arXiv subjects
Publications and source records attributed to G. Lopez Castro.
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.
We consider the isospin breaking corrections to charmless semileptonic decays of B mesons. Both, the recently measured branching ratios of exclusive decays by the CLEO Collaboration and the end-point reion of the inclusive lepton spectrum in form factor models, can be affected by these corrections. Isospin corrections can affect the determination of |V_ub| from exclusive semileptonic B decays at a level comparable to present statistical uncertainties.
Quantum field theory provides a consistent framework to deal with unstable particles. We present here an approach based on field theory to describe the production and decay of unstable $K^0-\bar{K^0}$ and $B^0-\bar{B^0}$ mixed systems. The formalism is applied to compute the time evolution amplitudes of $K^0$ and $\bar{K^0}$ studied in DAPHNE and CPLEAR experiments. We also introduce a new set of parameters that describe CP violation in $ K \to ππ$ decays without recourse to isospin decomposition of the decay amplitudes.
We consider the effects of anomalous magnetic dipole moments of vector mesons in the decay distribution of photons emitted in two-pseudoscalar decays of charged vector mesons. By choosing a kinematical configuration appropriate to isolate these effects from model-dependent and dominant bremsstrahlung contributions, we show that this method can provide a valid alternative for a measurement of the unknown magnetic dipole moments of charged vector mesons.
Two-body non-leptonic decays of B mesons into PT and VT modes are calculated using the non-relativistic quark model of Isgur et al.. The predictions obtained for $B \to πD^*_2, ρD_2^*$ are a factor of $3\sim 5$ below present experimental upper limits. Interesting patterns are obtained for ratios of B decays involving mesons with different spin excitations and their relevance for additional tests of forms factor models are briefly discussed.
We assume the stability of vacuum under radiative corrections in the context of the standard electroweak theory. We find that this theory behaves as a good effective model already at cut off energy scales as low as 0.7 TeV. This stability criterion allows to predict m_H= 318 +- 13 GeV for the Higgs boson mass.
The absorptive part of the $WWγ$ vertex induced by massive fermion loops is considered for different kinematical configurations. We show that the axial part of this vertex is different from zero not only when massive fermions are involved but also for massless fermion loops, if one of the W bosons is space-like and the other is time-like. We also discuss in what sense Low's soft photon theorem is satisfied.
We compute isospin corrections to the charmless semileptonic $B \rightarrow V$ transitions arising from $ρ-ω$ mixing and discuss its relevance in the determination of $V_{ub}$.
Based on the S-matrix approach, we introduce a modified formula for the $π^{\pm}$ electromagnetic form factor which describes very well the experimental data in the energy region $2m_π \leq \sqrt{s} \leq 1.1$ GeV. Using the CVC hypothesis we predict $B(\tpp) = (24.75 \pm 0.38)\% $, in excellent agreement with recent experiments.
We study a model with two Higgs doublets where FCNC are allowed at tree-level. In this model, the interactions of charged Higgs with fermions ($H^\pm f \bar{f'}$), include a term that is not proportional to the fermion masses, which we constraint using the following low-energy processes: i) tau decays ($τ\to ν_τ eν_e, ν_τμν_μ, ν_τπ$), ii) leptonic decays of pseudoscalar mesons ($π, K \to \ellν_{\ell}$) and iii) semileptonic b-decays. With these constraints it is possible to make predictions; we illustrate this by presenting the rates for the (FCNC) decay $c\to u+γ$, the (second class-current) decay $τ\to ν_τ+ ηπ$, and also the theoretical value of the neutron life-time.
We determine the mass and width of the $Δ^{++}\ (Δ^0)$ resonance from data on $π^+ p\ (π^- p)$ scattering both, in the pole of the $S$-matrix and conventional Breit-Wigner approaches to the scattering amplitude. We provide a simple formula that relates the two definitions for the parameters of the $Δ$. Isospin symmetry breaking in the $\d0-\dm$ system depends on the definition of the resonant properties: we find $M_0-M_{++} = 0.40 \pm 0.57\ {\rm MeV},\ Γ_0 -Γ_{++} = 6.89 \pm 0.95\ {\rm MeV}$ in the pole approach while $\wt{M}_0-\wt{M}_{++} = 2.25 \pm 0.68\ {\rm MeV},\ \wtΓ_0 - \wtΓ_{++} = 8.45 \pm 1.11\ {\rm MeV}$ in the conventional approach.
We address the problem of properly defining the $W^{\pm}, Z^0$ propagator in the resonance region. Particular attention is paid to the longitudinal piece of this propagator. We also discuss the related renormalization procedures and the unitarity property.