arXiv subjects

G. Tavares-Velasco

Publications and source records attributed to G. Tavares-Velasco.

65 records · Page 4Linked to original sources

arXiv2002

Update on very light CP-odd scalar in the Two-Higgs-Doublet Model

In a previous work we have shown that a general two-Higgs-doublet model (THDM) with a very light CP-odd scalar can be compatible with electroweak precision data, such as the rho parameter, BR(b --> s gamma), R_b, A_b, BR(Upsilon --> A gamma), BR(eta --> A gamma), and (g-2) of muon. Prompted by the recent significant change in the theoretical status of the latter observable, we comment on the consequences for this model and update the allowed parameter region. It is found that the presence of a very light scalar with a mass of 0.2 GeV is still compatible with the new theoretical prediction of the muon anomalous magnetic moment.

hep-ph↗

arXiv2002

Constraints on neutrino-photon interactions from rare Z decays

It is shown that the rare decays $Z \to ν\bar{nu} γ$ and $Z \to ν\barν γγ$ are useful to put model-independent bounds on neutrino-one-photon and neutrino-two-photon interactions. The results are then used to constrain the $τ$ neutrino magnetic moment and the double radiative decay $ν_j \to ν_i γγ$. It is found that the decay $Z \to ν\bar{nu} γ$ gives a more stringent bound on the $τ$ neutrino magnetic moment than that obtained from $Z \to ν\barν γγ$; the latter decay in turn gives limits on the neutrino-two-photon interaction that are less stringent than those obtained for a sterile neutrino $ν_s$ from the analysis of $ν_μN \to ν_s N$ conversion.

hep-ph↗

arXiv2002

Effective Lagrangian description of the lepton flavor violating decays Z-->li lj

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.

hep-ph↗

arXiv2001

Bilepton gauge boson contribution to the static electromagnetic properties of the W boson in the minimal 331 model

We present a complete calculation of the singly and doubly charged gauge bosons (bileptons) contribution to the static properties of the W boson in the framework of the minimal 331 model, which accommodates the bileptons in an SU_L(2) doublet. A nonlinear R_ξgauge is used and a slightly modified version of the Passarino-Veltman reduction scheme is employed as in this case the Gram determinant vanishes. It is found that the bilepton contribution is of the same order of magnitude as those arising from other weakly coupled renormalizable theories, like the two-Higgs doublet model and supersymmetry. The heavy mass limit is explored and the nontrivial decoupling properties of bileptons are discussed. Although there is a close resemblance with the contribution of an SU_L(2) fermion doublet, in the case of the bilepton doublet the decoupling theorem does remain valid. As a by-product, we present a detailed study of the trilinear and the quartic vertices involving the bileptons and the standard model gauge bosons.

hep-ph↗

arXiv2001

Very light CP-odd scalar in the Two-Higgs-Doublet Model

We show that a general two-Higgs-doublet model (THDM) with a very light CP-odd scalar (A) can be compatible with the rho parameter, Br(b --> sγ), R_b, A_b, (g-2) of muon, Br(Upsilon --> A gamma), and the direct search via the Yukawa process at LEP. For its mass around 0.2 GeV, the muon (g-2) and Br(Upsilon --> A γ) data require tan(beta) to be about 1. Consequently, A can behave like a fermiophobic CP-odd scalar and predominantly decay into a photon pair ("gamma gamma"), which registers in detectors of high energy collider experiments as a single photon signature when the momentum of A is large. We compute the partial decay width of Z --> A A A and the production rate of f \bar{f} --> Z A A --> Z +"gamma gamma", f^' {\bar f} --> W^{\pm} A A --> W^\pm + "gamma gamma" and f \bar f --> H^+ H^- --> W^+ W^- A A --> W^+ W^- + "gamma gamma" at high energy colliders such as LEP, Tevatron, LHC, and future Linear Colliders. Other production mechanisms of a light A, such as gg --> h --> AA --> "gamma gamma", are also discussed.

hep-ph↗

arXiv2001

Photon photon scattering in a 3-3-1 model

We analyze the effects of a doubly charged vector bilepton as well as exotic quarks with charge 5/3 e and -4/3 e on light by light scattering. We consider mass values in the range 0.3--1 TeV, which would be reached at the planned future linear colliders. It is found that such exotic particles, especially the doubly charged vector bilepton, give raise to remarkable deviations from the standard model cross section. The virtual effects arising from these particles would provide an indirect test to a particular model which is based on the SU_c(3) X SU_L(3) X U_x(1) gauge symmetry, known as the 331 model, where such particles are a natural prediction.

hep-ph↗

arXiv2000

Trilinear Neutral Gauge Boson Couplings in Effective Theories

We list all the lowest dimension effective operators inducing off-shell trilinear neutral gauge boson couplings Z-Z-Photon, Z-Photon-Photon, and ZZZ within the effective Lagrangian approach, both in the linear and nonlinear realizations of the SU(2)_{L} X U(1)_Y gauge symmetry. In the linear scenario we find that these couplings can be generated only by dimension eight operators necessarily including the Higgs boson field, whereas in the nonlinear case they are induced by dimension six operators. We consider the impact of these couplings on some precision measurements such as the magnetic and electric dipole moments of fermions, as well as the Z boson rare decay Z -> neutrino+antineutrino+ photon. If the underlying new physics is of a decoupling nature, it is not expected that trilinear neutral gauge boson couplings may affect considerably any of these observables. On the contrary, it is just in the nonlinear scenario where these couplings have the more promising prospects of being perceptible through high precision experiments.

hep-ph↗

arXiv1999

Signature of exotic particles in light by light scattering

We discuss the implications on light by light scattering of two kind of exotic particles: doubly charged scalar bosons and doubly charged fermions; the virtual effects of a nonstandard singly charged gauge boson are also examined. These particles, if their masses lie in the range 0.1--1.0 TeV, will have a clear signature in the future linear colliders. The present analysis has the advantage that it depends only on electromagnetic symmetry, so it is applicable to any model which predicts this class of particles. In particular, our results have interesting consequences on left-right models and their supersymmetric extension.

hep-ph↗

arXiv1999

Testing flavor-changing neutral currents in the rare decays t->cViVj

We discuss the Flavor-Changing Neutral Current (FCNC) decays of the top quark t -> c Vi Vj (Vi=gamma, Z, g) in the framework of the Standard Model (SM) and in a two-higgs doublet model (2HDM) with tree-level FCNC couplings. While in the SM the expected branching ratios are extremelly small, in the 2HDM they may be sizable, of order 10^(-5) - 10^(-5), and thus accesible at the CERN LHC. We conclude with the interesting observation that the FCNC decay modes may not be equally suppressed as their corresponding decays t ->c Vi in this 2HDM.

hep-ph↗

arXiv1999

The decay Z -> neutrino antineutrino photon in the Standard Model

A complete study of the one-loop induced decay Z -> neutrino antineutrino photon is presented within the framework of the Standard Model. The advantages of using a nonlinear gauge are stressed. We have found that the main contributions come from the electric dipole and the magnetic dipole transitions of the Z gauge boson and the neutrino, respectively. We obtain a branching ratio B=7.16E-10, which is about four orders of magnitude smaller than the bound recentely obtained by the L3 collaboration and thus it leaves open a window to search for new physics effects in single-photon decays of the Z boson.

hep-ph↗

arXiv1998

A Direct Constraint on Dimension-Eight Operators from Z-->neutrino antinetrino gamma

We study the radiative decay Z-->neutrino antineutrino gamma whitin an effective Lagrangian approach. Using the search for energetic single-photon events in the data collected by the L3 collaboration, we get direct bounds on the dimension-six and the dimension-eight operators associated with the tau-neutrino magnetic moment and the anomalous electromagnetic properties of the Z boson. As a by-product of our calculation, we reproduce the L3 result for the bound on the tau-neutrino magnetic moment.

hep-ph↗