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J. Portoles

Publications and source records attributed to J. Portoles.

49 records · Page 3Linked to original sources
arXiv2002

New contributions to heavy quark sum rules

We analyse new contributions to the theoretical input in heavy quark sum rules and we show that the general theory of singularities of perturbation theory amplitudes yields the method to handle these specific features. In particular we study the inclusion of heavy quark radiation by light quarks at O(alpha_s^2) and non-symmetric correlators at O(alpha_s^3). Closely related, we also propose a solution to the construction of moments of the spectral densities at O(alpha_s^3) where the presence of massless contributions invalidates the standard approach. We circumvent this problem through a new definition of the moments, providing an infrared safe and consistent procedure.

hep-ph↗
arXiv2001

The vector form factor of the pion from unitarity and analyticity: a model-independent approach

We study a model-independent parameterization of the vector pion form factor that arises from the constraints of analyticity and unitarity. Our description should be suitable up to sqrt(s) ~ 1.2 GeV and allows a model-independent determination of the mass of the rho(770) resonance, M(rho) = (775.1 +- 0.5) MeV. We analyse the experimental data on tau(-) -> pion(-) pion(0) neutrino(tau), in this framework, and its consequences on the low-energy observables worked out by chiral perturbation theory. An evaluation of the two-pion contribution to the anomalous magnetic moment of the muon, a_muon, and to the fine structure constant, alpha(M(Z)^2), is also performed.

hep-ph↗
arXiv2000

Exclusive hadronic Tau decays in the resonance effective action of QCD

Until present the study of the form factors associated to the vector and axial-vector components of the hadronic weak current has been carried out with a plethora of modelizations of the underlying strong interactions. While of importance to get an understanding of the dynamics involved, the amount and quality of the experimental data start to show some discrepancies with the analysed models. Moreover, and from a theoretical point of view, most of these models are not consistent with quantum chromodynamics (QCD). We propose a QCD-based model-independent procedure to analyse those decays through the use of the resonance chiral theory, the low-energy effective action of QCD in the relevant resonance region. Within this framework we study the hadronic off-shell width of the Rho meson and the vector form factor of the pion in Tau(-) -> Pi(-) Pi(0) nu_tau. We also comment on the Tau(-) -> (Pi Pi Pi)(-) nu_tau decay.

hep-ph↗
arXiv2000

The hadronic off-shell width of meson resonances

Within the resonance chiral effective theory we study the dressed propagators of the spin-1 fields, as arise from a Dyson-Schwinger resummation perturbatively constructed from loop diagrams with absorptive contributions in the s-channel. We apply the procedure to the vector pion form factor and the elastic pion-pion scattering to obtain the off-shell width of the rho_0 meson. We adopt a definition of the off-shell width of spin-1 meson resonances that satisfies the requirements of analyticity, unitarity, chiral symmetry and asymptotic behaviour ruled by QCD. To fulfil these constraints the resummation procedure cannot consist only of self-energy diagrams. Our width definition is shown to be independent of the formulation used to describe the spin-1 meson resonances.

hep-ph↗
arXiv1999

Supersymmetric contributions to direct CP violation in K -> pi pi gamma decays

We analyze the supersymmetric contributions to direct-CP-violating observables in $K \to ππγ$ decays induced by gluino-mediated magnetic-penguin operators. We find that $\epsp_{+-γ}$ and the differential width asymmetry of $K^\pm \to π^\pm π^0 γ$ decays could be substantially enhanced with respect to their Standard Model values, especially in the scenario where $\epsp/\eps$ is dominated by supersymmetric contributions. These observables could therefore provide a useful tool to search for New Physics effects in $|ΔS|=1$ transitions, complementary to $\epsp/\eps$ and rare decays.

hep-ph↗
arXiv1998

The Decays $K \to π\ell^+ \ell^-$ beyond Leading Order in the Chiral Expansion

We present a model-independent analysis of $K^+\toπ^+\ell^+\ell^-$ and $K_S\toπ^0\ell^+\ell^-$ decays, including $K\to 3 π$ unitarity corrections and a general decomposition of the dispersive amplitude. From the existing data on $K^+\toπ^+ e^+ e^-$ we predict the ratio $R= B(K^+\toπ^+μ^+μ^-)/B(K^+\toπ^+ e^+ e^-)$ to be larger than 0.23, in slight disagreement with the recent measurement $R = 0.167 \pm 0.036$. Consequences for the $K^\pm \toπ^\pm e^+ e^-$ charge asymmetries and for the $K_L\toπ^0 e^+ e^-$ mode are also discussed.

hep-ph↗
arXiv1998

D^0-D^0(bar) mixing in the weak-gauged U(4)_L x U(4)_R Chiral Lagrangian Model

The D^0-D^0(bar) mixing is analyzed in a weak gauged U(4)_L x U(4)_R chiral lagrangian model where the electroweak interaction is introduced as a gauge theory over the meson degrees of freedom. This model allows a particular realization of the G.I.M. mechanism and then could be useful in the study of processes where G.I.M. suppression is effective. As a test of the model we have also analyzed the K^0-K^0(bar) mixing. We find Δm_K in good agreement with the experimental result and we show that the D^0-D^0(bar) mixing is very much suppressed in agreement with previous estimates in the Heavy Quark expansion framework.

hep-ph↗
arXiv1997

Can we extract short-distance information from B(K_L -> mu^+ mu^-)?

A new analysis of the long-distance two-photon dispersive amplitude of K_L -> mu^+ mu^- is presented. We introduce a phenomenological parameterization of the K_L -> gamma^* gamma^* form factor, constrained at low energies by K_L -> gamma l^+ l^- (l = e,mu) data and at high energies by perturbative QCD. Using this form factor we provide a reliable estimate of magnitude and relative uncertainty of the two-photon dispersive contribution to K_L -> mu^+ mu^-. We finally discuss the implications of this analysis for the extraction of short-distance information from B(K_L -> mu^+ mu^-).

hep-ph↗
arXiv1996

Standard vs. Generalized Chiral Perturbation Theory : the Chell-Olsson test

A way of testing the $ππ$ predictions of Chiral Perturbation Theory against experimental data is to use dispersion relations to continue experimental information into the subthreshold region where the theory should unambiguously apply. Chell and Olsson have proposed a test of the subthreshold behaviour of chiral expansions which highlights potential differences between the Standard and the Generalized forms of the theory. We illustrate how, with current experimental uncertainties, data cannot distinguish between these particular {\em discriminatory} coefficients despite their sensitivity. Nevertheless, the Chell--Olsson test does provide a consistency check of the chiral expansion, requiring that the ${\cal O}(p^6)$ corrections to the {\em discriminatory} coefficients in the Standard theory must be $\sim 100%$. Indeed, some of these have been deduced from the new ${\cal O}(p^6)$ computations and found to give such large corrections. One can then check that the ${\cal O}(p^8)$ corrections must be much smaller. We conclude that this test, like others, cannot distinguish between the different forms of Chiral Symmetry Breaking embodied in the alternative versions of Chiral Perturbation Theory without much more precise experimental information near threshold.

hep-ph↗
arXiv1996

Unitarity and vector meson contributions to K^+ -> pi^+ gamma gamma

We compute the one-loop unitarity corrections O(p^6) from K^+ -> pi^+ pi^+ pi^- to K^+ -> pi^+ gamma gamma and we find they are relevant, increasing the leading order prediction for the width in a 30-40%. The contributions of local O(p^6) amplitudes, generated by vector meson exchange, are discussed in several models and we conclude that the vector resonance contribution should be negligible compared to the unitarity corrections.

hep-ph↗
arXiv1995

Parameter free calculation of non-resonant three body decays of charmed mesons in a weak gauged U(4)_L x U(4)_R Chiral Lagrangian Model

Non--resonant decays of charmed mesons into three pseudoscalars are analyzed in a weak gauged $U(4)_L \otimes U(4)_R$ chiral lagrangian model. The calculation is free of unknown parameters and only requires the masses of pseudoscalar mesons as hadronic inputs. When comparison with experimental data is possible we find that in some processes we have good agreement and in others we are an order of magnitude below. This may be due to the absence in our calculation of final state interactions, presumably important here, and the manifest difficulty in extracting the non--resonant contribution from an experimental point of view.

hep-ph↗
arXiv1994

The Chiral Lagrangian parameters, $\overline{\ell}_1$, $\overline{\ell}_2$, are determined by the $ρ$--resonance

The all--important consequence of Chiral Dynamics for $ππ$ scattering is the Adler zero, which forces $ππ$ amplitudes to grow asymptotically. The continuation of this subthreshold zero into the physical regions requires a $P$--wave resonance, to be identified with the $ρ$. It is a feature of $ππ$ scattering that convergent dispersive integrals for the $I=1$ channel are essentially saturated by the $ρ$--resonance and are much larger than those with $I=2$ quantum numbers. These facts predict the parameters $\overline{\ell}_1$, $\overline{\ell}_2$ of the Gasser--Leutwyler Chiral Lagrangian, as well as reproducing the well--known KSFR relation and self-consistently generating the $ρ-$resonance.

hep-ph↗