiv2007

Determining chiral couplings at NLO

We present a general method that allows to estimate the low-energy constants of Chiral Perturbation Theory up to next-to-leading corrections in the 1/N(C) expansion, that is, keeping full control of the renormalization scale dependence. As a first step we have determined L(8) and C(38), the couplings related to the difference of the two-point correlation functions of two scalar and pseudoscalar currents, L(8)(mu_0) = (0.6 \pm 0.4) 10^{-3} and C(38)(mu_0) = (2 \pm 6) 10^{-6}$, with mu_0 = 0.77 GeV. As in many effective approaches, one of the main ingredients of this method is the matching procedure: some comments related to this topic are presented here.

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arXiv2007

Vanishing chiral couplings in the large-N_C resonance theory

The construction of a resonance theory involving hadrons requires implementing the information from higher scales into the couplings of the effective Lagrangian. We consider the large-Nc chiral resonance theory incorporating scalars and pseudoscalars, and we find that, by imposing LO short-distance constraints on form factors of QCD currents constructed within this theory, the chiral low-energy constants satisfy resonance saturation at NLO in the 1/Nc expansion.

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arXiv2007

pi/K -> e nu branching ratios to O(e^2 p^4) in Chiral Perturbation Theory

We calculate the ratios R_{e/mu}^{(P)} = Gamma(P -> e nu)/Gamma (P -> mu nu) (P=pi,K) in Chiral Perturbation Theory to order e^2 p^4. We complement the one- and two-loop effective theory results with a matching calculation of the local counterterm, performed within the large-$N_C$ expansion. We find R_{e/mu}^{(π)} = (1.2352 \pm 0.0001)*10^{-4} and R_{e/mu}^{(K)} = (2.477 \pm 0.001)*10^{-5}, with uncertainty induced by the matching procedure and chiral power counting. Given the sensitivity of upcoming new measurements, our results provide a clean baseline to detect or constrain effects from weak-scale new physics in these rare decays. As a by-product, we also update the theoretical analysis of the individual pi/K -> \ell nu modes.

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