arXiv subjects
Publications and source records attributed to G. Lopez Castro.
An empirical formula relating the physical masses of elementary particles and the Fermi constant is proposed. Although no mechanism or theoretical model behind this formula is advocated, we seek for a possible physical interpretation. If not a simple numerical coincidence, this formula may motivate theorists to search for relations among the different sectors of the Electroweak Standard model.
The study of neutrinoless double beta decays of nuclei and hyperons require the calculation of hadronic matrix elements of local four-quark operators that change the total charge by two units ΔQ=2 . Using a low energy effective Lagrangian that induces these transitions, we compute these hadronic matrix elements in the framework of the MIT bag model. As an illustrative example we evaluate the amplitude and transition rate of Σ- -> p e- e-, a decay process that violates lepton number by two units (ΔL=2). The relevant matrix element is evaluated without assuming the usual factorization approximation of the four-quark operators and the results obtained in both approaches are compared.
Lepton number violation (LNV) can be induced by Majorana neutrinos in four-body decays of the neutral B meson and the top quark. We study the effects of Majorana neutrinos in these |ΔL|=2 decays in an scenario where a single heavy neutrino can enhance the amplitude via the resonant mechanism. Using current bounds on heavy neutrino mixings, the most optimistic branching ratios turn out to be at the level of 10^{-6} for \bar{B} -> D^+e^-e^-π^+ and t -> bl^+l^+W^- decays. Searches for these LNV decays at future facilities can provide complementary constraints on masses and mixings of Majorana neutrinos.
Light tensor mesons (T = a_2, f_2 and K_2^*) can be produced in decays of tau leptons. In this paper we compute the branching ratios of tau --> T pi nu decays by assuming the dominance of intermediate virtual states to model the form factors involved in the relevant hadronic matrix element. The exclusive f_2(1270) pi^- decay mode turns out to have the largest branching ratio, of O(10^-4) . Our results indicate that the contributions of tensor meson intermediate states to the three-pseudoscalar channels of tau decays are rather small.
We describe some recent results on isospin breaking corrections which are of relevance for predictions of the leading order hadronic contribution to the muon anomalous magnetic moment (amuhadLO) when using tau lepton data. When these corrections are applied to the new combined data on the pi^+pi^0 spectral function, the prediction for amuhadLO based on tau lepton data gets closer to the one obtained using e^+e^- data.
We revisit the procedure for comparing the pi pi spectral function measured in tau decays to that obtained in e+e- annihilation. We re-examine the isospin-breaking corrections using new experimental and theoretical input, and find improved agreement between the tau- --> pi- pi0 nu_tau branching fraction measurement and its prediction using the isospin-breaking-corrected e+e- --> pi+pi- spectral function, though not resolving all discrepancies. We recompute the lowest order hadronic contributions to the muon g-2 using e+e- and tau data with the new corrections, and find a reduced difference between the two evaluations. The new tau-based estimate of the muon magnetic anomaly is found to be 1.9 standard deviations lower than the direct measurement.
In the standard model, the top quark decay width Γ_t is computed from the exclusive t -> bW decay. We argue in favor of using the three body decays t-> bf_i\bar{f}_j to compute Γ_t as a sum over these exclusive modes. As dictated by the S-matrix theory, these three body decays of the top quark involve only asymptotic states and incorporate the width of the W boson resonance in a natural way. The convolution formula (CF) commonly used to include the finite width effects is found to be valid, in the general case, when the intermediate resonance couples to a conserved current (limit of massless fermions in the case of W bosons). The relation Gamma_t=Γ(t-> bW) is recovered by taking the limit of massless fermions followed by the W boson narrow width approximation. Although both calculations of Γ_t are different at the formal level, their results would differ only by tiny effects induced by light fermion masses and higher order radiative corrections.
Current predictions for the ratio of phi -> K^+K^-/K_LK_S decay rates exceed the corresponding experimental value in about five standard deviations. By far, the dominant sources of isospin breaking to this ratio are the phase-space (52%) and the electromagnetic radiative (4.3%, computed within scalar QED) corrections. Here we estimate the effects of the electromagnetic structure of kaons and other model-dependent contributions into the radiative corrections.
The τ-> (ω, ϕ)P^-νdecays, where P^-=π^-, K^-, are considered within a phenomenological model with dominance of meson intermediate states. We assume SU(3) flavor symmetry to fix some of the unknown strong interaction couplings. Our predictions for the τ-> ϕ(π^-, K^-)νbranching fractions are in good agreement with recent measurements of the BABAR and BELLE Collaborations.
We compute the rates of semileptonic B_A \to B_Bl^-l^- (l=e or μ) hyperon transitions in a model where intermediate states involve loops of baryons and a Majorana neutrino. These rates turn out to be well below present experimental bounds and other theoretical estimates. From the experimental upper limit on the Ξ^- \to pμ^-μ^- decay, we derive the bound < 22 TeV for the effective Majorana mass of the muon neutrino. Also, an estimate of background contributions for these decays due to the allowed B_A \to B_Bl^-l^-\barν\barν decays are provided.
We compute the difference in decay widths of charged and neutral ρ(770) vector mesons. The isospin breaking arising from mass differences of neutral and charged πand ρmesons, radiative corrections to ρ-> ππ, and the ρ-> ππγdecays are taken into account. It is found that the width difference ΔΓ_rho is very sensitive ot the isospin breaking in the $ρ$ meson mass Δm_ρ. This result can be useful to test the correlations observed between the values of these parameters extracted from experimental data.
Based on the description of unstable $K_{L,S}$ particles in quantum field theory (QFT), we compute the time-dependent probabilities for transitions between asymptotic states in $τ^{\pm} \to [π^+π^-]_Kπ^{\pm}ν$ decays, where the pair $[π^+π^-]_K$ is the product of (intermediate state) neutral kaon decays. Then we propose a definition of $τ$ decays into $K_L$ and $K_S$ states, which reflects into the cancellation between their CP rate asymmetries, thus solving in a natural way the paradox pointed out previously in litterature. Since our definition of $K_{L,S}$ final states in $τ$ decays is motivated on experimental grounds, our predictions for the integrated CP rate asymmetries can be tested in a dedicated experiment.
The long-distance electromagnetic radiative corrections to tau- -> pi- pi0 nu are re-evaluated. A meson dominance model is used to describe the emission of real photons in this decay. Results obtained for the hadronic spectrum and the decay rate in photon inclusive reactions are compared with previous calculations based on the chiral resonance theory. Independent tests in tau -> pi pi nu gamma that can help to validate the predictions of one of the two models are briefly discussed.
We evaluate the model-dependent piece of O(alpha) long-distance radiative corrections to tau^- \to π^- π^0ν_τ 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.
We propose some empirical formulae relating the masses of the heaviest particles in the standard model (the W,Z,H bosons and the t quark) to the charge of the positron $e$ and the Higgs condensate v. The relations for the masses of gauge bosons m_W = (1+e)v/4 and m_Z=sqrt{(1+e^2)/2}*(v/2) are in excellent agreement with experimental values. By requiring the electroweak standard model to be free from quadratic divergencies at the one-loop level, we find: m_t=v/sqrt{2} and m_H=v/sqrt{2e}, or the very simple ratio (m_t/m_H)^2=e.
We compute the SUSY effective hamiltonian that describes the |ΔS|=1 semileptonic decays of tau leptons. We provide analytical expressions for supersymmetric contribution to tau --> u bar{s} nu_{tau} transition in mass insertion approximation. We show that SUSY contributions may enhance the CP asymmetry of tau --> K pi nu_{tau} decays by several orders of magnitude than the standard model expectations. However, the resulting asymmetry is still well below the current experimental limits obtained by CLEO collaborations. We emphasize that measuring CP rate asymmetry in this decay larger than 10^{-6} would be a clear evidence of physics beyond the supersymmetric extensions of the standard model.
We consider the bremsstrahlung and model-dependent contributions to the radiative decay $τ^- \to π^-π^0ν_τγ$ 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 $ρ^-(770)$ vector meson. Important numerical differences are found with respect to results obtained in the framework of chiral perturbation theory.
The leading order contribution to the direct CP asymmetry in tau^{+/-} -> K^{+/-} pi^0 nu_{tau} decay rates is evaluated within the Standard Model. The weak phase required for CP violation is introduced through an interesting mechanism involving second order weak interactions, which is also responsible for tiny violations of the Delta S= Delta Q rule in K_{l3} decays. The calculated CP asymmetry turns out to be of order 10^{-12}, leaving a large window for studying effects of non-standard sources of CP violation in this observable.