The $N to Delta$ weak vertex provides an important contribution to the one pion production in neutrino-nucleon and neutrino-nucleus scattering for $pi N$ invariant masses below 1.4 GeV. Beyond its interest as a tool in neutrino detection and their ba
ckground analyses, one pion production in neutrino-nucleon scattering is useful to test predictions based on the quark model and other internal symmetries of strong interactions. Here we try to establish a connection between two commonly used parametrizations of the weak $N to Delta$ vertex and form factors (FF) and we study their effects on the determination of the axial coupling $C_5^A(0)$, the common normalization of the axial FF, which is predicted to hold 1.2 by using the PCAC hypothesis. Predictions for the $ u_{mu} p to mu^- ppi^+$ total cross sections within the two approaches, which include the resonant $Delta^{++}$ and other background contributions in a coherent way, are compared to experimental data.
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 Delta Q=2 . Using a low energy effective Lagrangian that i
nduces 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 Sigma- -> p e- e-, a decay process that violates lepton number by two units (Delta L=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.
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.
We study flavor violating processes in the production or decay of a neutral pseudoscalar meson $P^0$ in the framework of a general two Higgs Doublet Model type III (2HDM-III). We use a version of the model where Yukawa interactions of neutral Higgs b
osons allow for flavor change at the tree-level, but conserves CP symmetry. We focus on all possible $tau^{pm} to l^{pm}P^0 $ and $P^0 to l^+l^-$ decay channels, where $l,l$ are charged leptons. We find that these processes provide complementary information on quark and lepton FCNC Yukawa couplings. In particular flavor violating parameters in the quark sector, $chi_{sb}$ and $chi_{db}$, are significantly constrained by present experimental data, whereas the corresponding parameters in the leptonic sector are less constrained.
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 elec
tromagnetic 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.
Top quark decays are of particular interest as a mean to test the standard model (SM) predictions, both for dominant (t -> b + W) and rare decays (t -> q + W, cV, cVV, c phi^{0}, bWZ). As the latter are highly suppressed, they become an excellent win
dow to probe the predictions of thories beyond the SM. In particular, we evaluate the corrections from new physics to the CKM-suppressed SM top quark decay t -> q + W (q = d, s), both within the an effective model with right-handed currents and the MSSM. We also discuss the perspectives to probe those predictions at the ILC.
The tau -> (omega, phi)P^- u decays, where P^-=pi^-, 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 predict
ions for the tau -> phi (pi^-, K^-) u branching 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 mu) 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 t
heoretical estimates. From the experimental upper limit on the Xi^- to pmu^-mu^- decay, we derive the bound <m_{mu mu}> < 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^-bar{ u}bar{ u} decays are provided.
We compute the difference in decay widths of 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 -> pipi, and the rho -> pipigamma decay
s are taken into account. It is found that the width difference Delta Gamma_rho is very sensitive ot the isospin breaking in the $rho$ meson mass Delta m_rho. This result can be useful to test the correlations observed between the values of these parameters extracted from experimental data.
