Ignasi Rosell

Quantum loops in the resonance chiral theory: The Vector form-factor

We present a calculation of the Vector Form Factor at the next-to-leading order in the 1/NC expansion, within the framework of Resonance Chiral Theory. The calculation is performed in the chiral limit, and with two dynamical quark flavours. The ultraviolet behaviour of quantum loops involving virtual resonance propagators is analyzed, together with the kind of counterterms needed in the renormalization procedure. Using the lowest-order equations of motion, we show that only a few combinations of local couplings appear in the final result. The low-energy limit of our calculation reproduces the standard Chiral Perturbation Theory formula, allowing us to determine the resonance contribution to the chiral low-energy couplings, at the next-to-leading order in 1/NC, keeping a full control of their renormalization scale dependence.

π/K → eν̄e branching ratios to O(e2p4) 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-

Two-loop effective theory analysis of pi (K) ---> e anti-nu/e [gamma] branching ratios

N_C

The Standard Model prediction for R_{e/mu}^{(pi,K)}

expansion. We find R_{e/mu}^{(\pi)} = (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.

One-loop renormalization of Resonance Chiral Theory: scalar and pseudoscalar resonances

We study the ratios R_{e/mu}^{(P)} = Gamma(P -> e nu [gamma])/Gamma(P -> mu nu [gamma]) (P=pi,K) in Chiral Perturbation Theory to order e^2 p^4. We complement the two-loop effective theory results with a matching calculation of the counterterm, finding R_{e/mu}^{(pi)} = (1.2352 \pm 0.0001)*10^(-4) and R_{e/mu}^{(K)} = (2.477 \pm 0.001)*10^(-5).

Form-factors and current correlators: chiral couplings L10r(μ) and C87r(μ) at NLO in 1/NC

We study the ratios R_{e/mu}^{(P)} = Gamma(P -> e nu [gamma])/Gamma(P -> mu nu [gamma]) (P=pi,K) in Chiral Perturbation Theory to order e^2 p^4. We complement the two-loop effective theory results with a matching calculation of the counterterm, finding R_{e/mu}^{(pi)} = (1.2352 \pm 0.0001)*10^(-4) and R_{e/mu}^{(K)} = (2.477 \pm 0.001)*10^(-5).

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

We consider the Resonance Chiral Theory with one multiplet of scalar and pseudoscalar resonances, up to bilinear couplings in the resonance fields, and evaluate its β-function at one-loop with the use of the background field method. Thus we also provide the full set of operators that renormalize the theory at one loop and render it finite.

Towards a determination of the chiral couplings at NLO in 1/NC: L8r(μ) and C38r(μ)

Using the resonance chiral theory Lagrangian, we perform a calculation of the vector and axial-vector two-point functions at the next-to-leading order (NLO) in the 1/NC expansion. We have analyzed these correlators within the single-resonance approximation and have also investigated the corrections induced by a second multiplet of vector and axial-vector resonance states. Imposing the correct QCD short-distance constraints, one determines the difference of the two correlators Π(t) ≡ ΠVV(t)−ΠAA(t) in terms of the pion decay constant and resonance masses. Its low momentum expansion fixes then the low-energy chiral couplings L10 and C87 at NLO, keeping full control of their renormalization scale dependence. At μ0 = 0.77 GeV, we obtain L10r(μ0) = (−4.4±0.9) 10−3 and C87r(μ0) = (3.1±1.1) 10−5.

Oblique S and T constraints on electroweak strongly-coupled models with a light Higgs

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) [Symbol: see text]SU(2)(R) → SU(2)(L+R) with Goldstone bosons, 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 behavior, 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.

One-Loop Calculation of the Oblique S Parameter in Higgsless Electroweak Models

We present a dispersive method which allows to investigate the low-energy couplings of chiral perturbation theory at the next-to-leading order (NLO) in the 1/NC expansion, keeping full control of their renormalization scale dependence. Using the res- onance chiral theory Lagrangian, we perform a NLO calculation of the scalar and pseu- doscalar two-point functions, within the single-resonance approximation. Imposing the cor- rect QCD short-distance constraints, one determines their difference |(t) ´ |S(t)i|P(t) in terms of the pion decay constant and resonance masses. Its low momentum expan- sion fixes then the low-energy chiral couplings L8 and C38. At µ0 = 0.77GeV, we obtain L r (µ0) SU(3) = (0.6 ± 0.4) · 10 i3 and C r (µ0) SU(3) = (2 ± 6) · 10 i6 .