arXiv subjects

J. J. Toscano

Publications and source records attributed to J. J. Toscano.

At least 37 records · Page 2Linked to original sources

arXiv2015

Decays $Z\toγγ$ and $Z\to gg$ in the Standard Model Extension

The $Z\to γγ$ and $Z\to gg$ decays are studied in the context of the renormalizable version of the Standard Model Extension. The $CPT$-odd $\barψγ_5 γ^μb_μψ$ bilinear interaction, which involves the constant background field $b_α$ and which has been a subject of interest in literature, is considered. It is shown that the $Z\to γγ$ and $Z\to gg$ decays, which are strictly zero in the standard model, can be generated radiatively at the one-loop level. It is found that these decays are gauge invariant and free of ultraviolet divergences, and that the corresponding decay widths only depend on the spatial component of the background field $b$.

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arXiv2015

Revisiting the flavor changing neutral current Higgs decays $H\to q_i q_j$ in the Standard Model

An exact calculation of the Higgs boson decays $H\to q_i q_j$ mediated by flavor changing neutral currents is presented in the context of the Standard Model. Using up-to-date experimental data, branching ratios of the order of $10^{-7}$, $10^{-8}$, $10^{-8}$, and $10^{-15}$ are found for the $\bar{b}s+\bar{s}b$, $\bar{b}d+\bar{d}b$, $\bar{s}d+\bar{d}s$, and $\bar{c}u+\bar{u}c$ decay modes, respectively.

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arXiv2015

CP-odd contributions to the $ZZ^\astγ$, $ZZγ^\ast$, and $ZZZ^\ast$ vertices induced by nondiagonal charged scalar boson couplings

In models with extended scalar sectors with several Higgs multiplets, such as Higgs triplet models, the $Z$ gauge boson can have nondiagonal couplings to charged Higgs bosons. In a model-independent way, we study the potential contributions arising from such theories to the CP-violating trilinear neutral gauge boson couplings $ZZ^*γ$, $ZZγ^*$, and $ZZZ^*$, which are parametrized by four form factors: $h^Z_{1,2}$, $f_4^γ$, and $f_4^Z$, respectively. Such form factors can only be induced if there are at least two nondegenerate charged Higgs scalars and an imaginary phase in the coupling constants. For the masses of the charged scalar bosons we consider values above 300 GeV and find that the form factors can reach the following orders of magnitude: $|h_1^Z|\sim 10^{-5}-10^{-4}$, $|h_2^Z|\sim 10^{-7}-10^{-6}$, $|f_4^γ|\sim 10^{-5}-10^{-3}$ and $|f_4^Z|\sim 10^{-6}-10^{-5}$, though there could be an additional suppression factor arising from the coupling constants. We also find that the form factors decouple at high energies and are not very sensitive to a change in the masses of the charged scalar bosons. Apart from a proportionality factor, our results for the $f_4^Z$ form factor, associated with the $ZZZ^*$ vertex, is of the same order of magnitude than that induced via nondiagonal neutral scalar boson couplings in the framework of a two-Higgs doublet model.

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arXiv2015

Yang-Mills theories with an arbitrary number of compactified extra dimensions

The well-known Yang-Mills theory with one $ S^{1} / Z_{2}$ universal extra dimension (UED) is generalized to an arbitrary number of spatial extra dimensions through a novel compactification scheme. In this paper, the Riemannian flat based manifold under consideration contains $ n $ spatial extra dimensions defined by $ n $ copies of the orbifold $ S^{1} / Z_{2}$. In this approach, we present the gauge structure and the mass spectrum of the effective four-dimensional theory. We introduce the concept of standard and nonstandard gauge transformations of the effective theory, and explicitly identify the emergence of massive vector fields in the same number as massless ('pseudo-Goldstone') scalars in the compactified theory, verifying that a Higgs-like mechanism operates in the compactification process. It is found that, in contrast with the one UED scenario, in cases with two or more UEDs there emerge massive scalar fields. Besides, at phase-space level, the Hamiltonian analysis yields that the higher-dimensional and compactified theories are classically equivalent using the fundamental concept of canonical transformation. This work lays the ground for a wider study on these theories concerning their quantization and predictive power at the level of quantum fluctuations.

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arXiv2014

Implications of a nonvanishing $Z γγ$ vertex on the $H\to γγγ$ decay

The $Z\to γγ$ and $H\to γγγ$ decays are strictly forbidden in the Standard Model, but they can be induced by theories that violate Lorentz symmetry or the CPT theorem. By assuming that a nonvanishing $Zγγ$ vertex is induced in some context of new physics, and by analyzing the reaction $H\to γZ^*\to γγγ$ in the $Z$ resonance, we obtain an estimation for the branching ratio of the $H\to γγγ$ decay. Specifically, it is found that $BR(H\to γγγ)\lesssim BR(H\to γZ)BR_{Exp}(Z\to γγ)$, which is of the order of $10^{-7}$.

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arXiv2014

One-loop nonbirefringent effects on the electromagnetic vertex in the Standard Model Extension

Lorentz violation emerged from a fundamental description of nature may impact, at low energies, the Maxwell sector, so that contributions from such new physics to the electromagnetic vertex would be induced. Particularly, nonbirefringent $CPT$-even effects from the electromagnetic sector modified by the Lorentz- and $CPT$-violating Standard Model Extension alter the structure of the free photon propagator. We calculate Lorentz-violating contributions to the electromagnetic vertex, at the one-loop level, by using a modified photon propagator carrying this sort of effects. We take the photon off shell, and find an expression that involves both isotropic and anisotropic effects of nonbirefringent violation of Lorentz invariance. Our analysis of the one-loop vertex function includes gauge invariance, transformation properties under $C$, $P$, and $T$, and tree-level contributions from Lorentz-violating nonrenormalizable interactions. These elements add to previous studies of the one-loop contributions to the electromagnetic vertex in the context of Lorentz violation in the photon sector. Finally, we restrict our analysis to the isotropic case and derive a finite contribution from isotropic Lorentz violation to the anomalous magnetic moment of fermions that coincides with the result already reported in the literature.

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arXiv2014

Gauge invariant electromagnetic properties of fermions induced by CPT violation in the Standard Model Extension

Low-energy Lorentz-invariant quantities could receive contributions from a fundamental theory producing small Lorentz-violating effects. Within the Lorentz-violating extension of quantum electrodynamics, we investigate, perturbatively, the contributions to the one-loop $ffγ$ vertex from the $CPT$-violating axial coupling of a vector background field to fermions. We find that the resulting vertex function has a larger set of Lorentz structures than the one characterizing the usual, Lorentz invariant, parametrization of the $ffγ$ vertex. We prove gauge invariance of the resulting one-loop expression through a set of gauge invariant nonrenormalizable operators introducing new-physics effects at the first and second orders in Lorentz violation, and which generate tree-level contributions to the $ffγ$ vertex. Whereas loop contributions involving parameters that violate Lorentz invariance at the first order are $CPT$-odd, those arising at the second order are $CPT$-even, so that contributions to low-energy physics are restricted to emerge for the first time at the second order. In this context, we derive a contribution to anomalous magnetic moment of fermions, which we use to set a bound on Lorentz violation.

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arXiv2013

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.

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arXiv2013

Trilinear gauge boson couplings in the standard model with one universal extra dimension

One-loop effects of Standard Model (SM) extensions comprising universal extra dimensions are essential as a consequence of Kaluza-Klein (KK) parity conservation, for they represent the very first presumable virtual effects on low-energy observables. In this paper, we calculate the one-loop CP-even contributions to the SM WWgamma and WWZ gauge couplings produced by the KK excited modes that stand for the dynamical variables of the effective theory emerged from a generalization of the SM to five dimensions, in which the extra dimension is assumed to be universal, after compactification. The employment of a covariant gauge-fixing procedure that removes gauge invariance associated to gauge KK excited modes, while keeping electroweak gauge symmetry manifest, is a main feature of this calculation, which is performed in the Feynman 't Hooft gauge and yields finite results that consistently decouple for a large compactification scale. After numerical evaluation, our results show to be comparable with the one-loop SM contributions and well within the reach of a next linear collider.

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arXiv2013

Lorentz-violating effects on pair production of W bosons in photon collisions

We examine Lorentz-violating effects that could appear through deviations of the Standard Model gauge couplings WWgamma and WWgammagamma. These new physics effects are explored on the gamma gamma -> WW reaction at the International Linear Collider. In particular, the associated helicity amplitudes are computed in the context of the Standard Model Extension (which is a model-independent approach to CPT and Lorentz violation) and the Effective Lagrangian Model (which incorporates new physics effects that respect CPT and Lorentz violation). We perform an exhaustive study of the polarized differential cross sections to stand out effects related to Lorentz symmetry violation, where it is evidenced that the effects of Lorentz symmetry violation are more sensitive to the presence of the b constant background field. We found that for the $(\pm,\pm,(L,T+T,L))$ polarization state, only Standard Model Extension and Effective Lagrangian Model contribute at the lowest order, however, both types of new physics effects are clearly distinguished, being dominant the convoluted cross section of the Standard Model Extension in around 4 orders of magnitude. For this polarization state, at the last stage of operation of International Linear Collider, it is expected an integrated luminosity of 10^3 fb^{-1}, finding around of 2 events for a Lorentz-violating energy scale of 32 TeV.

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arXiv2013

Effects of Lorentz violation through the $γe\to Wν_e$ process in the Standard Model Extension

Physics beyond the Fermi scale could show up through deviations of the gauge couplings predicted by the electroweak Yang-Mills sector. This possibility is explored in the context of the International Linear Collider (ILC) through the helicity amplitudes for the gamma e -> W nu_e reaction to which contributes the trilinear WWgamma coupling. The new physics effects on this vertex are parametrized in a model-independent fashion through an effective electroweak Yang-Mills sector, which is constructed by considering two essentially different sources of new physics. In one scenario, Lorentz violation will be considered exclusively as the source of new physics effects. This type of new physics is considered in an extension of the Standard Model that is known as the Standard Model Extension (SME), which is an effective field theory that contemplates CPT and Lorentz violation in a model-independent fashion. Any source of new physics that respects the Lorentz symmetry, will be considered within the general context of the well known Conventional Effective Standard Model (CESM) extension. Both the SME and the CESM descriptions include gauge invariant operators of dimension higher than four, which, in general, transform as Lorentz tensors of rank higher than zero. Whereas in the former theory observer Lorentz invariants are constructed by contracting these operators with constant Lorentz tensors, in the latter the corresponding Lorentz invariant interactions are obtained contracting such operators with products of the metric tensor. We focus our study on the possibility of experimentally distinguish both types of new physics effects on the WWgamma vertex. It is found that for a new physics scale of the same order of magnitude and under determined circumstances, both types of new physics effects will be clearly distinguished.

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arXiv2012

Study of the lepton flavor-violating $Z'\toτμ$ decay

The lepton flavor violating $Z^{\prime}\toτμ$ decay is studied in the context of several extended models that predict the existence of the new gauge boson named $Z^\prime$. A calculation of the strength of the lepton flavor violating $Z^\primeμτ$ coupling is presented by using the most general renormalizable Lagrangian that includes lepton flavor violation. We used the experimental value of the muon magnetic dipole moment to bound this coupling, from which the $\mathrm{Re}(Ω_{Lμτ}Ω^\ast_{Rμτ})$ parameter is constrained and it is found that $\mathrm{Re}(Ω_{Lμτ}Ω^\ast_{Rμτ})\sim 10^{-2}$ for a $Z^\prime$ boson mass of 2 TeV. Alongside, we employed the experimental restrictions over the $τ\toμγ$ and $τ\toμμ^+μ^-$ processes in the context of several models that predict the existence of the $Z^\prime$ gauge boson to bound the mentioned coupling. The most restrictive bounds come from the calculation of the three-body decay. For this case, it was found that the most restrictive result is provided by a vector-like coupling, denoted as $|Ω_{μτ}|^2$, for the $Z_χ$ case, finding around $10^{-2}$ for a $Z^\prime$ boson mass of 2 TeV. We used this information to estimate the branching ratio for the $Z^\prime\toτμ$ decay. According to the analyzed models the least optimistic result is provided by the Sequential $Z$ model, which is of the order of $10^{-2}$ for a $Z^\prime$ boson mass around 2 TeV.

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arXiv2012

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.

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arXiv2012

Decays Z' -> γγγ and Z -> γγγ in the minimal 331 model

The possibility of a significant effect of exotic particles on the Z'->γγγ and Z->γγγ decays is investigated in the context of the minimal 331 model. This model, which is based in the SU_C(3)xSU_L(3)xU_X(1) gauge group, predicts the existence of many exotic charged particles that can significantly enhance the decay widths. It is found that the standard model prediction for the Z->γγγ decay remains essentially unchanged, as the new physics effects quickly decouples. On the other hand, it is found that the contributions of the new exotic quarks and gauge bosons predicted by this model lead to a branching fraction for the Z'->γγγ decay of about 10^(-6), which is about three orders of magnitude larger than that of the Z->γγγ decay.

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arXiv2011

The γγ\to ϕ_iϕ_j processes in the type-III two-Higgs-doublet model

We discuss the implications of assuming a four-zero Yukawa texture and a general Higgs potential for the production of neutral Higgs boson pairs at γγcolliders through the γγ\to ϕ_iϕ_j (ϕ_i = h, H, A) reaction within the context of the two Higgs doublet model type III. Exact analytical expressions for the γγ\to ϕ_iϕ_j reaction are presented. The use of a nonlinear R_ξ-gauge, which considerably simplifies the loop calculations and renders compact analytical expressions, is stressed. We show that these processes are very sensitive to a general structure of the Higgs potential that impact the triple and quartic couplings of the scalar sector. We present results for scenarios of the parameters of the model that are still consistent with current experimental constraints. It is found that the cross sections for the γγ\to ϕ_iϕ_j processes can be up to two orders of magnitude larger than those gotten in 2HDM type I and type II. The possibility of a light CP--scalar is also studied.

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arXiv2011

Integration of Kaluza-Klein modes in Yang-Mills theories

A five dimensional pure Yang--Mills theory, with the fifth coordinate compactified on the orbifold $S^1/Z_2$ of radius R, leads to a four dimensional theory which is governed by two types of local gauge transformations, namely, the well known standard gauge transformations (SGT) dictated by the $SU_4(N)$ group under which the zero Fourier modes transform as gauge fields, and a set of nonstandard gauge transformations (NSGT) determining the gauge nature of the Kaluza--Klein (KK) excitations. By using a SGT-covariant gauge-fixing procedure for removing the degeneration associated with the NSGT, we integrate out the KK excitations and obtain a low-energy effective Lagrangian expansion involving all of the independent canonical-dimension-six operators that are invariant under the SGT of the $SU_4(N)$ group and that are constituted by light gauge fields (zero modes), exclusively. It is shown that this effective Lagrangian is invariant under the SGT, but it depends on the gauge-fixing of the gauge KK excitations. Our result shows explicitly that the one-loop contributions of the KK excitations to light (standard) Green's functions are renormalizable.

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arXiv2011

Z'tc coupling from D0-D0 mixing

We bound the $Z^\prime tc$ coupling using the $D^{0}-\bar{D^{0}}$ meson mixing system. We obtained such coupling which is less than $5.75\times 10^{-2}$. We have studied the $Z^\prime$ boson resonance considering single top production in the $e^+e^-\to Z^\prime\to tc$ process. We obtained the number of events which is expected to be less than $10^7$ at the International Linear Collider scenario. We get a branching ratio of the order of $10^{-2}$ for the $Z^\prime\to tc$ decay.

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arXiv2011

About gauge invariance in compactified extra dimensions

Gauge theories formulated in a space-time manifold that includes compact extra dimensions can show a nontrivial gauge structure. Depending on whether the gauge parameters propagate or not in the extra dimensions, two different Kaluza--Klein theories can arise when the extra dimensions are compactified. A comparison between these two possibilities, in the context of a five dimensional theory, is presented from both the theoretical and phenomenological viewpoints. The phenomenological implications of these theories are contrasted by discussing the one--loop decay of the Higgs boson into two photons. It is shown that the amplitude for this decay differs substantially from one approach to the other and that such a difference is intimately related to gauge invariance.

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