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J. J. Toscano

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

At least 73 records · Page 4Linked to original sources
arXiv2007

Bounding the top and bottom electric dipole moments from neutron experimental data

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, $d_n$. The current experimental limit on $d_n$ is used then to derive the uppers bounds $|d_t|<3.06\times 10^{-15}$ e-cm, $|d_b|<1.22\times 10^{-13}$ e-cm.

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arXiv2007

Bound on the anomalous tbW coupling from two-loop contribution to neutron electric dipole moment

The two-loop contribution to the electric dipole moment (EDM) and the chromo electric dipole moment (CEDM) of an arbitrary fermion f induced by the most general renormalizable tbW coupling with complex left- and right-handed components (a_L and a_R) is calculated. The analytical expressions are numerically evaluated and the current experimental constraints on the electron, neutron and mercury atom EDMs are used to obtain a bound on the complex phase Im(a^*_La_R). It is found that the most stringent constraint, Im(a^*_La_R)<2.33 X 10^{-2}, arises from the neutron EDM.

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arXiv2007

CP-odd static electromagnetic properties of the W gauge boson and the t quark via the anomalous tbW coupling

In the framework of the electroweak chiral Lagrangian, the one-loop induced effects of the anomalous $tbW$ coupling, which includes both left- and right-handed complex components, on the static electromagnetic properties of the $W$ boson and the $t$ quark are studied. The attention is focused mainly on the CP-violating electromagnetic properties. It is found that the $tbW$ anomalous coupling can induce both CP-violating moments of the $W$ boson, namely, its electric dipole ($\tildeμ_W$) and magnetic quadrupole ($\tilde{Q}_W$) moments. As far as the $t$ quark is concerned, a potentially large electric dipole moment $(d_t)$ can arise due to the anomalous $tbW$ coupling. The most recent bounds on the left- and right-handed parameters from $B$ meson physics lead to the following estimates $\tildeμ_W ~ 10^{-23}-10^{-22}$ e-cm and $\tilde{Q}_W~ 10^{-38}-10^{-37}$ e-cm$^2$, which are 7 and 14 orders of magnitude larger than the standard model (SM) predictions, whereas $d_t$ may be as large as $10^{-22}$ e-cm, which is about 8 orders of magnitude larger than its SM counterpart.

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arXiv2007

Bilepton effects on the WWV^* vertex in the 331 model with right-handed neutrinos via a SU_L(2)XU_Y(1) covariant quantization scheme

In a recent paper, we investigated the effects of the massive charged gauge bosons (bileptons) predicted by the minimal 331 model on the off-shell vertex WWV^* (V=gamma, Z) using a SU_L(2) X U_Y(1) covariant gauge-fixing term for the bileptons. We proceed along the same lines and calculate the effects of the gauge bosons predicted by the 331 model with right-handed neutrinos. It is found that the bilepton effects on the WWV^* vertex are of the same order of magnitude than those arising from the SM and several of its extensions, provided that the bilepton mass is of the order of a few hundred of GeVs. For heavier bileptons, their effects on the WWV^* vertex are negligible. The behavior of the form factors at high energies is also discussed as it is a reflect of the gauge invariance and gauge independence of the WWV^* Green function obtained via our quantization method.

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arXiv2006

Production of doubly charged vector bilepton pairs at $γγ$ colliders

The production of pairs of doubly charged vector bileptons is studied at future $γγ$ colliders. The unpolarized cross--section for the $γγ\to Y^{--}Y^{++}$ subprocess is analytically calculated and convoluted to predict the number of events in the complete $e^+e^-\to γγ\to Y^{--}Y^{++}$ process. The gauge or non--gauge character of the vector bilepton $Y^{\pm \pm}$ is discussed. It is found that as a consequence of its spectacular signature, as it decays dominantly into two identical charged leptons, and also due to its charge contents, which significantly enhance the cross--section, the detection of this class of particles with mass in the sub--TeV region can be at the reach of these colliders. The model--independent nature of our results is stressed.

hep-ph↗
arXiv2006

Rare top quark decay t --> u_1 \bar{u}_2 u_2 in the standard model

The one-loop induced top quark decay $t\to u_1\bar{u}_2u_2$ ($u_i=u,c$) is calculated in the context of the standard model. The dominant contribution to this top quark decay arises from the Feynman diagrams induced by the off-shell $tu_1g^*$ vertex, whereas the box diagrams are negligibly small. In contrast with the on-shell $tu_1g$ vertex, which only gives rise to a pure dipolar effect, the off-shell $tu_1g^*$ coupling also involves a monopolar term, which gives a larger contribution than the dipolar one. It is found that the branching ratio for the three-body decay $t\to u_1\bar{u}_2u_2$ is about of the same order of magnitude of the two-body decay $t\to u_1 g$, the latter being slightly larger, which stems from the fact that the three-body decay is dominated by a monopolar term.

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arXiv2006

Rare three-body decay t -> c h gamma in the standard model and the two-Higgs doublet model

A complete calculation of the rare three-body decay t --> c h gamma is presented in the framework of the standard model. In the unitary gauge, such a calculation involves about 20 Feynman diagrams. We also calculate this decay in the general two-Higgs doublet model (model III), in which it arises at the tree-level. While in the standard model the decay t --> c h gamma is extremely suppressed, with a branching fraction of the order of $10^{-15}$ for a Higgs boson mass of the order of 115 GeV, in the model III it may have a branching ratio up to $10^{-5}$. We also discuss the crossed decay h --> b \bar{s} gamma.

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arXiv2005

Implications of a very light pseudoscalar boson on lepton flavor violation

A long-lived very light pseudoscalar boson would favor lepton flavor violating transitions of charged leptons. Its implications on the l_i --> l_j γγ, and l_i --> l_j e^+ e^- transitions are investigated. Assuming 2 m_e < m_ϕ l_j γγ) l_j e^+ e^-) is hold. The experimental constraints on the decays l_i --> l_j γ, l_i --> l_j l_k l_k, and l_i --> l_j γγare used to bound the ϕl_i l_j couplings.

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arXiv2005

Electric dipole and magnetic quadrupole moments of the $W$ boson via a CP-violating $HWW$ vertex in effective Lagrangians

The possibility of nonnegligible $W$ electric dipole ($\widetildeμ_W$) and magnetic quadrupole ($\widetilde{Q}_W$) 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 $SU_L(2)\times U_Y(1)$-invariant dimension-six operator. This anomalous contribution, when combined with the standard model CP-even contribution, lead to CP-odd electromagnetic properties of the $W$ boson, which are characterized by the form factors $Δ\widetildeκ$ and $Δ\widetilde{Q}$. It is found that $Δ\widetildeκ$ is divergent, whereas $Δ\widetilde{Q}$ 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 $\widetildeμ_W\sim 3-6\times 10^{-21}$ e-cm, which is eight 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-studied $\widetilde{Q}_W$ moment is of the order of $-10^{-36}$ e-cm^2, which is fifteen orders of magnitude above the SM contribution.

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arXiv2005

SU_L(2) X U_Y(1)-invariant description of the bilepton contribution to the WWV vertex in the minimal 331 model

We study the one-loop sensitivity of the WWV (V=gamma, Z) vertex to the new massive gauge bosons predicted by the minimal SU_L(3) X U_X(1) model, which have unusual couplings to the standard model (SM) gauge bosons. A gauge-fixing procedure covariant under the SU_L(2) X U_Y(1) group was introduced for these new gauge bosons (dubbed bileptons) in order to generate gauge-invariant Green functions. The similarities between this procedure and the nonconventional quantization scheme of the background field method are discussed. It is found that, for relatively light bileptons, with a mass ranging from 2m_W to 6m_W, the radiative corrections to the form factors associated with the WWV vertex can be of the same order of magnitude than the SM one. In the case of heavier bileptons, their contribution is smaller by about one and two orders of magnitude than their SM counterpart.

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arXiv2005

Effects of an extra $Z'$ gauge boson on the top quark decay $t --> c γ$

The effects of an extra $Z'$ gauge boson with family nonuniversal fermion couplings on the rare top quark decay $t --> c $gamma$ are first examined in a model independent way and then in the minimal 331 model. It is found that the respective branching fraction is at most of the order of $10^{-8}$ for $m_{Z'}=500$ GeV and dramatically decreases for a heavier $Z'$ boson. This results is in sharp contrast with a previous evaluation of this decay in the context of topcolor assisted technicolor models, which found that $B(t --> c γ)\sim 10^{-6}$ for $m_{Z'}=1$ TeV.

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arXiv2004

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=γ, 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.

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arXiv2004

Static quantities of a neutral bilepton in the 331 model with right-handed neutrinos

A neutral vector boson can possess static electromagnetic properties provided that the associated field is no self-conjugate. This possibility is explored in the $SU_C(3) X SU_L(3) X U_N(1)$ model with right-handed neutrinos, which predicts a complex neutral gauge boson $Y^0$ in a nontrivial representation of the electroweak group. In this model the only nonvanishing form factors are the CP-even ones, which arise from both the quark and gauge sectors, and contribute to the magnetic dipole and the electric quadrupole moments of this neutral particle.

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arXiv2004

Fermion contribution to the static quantities of arbitrarily charged vector bosons

We present an analysis of the one-loop contribution from left- and right-handed fermions to the static electromagnetic properties of an arbitrarily charged no self-conjugate vector boson $V$. Particular emphasis is given to the case of a no self-conjugate neutral boson $V^0$. Regardless the electric charge of the $V$ boson, a fermionic loop can induce the two CP-even form factors but only one CP-odd. As a result the corresponding electric dipole moment is directly proportional to the magnetic quadrupole moment. The CP-odd form factor might be severely suppressed since it requires the presence of both left- and right-handed fermions. The behavior of the form factors is analyzed for several scenarios of the fermion masses in the context of the decoupling theorem.

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arXiv2004

Static quantities of the W boson in the SU_L(3) X U_X(1) model with right-handed neutrinos

The static electromagnetic properties of the $W$ boson, $Δκ$ and $ΔQ$, are calculated in the $SU_L(3)} \times U_X(1)$ model with right-handed neutrinos. The new contributions from this model arise from the gauge and scalar sectors. In the gauge sector there is a new contribution from a complex neutral gauge boson $Y^0$ and a singly-charged gauge boson $Y^\pm$. The mass of these gauge bosons, called bileptons, is expected to be in the range of a few hundreds of GeV according to the current bounds from experimental data. If the bilepton masses are of the order of 200 GeV, the size of their contribution is similar to that obtained in other weakly coupled theories. However the contributions to both $ΔQ$ and $Δκ$ are negligible for very heavy or degenerate bileptons. As for the scalar sector, an scenario is examined in which the contribution to the $W$ form factors is identical to that of a two-Higgs-doublet model. It is found that this sector would not give large corrections to $Δκ$ and $ΔQ$.

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arXiv2004

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

A new evaluation of the charged Higgs boson decay $H^+ -> W^+γ$ is presented in the context of the general two-Higgs doublet model. A nonlinear $R_ξ$-gauge which considerably simplifies the calculation is introduced and simple expressions are obtained for the fermionic and bosonic contributions. The $H^+ -> W^+γ$ 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 $b -> sγ$, $g-2$ of the muon, $Z -> b \bar b$, and the $ρ$ 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.

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arXiv2004

Two-body $Z'$ decays in the minimal 331 model

The two-body decays of the extra neutral boson Z_2 predicted by the minimal 331 model are analyzed. At the three-level it can decay into standard model particles as well as exotic quarks and the new gauge bosons predicted by the model. The decays into a lepton pair are strongly suppressed, with $Br(Z_2 --> l^+l^-) ~ 10^{-2}$ and $Br(Z_2 --> \barν_l ν) ~ 10^{-3}$. In the bosonic sector, Z_2 would decay mainly into a pair of bilepton gauge bosons, with a branching ratio below the 0.1 level. The Z_2 boson has thus a leptophobic and bileptophobic nature and it would decay dominantly into quark pairs. The anomaly-induced decays $Z_2 --> Z_1γ$ and $Z_2 --> Z_1Z_1$, which occurs at the one-loop level are studied. It is found that $Br(Z_2 --> Z_1γ) ~ 10^{-9}$ and $Br(Z_2 --> Z_1Z_1) ~ 10^{-6}$ at most. As for the $Z_2 --> W^+W^-$ and $Z_2 --> Z_1H$ decays, with H a relatively light Higgs boson, they are induced via Z'-Z mixing. It is obtained that $Br(Z_2 --> W^+W^-) ~ 10^{-2}$ and $Br (Z_2 --> Z_1H) ~ 10^{-5}$. We also examine the flavor changing neutral current decays $Z_2 --> tc$ and $Z_2 --> tu$, which may have branching fractions as large as $10^{-3}$ and $10^{-5}$, respectively, and thus may be of phenomenological interest.

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