About heavy neutrinos: Lepton-flavor violation in decays of charged leptons

The fundamental description of nature, beyond the Standard Model (SM), may include heavy neutrinos that mix and thus allow processes in which lepton flavor is not preserved. We investigate the impact of charged currents that couple heavy gauge bosons to heavy neutrinos and SM leptons on lepton-flavor-violating decays of SM leptons into three charged leptons, with no final-state neutrinos. We implement our expressions for the leading contributions to ${\rm Br}(l_\alpha\to l_\beta\,l_\sigma\,l_\sigma)$, which hold for either Dirac or Majorana neutrinos, to the trilepton decay $\mu\to3e$, of the muon, and so determine sets of masses of heavy neutrinos and the heavy gauge boson, within GeVs to few TeVs, that are consistent with the upper bounds provided by the SINDRUM Collaboration. We find, however, that constraints dictated by the upper bound on ${\rm Br}(\mu\to e\gamma)$, from the MEG Collaboration, are more stringent. We utilize such parameters to find that the contributions to tau decays are $\sim10^{-15}-10^{-13}$, well below bounds from $B$ factories. The mixing of heavy and SM charged bosons is also investigated. We find that current experimental data from MEG and SINDRUM would allow mixing angles as large as $\sim10^{-2}$, for a relatively light new charged boson, but the expected sensitivity of the Mu3e experiment would be capable of setting an upper bound on this angle as small as $\sim10^{-4}$ if the mass of this boson is within the range of few TeVs.Comment: Accepted for publication in Journal of Physics G, 26 pages, 12 figures, 4 tables, several new elements were added for this version of the manuscript, the whole discussion was improved as wel

γγ→π0π0\gamma \gamma\to \pi^0 \pi^0 as a test for the existence of a light σ\sigma meson

We work out predictions of the Linear Sigma Model for the $\gamma \gamma \to\pi^0 \pi^0$ cross section. We consider the sigma width, which is introduced in a consistent way with chiral Ward identities. The results of Chiral Perturbation Theory are recovered in the $m_\sigma \to \infty$ limit. A fit to existing experimental results is consistent with a light and broad $\sigma$ meson.Comment: 7 pages, 3 figures. The sigma width is introduced in a consistent way with chiral symmetry. Results for the cross section are affected by the corresponding change

Implications of Lorentz violation on Higgs-mediated lepton flavor violation

The lepton flavor violating decay of the Higgs boson $H\to l_Al_B$ is studied within two qualitatively different extensions of the Yukawa sector: one renormalizable and the other nonrenormalizable; both incorporating Lorentz violation in a model-independent fashion. These extensions are characterized by Yukawa-like matrices, the former by a constant Lorentz 2-tensor $Y^{AB}_{\mu \nu}$, whereas the latter by a constant Lorentz vector $Y^{AB}_\mu$. It is found that the experimental constraints on the decays $l_A\to l_B\gamma$ severely restrict lepton flavor violating Higgs signals in the renormalizable scenario. In this context, it is found that $BR(H\to \mu^\pm e^\mp)$ and $BR(H\to \tau^\pm \mu^\mp)$ cannot be larger than $10^{-18}$ and $10^{-11}$, respectively. In the nonrenormalizable scenario, transitions mediated by the Higgs or the $Z$ gauge boson are induced at tree level, and we find mild restrictions on lepton flavor violation. Using the experimental limits on the three-body decays $l_A \to l_B \bar{l}_Cl_C$ to constraint the vector $Y^{AB}_\mu$, it is found that the branching ratio for the decays $H\to \mu^\pm e^\mp$ is of about $4\times 10^{-9}$, more important, a branching ratio of $7\times 10^{-4}$ is found for the $\tau^\pm \mu^\mp$ mode. Accordingly, the decay $H \to \tau^\pm \mu^\mp$ could be at the reach of future measurements. The lepton flavor violating decays of the $Z$ gauge boson were also studied. In the renormalizable scenario, it was found the undetectable branching ratios $BR(Z\to \mu^\pm e^\mp)<5.7\times 10^{-21}$ and $BR(Z\to \tau^\pm \mu^\mp)<2.0\times 10^{-12}$. In the nonrenormalizable scenario, it was found that $BR(Z\to \mu^\pm e^\mp)<0.67\times 10^{-12}$ and $BR(Z\to \tau^\pm \mu^\mp)<1.12\times 10^{-7}$. Although the latter branching ratio is relatively large, it still could not be within the range of future measurements.Comment: Updated to essentially match published versio

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.Comment: 21 pages, 2 figures, RevTex formatte

Decay H^+ -> W^+ gamma in a nonlinear R_xi-gauge

A new evaluation of the charged Higgs boson decay $H^+ -> W^+\gamma$ is presented in the context of the general two-Higgs doublet model. A nonlinear $R_\xi$-gauge which considerably simplifies the calculation is introduced and simple expressions are obtained for the fermionic and bosonic contributions. The $H^+ -> W^+\gamma$ branching ratio is analyzed for several values of the parameters of the model. Although this decay can have a branching fraction as large as $10^{-4}$ in a certain region of the parameter space, it is found that such a region is disfavored by the most recent constraints on $b -> s\gamma$, $g-2$ of the muon, $Z -> b \bar b$, and the $\rho$ parameter, along with the exclusions from direct searches at the CERN $e^-e^+$ LEP collider. The possibility of detecting this decay at future colliders is discussed.Comment: 12 pages, 9 figure

The anapole moment in scalar quantum electrodynamics

The anapole moment of a charged scalar particle is studied in a model independent fashion, using the effective Lagrangian technique, as well as radiatively within the context of scalar quantum electrodynamics (SQED). It is shown that this gauge structure is characterized by a non renormalizable interaction, which is radiatively generated at the one--loop. It is found that the resulting anapole moment for off-shell particles, though free of ultraviolet divergences, is gauge dependent and thus it is not a physical observable. We also study some of its kinematical limits. In particular, it is shown that its value comes out to be zero when all particles are on--shell.Comment: 4 pages, 1 figur

Effective Lagrangian approach to Higgs-mediated FCNC top quark decays

The flavor changing neutral current (FCNC) transitions t --> q'H and t --> q'V_i (V_i=\gamma, 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) X U_Y(1)-invariant operators, which generate the most general CP-even and CP-odd tq'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'V_i decays are enhanced by two or three 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 LHC.Comment: 11 pages, 6 figures, submitted to Phys. Rev.

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.Comment: RevTeX, 2 epsi figures, 10 pgs. Comments and references added. Submitted to Physical Review

The Standard Model with one universal extra dimension

Effects of universal extra dimensions on Standard Model observables first arise at the one-loop level. The quantization of this class of theories is therefore essential in order to perform predictions. A comprehensive study of the Standard Model defined in a space-time manifold with one universal extra dimension, compactified on the oribifold $S^1/Z_2$, is presented. The fact that the four-dimensional Kaluza-Klein theory is subject to two types of gauge transformations is stressed and its quantization under the basis of the BRST symmetry discussed. A $SU_C(3) x SU_L(2) x U_Y(1)$-covariant gauge-fixing procedure for the Kaluza-Klein excitations is introduced. The connection between gauge and mass eigenstates fields is established in an exact way. An exhaustive list of the explicit expressions for all physical couplings induced by the Yang-Mills, Currents, Higgs, and Yukawa sectors is presented. The one-loop renormalizability of the standard Green's functions, which implies that the Standard Model observables do not depend on a cutoff scale, is stressed.Comment: 51 page

Hidden symmetries induced by a canonical transformation and gauge structure of compactified Yang-Mills theories

Compactified Yang-Mills theories with one universal extra dimension were found [arXiv:1008.4638] to exhibit two types of gauge invariances: the standard gauge transformations (SGTs) and the nonstandard gauge transformations (NSGTs). In the present work we show that these transformations are not exclusive to compactified scenarios. Introducing a notion of hidden symmetry, based on the fundamental concept of canonical transformation, we analyse three different gauge systems, each of which is mapped to a certain effective theory that is invariant under the so-called SGTs and NSGTs. The systems under discussion are: (i) four dimensional pure $SU(3)$ Yang-Mills theory, (ii) four dimensional $SU(3)$ Yang-Mills with spontaneous symmetry breaking, and (iii) pure Yang-Mills theory with one universal compact extra dimension. The canonical transformation, that induces the notion of hidden symmetry, maps objects with well defined transformation laws under a gauge group $G$ to well defined objects under a non-trivial subgroup $H \subset G$. In the case where spontaneous symmetry breaking is present, the set of SGTs corresponds to the group into which the original gauge group is broken into, whereas the NSGTs are associated to the broken generators and can be used to define the unitary gauge. For the system (iii), the SGTs coincide with the gauge group $SU(N,\cal{M}^{4})$, whereas the NSGTs do not form a group; in this system the 'fundamental' theory and the effective one are shown to be classically equivalent.Comment: 26 pages, title has changed, some comments and clarifications added, some references adde