The lowest resonance in QCD from low--energy data

We show that a generalization of $su(2)$ Chiral Perturbation Theory, including a perturbative singlet scalar field, converges faster towards the physical value of sensible low--energy observables. The physical mass and width of the scalar particle are obtained through a simultaneous analysis of the pion radius and the gamma gamma -> pi^0 pi^0 cross-section. Both values are statistically consistent with the ones obtained by using Roy equations in pi-pi scattering. In addition we find indications that the photon-photon-singlet coupling is quite small.Comment: pdfLatex 15 pages, 3 figure

Insights into Q2Qˉ2Q^2 \bar{Q}^2 states from an effective perspective

We discuss the two photon coupling of the lightest scalar meson on the basis of an extension of $\chi$PT. Using low energy data on the pion form-factor and the $\gamma\gamma\to \pi^+\pi^-(\pi^0\pi^0)$ cross-sections as inputs, we find $\Gamma(\sigma\to\gamma\gamma) \cong 0.126~\rm{keV}$. The smallness of the result and the relative weight between its components, ${\Gamma_{\gamma\gamma\to S_1}\over \Gamma{ \gamma\gamma\to\pi\pi\to S_1}} < 1$, suggests that the scalar $0^{++}$ meson is mainly a $Q^2\bar{Q}^2$ state.Comment: 14 pages, 6 figures. Part of the paper rewritten and enlarged, including a discussion on pi-pi phase shifts. Added reference

Vector Meson Masses in Chiral Perturbation Theory

We discuss the vector meson masses within the context of Chiral Perturbation Theory performing an expansion in terms of the momenta, quark masses and 1/Nc. We extend the previous analysis to include isospin breaking effects and also include up to order $p^4$. We discuss vector meson chiral perturbation theory in some detail and present a derivation from a relativistic lagrangian. The unknown coefficients are estimated in various ways. We also discuss the relevance of electromagnetic corrections and the implications of the present calculation for the determination of quark masses.Comment: 17 pages, LaTeX, 2 figures. Revised version to appear in Nuclear Physics B. One reference added, some misprints, mainly in the appendix, correcte

Matching the Heavy Vector Meson Theory

We show how to obtain a ``heavy'' meson effective lagrangian for the case where the number of heavy particles is not conserved. We apply the method in a simple example at tree level and perform then the reduction for the case of vector mesons in Chiral Perturbation Theory in a manifestly chiral invariant fashion. Some examples showing that ``heavy'' meson effective theory also works at the one-loop level are included.Comment: 11 pages, 2 figures, revte

The Vector and Scalar Form Factors of the Pion to Two Loops

We calculate the vector and scalar form factors of the pion to two loops in Chiral Perturbation Theory. We estimate the unknown O(p^6) constants using resonance exchange. We make a careful comparison to the available data and determine two O(p^4) constants rather precisely, and two O(p^6) constants less precisely. We also use Chiral Perturbation Theory to two loops to extract in a model--independent manner the charge radius of the pion from the available data, and obtain \rpiV=0.437\pm 0.016 fm^2.Comment: Latex2e, JHEP.cls, 30 pages 5 figure

Low Energy Constants from Kl4 Form-Factors

We have calculated the form-factors F and G in K ---> pi pi e nu decays (Kl4) to two-loop order in Chiral Perturbation Theory (ChPT). Combining this together with earlier two-loop calculations an updated set of values for the L's, the ChPT constants at p^4, is obtained. We discuss the uncertainties in the determination and the changes compared to previous estimates.Comment: 6 pages, uses axodraw.sty, 2 figures, bug in program corrected, new numerical result

Chiral Perturbation Theory with Virtual Photons and Leptons

We construct a low-energy effective field theory which allows the full treatment of isospin-breaking effects in semileptonic weak interactions. To this end, we enlarge the particle spectrum of chiral perturbation theory with virtual photons by including also the light leptons as dynamical degrees of freedom. Using super-heat-kernel techniques, we determine the additional one-loop divergences generated by the presence of virtual leptons and give the full list of associated local counterterms. We illustrate the use of our effective theory by applying it to the decays pi -> l nu_{l} and K -> l nu_{l}.Comment: 22 pages, no figure

Radiative corrections to K_{l3} decays

We present a complete calculation of the K_{l3} decays K^+ --> pi^0 l^+ nu_l and K^0 --> pi^- l^+ nu_l to O(p^4, (m_d-m_u) p^2, e^2 p^2) in chiral perturbation theory with virtual photons and leptons. We introduce the concept of generalized form factors and kinematical densities in the presence of electromagnetism, and propose a possible treatment of the real photon emission in K^+_{l3} decays. We illustrate our results by applying them to the extraction of the Kobayashi--Maskawa matrix element |V_{us}| from the experimental K^+_{e3} decay parameters.Comment: 13 page

Kant y la idea del progreso indefinido de la humanidad

The idea of progress that underlies Modernity (and which Kant praises) has a very singular trait: its allegedly linear, irreversible and necessary character. Here we seek to analyze the fundamental dimensions of the Kantian conception of progress, and to trace its limits, appealing to a reformulation of the notion of progress, not based on autonomy, understood in terms of self-consciousness and self-sufficiency, but on the radical interdependence and solidarity of the human being and on the recovery of the idea of the “common good”