We show that a general two-Higgs-doublet model (THDM) with a very light CP-odd scalar (A) can be compatible with ρ parameter, Br(b → sγ), R b , A b , (g − 2)µ of muon, Br(Υ → Aγ), and the direct search via the Yukawa process at LEP. For its mass around 0.2 GeV, the muon (g − 2)µ and Br(Υ → Aγ) data require tan β to be about 1. Consequently, A can behave like a fermiophobic CP-odd scalar and predominantly decay into a γγ pair, 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 → AAA and the production rate of ff → ZAA → Z + "γγ",γγ" 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 → "γγ", are also discussed.

The possibility of nonnegligible W electric dipole ( W ) and magnetic quadrupole (Q W ) moments induced by the most general HWW vertex is examined via the effective Lagrangian technique. It is assumed that new heavy fermions induce an anomalous CP-odd component of the HWW vertex, which can be parametrized by an SU L 2 U Y 1-invariant dimension-six operator. This anomalous contribution, when combined with the standard model CP-even contribution, leads to CP-odd electromagnetic properties of the W boson, which are characterized by the form factors and Q. It is found that is divergent, whereas Q is finite, which reflects the fact that the latter cannot be generated at the one-loop level in any renormalizable theory. Assuming reasonable values for the unknown parameters, we found that W 3 ÿ 6 10 ÿ21 e cm, which is 8 orders of magnitude larger than the SM prediction and close to the upper bound derived from the neutron electric dipole moment. The estimated size of the somewhat less-studiedQ W moment is of the order of ÿ10 ÿ36 e cm 2 , which is 15 orders of magnitude above the SM contribution.

A new evaluation of the charged Higgs boson decay H ϩ →W ϩ ␥ is presented in the context of the general two-Higgs doublet model. A nonlinear R gauge which considerably simplifies the calculation is introduced and simple expressions are obtained for the fermionic and bosonic contributions. The H ϩ →W ϩ ␥ branching ratio is analyzed for several values of the parameters of the model. Although this decay can have a branching fraction as large as 10 Ϫ4 in a certain region of the parameter space, it is found that such a region is disfavored by the most recent constraints on b→s␥, gϪ2 of the muon, Z→bb , and the parameter, along with the exclusions from direct searches at the CERN e Ϫ e ϩ LEP collider. The possibility of detecting this decay at future colliders is discussed.

The flavor changing neutral current (FCNC) transitions t ! q 0 H and t ! q 0 V i V i ; g; Z are studied in the context of the effective Lagrangian approach. We focus on the scenario in which these decays are predominantly induced by new physics effects arising from the Yukawa sector extended with dimension-six SU L 2 U Y 1-invariant operators, which generate the most general CP-even and CP-odd tq 0 H vertex at the tree level. For the unknown coefficients, we assume a slightly modified version of the Cheng-Sher ansatz. We found that the branching ratio for the Higgs-mediated FCNC t ! q 0 V i decays are enhanced by two or 3 orders of magnitude with respect to the results expected in models with extended Higgs sectors, such as the general two-Higgs doublet model. We discuss the possibilities of detecting this class of decays at the large hadron collider (LHC).

It is shown that the rare decays Z → ννγ and Z → ννγγ are useful to put modelindependent bounds on neutrino-one-photon and neutrino-two-photon interactions. The results are then used to constrain the τ neutrino magnetic moment µ ντ and the double radiative decay ν j → ν i γγ. It is found that the decay Z → ννγ gives a more stringent bound on µ ντ than that obtained from Z → ννγγ; 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 µ N → ν s N conversion.

We list all the lowest dimension effective operators inducing off-shell trilinear neutral gauge boson couplings ZZ␥, Z␥␥, and ZZZ within the effective Lagrangian approach, both in the linear and nonlinear realizations of SU(2) L ϫ U(1) Y gauge symmetry. In the linear scenario we find that these couplings can be generated only by dimension-8 operators necessarily including the Higgs boson field, whereas in the nonlinear case they are induced by dimension-6 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→ ␥. 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.1 Throughout this work we consider the general case of off-shell bosons, unless stated otherwise, but they will be denoted by V rather than V*.

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 SUL(2) doublet. A nonlinear R ξ gauge 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 SUL(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.

Heavy quarks, namely, the top and bottom quarks, may show great sensitiveness to new physics effects. In particular, they might have unusually large electric dipole moments. This possibility is analyzed via the corresponding one-loop correction to the neutron electric dipole moment, dn. The current experimental limit on dn is used then to derive the uppers bounds |dt| < 3.06 × 10 −15 e-cm, |d b | < 1.22 × 10 −13 e-cm.