G. Lopez Castro

Long distance radiative corrections to the decay τ- → π- π0ντ

We evaluate the model-dependent piece of O(alpha) long-distance radiative corrections to tau^- \to \pi^- \pi^0\nu_{\tau} decays by using a meson dominance model. We find that these corrections to the di-pion invariant mass spectrum are smaller than in previous calculations based on chiral perturbation theory. The corresponding correction to the photon inclusive rate is tiny (-0.15%) but it can be of relevance when new measurements reach better precision.

Elastic and radiative π+p scattering and properties of the Δ++ resonance

Abstract We study the Δ ++ contributions to elastic and radiative π + p scattering within an effective Lagrangian model which incorporates the Δ , N, ρ and σ meson degrees of freedom. This model provides a description of the Δ resonance and its interactions that respects electromagnetic gauge invariance and invariance under contact transformations when finite width effects are incorporated. Following recent developments in the description of unstable gauge bosons, we use a complex mass scheme to introduce the finite width of the Δ ++ without spoiling gauge invariance. The total cross section of elastic π + p scattering, whose amplitude exhibits the resonant plus background structure of S -matrix theory, is used to fix the mass, width and strong coupling of the Δ resonance. The differential cross section of elastic scattering is found in very good agreement with experimental data. The magnetic dipole moment of the Δ ++ , μ Δ , is left as the only adjustable parameter in radiative π + p scattering. From a fit to the most sensitive configurations for photon emission in this process, we obtain μ Δ =(6.14±0.51) e /2 m p , in agreement with predictions based on the SU (6) quark model.

Gauge invariance and finite width effects in radiative two pion tau lepton decay

The contribution of the rho^{\pm} vector meson to the tau -> pi pi nu gamma decay is considered as a potential source for the determination of the magnetic dipole moment of this light vector meson. In order to keep gauge-invariance of the whole decay amplitude, a procedure similar to the fermion loop-scheme for charged gauge bosons is implemented to incorporate the finite width effects of the rho^{\pm} vector meson. The absorptive pieces of the one-loop corrections to the propagators and electromagnetic vertices of the rho^{\pm} meson and W^{\pm} gauge boson have identical forms in the limit of massless particles in the loops, suggesting this to be a universal feature of spin-one unstable particles. Model-dependent contributions to the tau -> pi pi nu gamma decay are suppressed by fixing the two-pion invariant mass distribution at the rho meson mass value. The resulting photon energy and angular distribution is relatively sensitive to the effects of the rho magnetic dipole moment.

The Width difference of rho vector mesons

We compute the difference in decay widths between charged and neutral {rho}(770) vector mesons. The isospin breaking arising from mass differences of neutral and charged {pi} and {rho} mesons, radiative corrections to {rho}{yields}{pi}{pi}, and the {rho}{yields}{pi}{pi}{gamma} decays are taken into account. It is found that the width difference {delta}{gamma}{sub {rho}} is very sensitive to the isospin breaking in the {rho} meson mass, {delta}m{sub {rho}}. This result can be useful to test the correlations observed between the values of these parameters extracted from experimental data.

Radiative two-pion decay of the tau lepton

We consider the bremsstrahlung and model-dependent contributions to the radiative decay

Tests for the asymptotic behaviour of the γ∗→γπ0 form factor

{\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{\ensuremath{\nu}}_{\ensuremath{\tau}}\ensuremath{\gamma}

Tests for the asymptotic behavior of the gamma* ---> gamma pi0 form-factor

in the context of a meson dominance model. We focus on several observables related to this decay, including the branching ratio and the photon and di-pion spectra. Particular attention is paid to the sensitivity of different observables upon the effects of model-dependent contributions and of the magnetic dipole moment of the

Behaviour of the absorptive part of the W± electromagnetic vertex

{\ensuremath{\rho}}^{\ensuremath{-}}(770)

Pion - proton scattering and isospin breaking in the Delta0 - Delta++ system

vector meson. Important numerical differences are found with respect to results obtained in the framework of chiral perturbation theory.

W+- and Z0 propagators on the resonance region

The gamma^* --> gamma pi^0 transition measured at different photon virtualities already provides us with a clean test for the behaviour of the pi^0 gamma^* gamma off-shell axial anomaly at large time-like squared momenta. It also allows reliable predictions for the branching ratio of heavy quarkonium decays into omega pi^0. Comment: Latex, 7 pages + 1 .ps figure, comments on the Brodsky-Lepage limit and 5 new references added