Search

The long-distance electromagnetic radiative corrections to τ − → π − π 0 ν τ 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 τ → π π ν γ that can help to validate the predictions of one of the two models are briefly discussed.

We study the five-body decays $\mu^- \to e^- e^+ e^- \nu_{\mu} \bar{\nu}_{e}$ and $\tau^- \to \ell^- \ell'^+\ell'^-\nu_{\tau}\bar{\nu}_{\ell}$ for $\ell, \ell'=e, \mu$ within the Standard Model (SM) and in a general effective field theory description of the weak interactions at low energies. We compute the branching ratios and compare our results with two previous, mutually discrepant, SM calculations. By assuming a general structure for the weak currents we derive the expressions for the energy and angular distributions of the three charged leptons when the decaying lepton is polarized, which will be useful in precise tests of the weak charged current at Belle II. In these decays, leptonic $\mathbf{T}$-odd correlations in triple products of spin and momenta --which may signal time reversal violation in the leptonic sector-- are suppressed by the tiny neutrino masses. Therefore, a measurement of such $\mathbf{T}$-violating observables would be associated to neutrinoless lepton flavor violating (LFV) decays, where this effect is not extremely suppressed. We also study the backgrounds that the SM five-lepton lepton decays constitute to searches of LFV $L^- \to \ell^- \ell'^+\ell'^-$ decays. Searches at high values of the invariant mass of the $\ell'^+\ell'^-$ pair look the most convenient way to overcome the background., Comment: 40 pages, 6 figures, extended discussion of results in the case of the effective lagrangian approach; other minor corrections/modifications are added

The one-loop contribution of the excited Kaluza-Klein (KK) modes of the $S{U}_{L}(2)$ gauge group on the off-shell ${W}^{\ensuremath{-}}{W}^{+}\ensuremath{\gamma}$ and ${W}^{\ensuremath{-}}{W}^{+}Z$ vertices is calculated in the context of a pure Yang-Mills theory in five dimensions and its phenomenological implications discussed. The use of a gauge-fixing procedure for the excited KK modes that is covariant under the standard gauge transformations of the $S{U}_{L}(2)$ group is stressed. A gauge-fixing term and the Faddeev-Popov ghost sector for the KK gauge modes that are separately invariant under the standard gauge transformations of $S{U}_{L}(2)$ are presented. It is shown that the one-loop contributions of the KK modes to the off-shell ${W}^{\ensuremath{-}}{W}^{+}\ensuremath{\gamma}$ and ${W}^{\ensuremath{-}}{W}^{+}Z$ vertices are free of ultraviolet divergences and well-behaved at high energies. It is found that for a size of the fifth dimension of ${R}^{\ensuremath{-}1}\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$, the one-loop contribution of the KK modes to these vertices is about 1 order of magnitude lower than the corresponding standard model radiative correction. This contribution is similar to the one estimated for new gauge bosons contributions in other contexts. Tree-level effects on these vertices induced by operators of higher canonical dimension are also investigated. It is found that these effects are lower than those generated at the one-loop order by the KK gauge modes.

We perform a complete calculation at the one-loop level for the $Zggg$ and ${Z}^{\ensuremath{'}}ggg$ couplings in the context of the minimal 331 model, which predicts the existence of a new ${Z}^{\ensuremath{'}}$ gauge boson and new exotic quarks. Bose symmetry is exploited to write a compact and manifest $S{U}_{C}(3)$-invariant vertex function for the $Vggg$ ($V=Z$, ${Z}^{\ensuremath{'}}$) coupling. Previous results on the $Z\ensuremath{\rightarrow}ggg$ decay in the standard model are reproduced. It is found that this decay is insensitive to the effects of the new exotic quarks. This in contrast with the ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}ggg$ decay, which is sensitive to both the standard model and exotic quarks, whose branching ratio is larger than that of the $Z\ensuremath{\rightarrow}ggg$ transition by about a factor of 4.

Higgs mediated flavor violating electromagnetic interactions, induced at the one--loop level by a nondiagonal $Hf_if_j$ vertex, with $f_i$ and $f_j$ charged leptons or quarks, are studied within the context of a completely general effective Yukawa sector that comprises $SU_L(2)\times U_Y(1)$--invariant operators of up to dimension--six. Exact formulae for the one--loop $\gamma f_if_j$ and $\gamma \gamma f_if_j$ couplings are presented and their related processes used to study the phenomena of Higgs mediated lepton flavor violation. The experimental limit on the $\mu \to e\gamma$ decay is used to derive a bound on the branching ratio of the $\mu \to e\gamma \gamma$ transition, which is 6 orders of magnitude stronger than the current experimental limit. Previous results on the $\tau \to \mu \gamma$ and $\tau \to \mu \gamma \gamma$ decays are reproduced. The possibility of detecting signals of lepton flavor violation at $\gamma \gamma$ colliders is explored through the $\gamma \gamma \to l_il_j$ reaction, putting special emphasis on the $\tau \mu$ final state. Using the bound imposed on the $H\tau \mu$ vertex by the current experimental data on the muon anomalous magnetic moment, it is found that about half a hundred events may be produced in the International Linear Collider., Comment: 18 pages, 12 figures

The {tau}{sup -}{yields}({omega},{phi})P{sup -}{nu}{sub {tau}} decays, where P{sup -}={pi}{sup -} or K{sup -}, 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 {tau}{sup -}{yields}{phi}({pi}{sup -},K{sup -}){nu}{sub {tau}} branching fractions are in good agreement with recent measurements of the BABAR and BELLE Collaborations.

We evaluate the model-dependent piece of O({alpha}) long-distance radiative corrections to {tau}{sup -}{yields}{pi}{sup -}{pi}{sup 0}{nu}{sub {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.

We consider the bremsstrahlung and model-dependent contributions to the radiative decay $\tau^- \to \pi^-\pi^0\nu_{\tau}\gamma$ 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 $\rho^-(770)$ vector meson. Important numerical differences are found with respect to results obtained in the framework of chiral perturbation theory., Comment: 14 pages, 8 figures, submitted for publication

A comprehensive analysis of the lepton flavor violating (LFV) decays Z-->li lj is presented within the effective Lagrangian approach. Both the decoupling and nondecoupling scenarios are explored. The experimental constraints from li --> lj lk \bar{lk} and li -->lj gamma as well as some relationships arising from the gauge invariance of the effective Lagrangian are used to put constraints on Z-->li lj. It is found that while current experimental data impose very strong constraints on Z-->mu e, the channel Z --> tau mu (e)still may be at the reach of the planned TESLA collider., Comment: References added, final version to appear in Physical Review D