arXiv subjects

J. J. Toscano

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

At least 55 records · Page 3Linked to original sources

arXiv2011

Bounding the $Z^\prime tc$ coupling from $D^{0}-\bar{D^{0}}$ mixing and single top production at the ILC

In the present work the $Z^\prime tc$ coupling is bounded by using the current experimental data on the $D^{0}-\bar{D^{0}}$ meson-mixing system. It is found that the strength associated to this coupling is less than $5.75\times 10^{-2}$. The single top production through the $e^+e^-\to Z^\prime\to tc$ process at the $Z^\prime$ boson resonance is studied and we found that around $10^7$ $tc$ events will be expected at the International Linear Collider. For the $Z^\prime\to tc$ decay, we predict a branching ratio of $10^{-2}$.

hep-ph↗

arXiv2011

One-loop effects of extra dimensions on the WWγand WWZ vertices

The one-loop contribution of the excited Kaluza-Klein (KK) modes of the $SU_L(2)$ gauge group on the off-shell $WWγ$ and $WWZ$ vertices is calculated in the context of a pure Yang-Mills theory in five dimensions and its phenomenological implications discussed. The use of a gauge-fixing procedure for the excited KK modes that is covariant under the standard gauge transformations of the $SU_L(2)$ group is stressed. A gauge-fixing term and the Faddeev-Popov ghost sector for the KK gauge modes that are separately invariant under the standard gauge transformations of $SU_L(2)$ are presented. It is shown that the one-loop contributions of the KK modes to the off-shell $WWγ$ and $WWZ$ vertices are free of ultraviolet divergences and well-behaved at high energies. It is found that for a size of the fifth dimension of $R^{-1}\sim 1TeV$, the one-loop contribution of the KK modes to these vertices is about one order of magnitude lower than the corresponding standard model radiative correction. This contribution is similar to the one estimated for new gauge bosons contributions in other contexts. Tree-level effects on these vertices induced by operators of higher canonical dimension are also investigated. It is found that these effects are lower than those generated at the one-loop order by the KK gauge modes.

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arXiv2011

Gauge invariance and radiative corrections in an extra dimensional theory

The gauge structure of the four dimensional effective theory originated in a pure five dimensional Yang-Mills theory compactified on the orbifold $S^1/Z_2$, is discussed on the basis of the BRST symmetry. If gauge parameters propagate in the bulk, the excited Kaluza-Klein (KK) modes are gauge fields and the four dimensional theory is gauge invariant only if the compactification is carried out by using curvatures as fundamental objects. The four dimensional theory is governed by two types of gauge transformations, one determined by the KK zero modes of the gauge parameters and the other by the excited ones. Within this context, a gauge-fixing procedure to quantize the KK modes that is covariant under the first type of gauge transformations is shown and the ghost sector induced by the gauge-fixing functions is presented. If the gauge parameters are confined to the usual four dimensional space-time, the known result in the literature is reproduced with some minor variants, although it is emphasized that the excited KK modes are not gauge fields, but matter fields transforming under the adjoint representation of $SU_4(N)$. A calculation of the one-loop contributions of the excited KK modes of the electroweak gauge group on the off-shell WWV, with V a photon or a Z boson, is exhibited. Such contributions are free of ultraviolet divergences and well-behaved at high energies.

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arXiv2011

Gauge invariance and quantization of Yang-Mills theories in extra dimensions

The gauge structure of the four dimensional effective theory (4DET) arising from a pure Yang-Mills theory in five dimensions compactified on the orbifold $S^1/Z_2$ is reexamined on the basis of the BRST symmetry. The two scenarios that can arise are analyzed: if the gauge parameters propagate in the bulk, the excited Kaluza-Klein (KK) modes are gauge fields, but they are matter vector fields if these parameters are confined in the 3-brane. In the former case, it is shown that the 4DET is gauge invariant only if the compactification is carried out by using curvatures instead of gauge fields as fundamental objects. It is shown that the 4DET is governed by two types of gauge transformations, one determined by the KK zero modes of the gauge parameters and the other by the excited KK modes. The Dirac's method and the proper solution of the master equation are employed to show that the theory is subject to first class constraints. A gauge-fixing procedure to quantize the KK modes, that is covariant under the first type of gauge transformations, is introduced. The ghost sector induced by these gauge-fixing functions is derived on the basis of the BRST formalism. The effective quantum Lagrangian that links the interactions between light physics and heavy physics is presented. Concerning the radiative corrections of the excited KK modes on the light Green functions, the predictive character of this Lagrangian is stressed. In the case of the gauge parameters confined to the 3-brane, the known result in the literature is reproduced with some minor variants, although it is emphasized that the excited KK modes are not gauge fields, but matter fields that transform under the adjoint representation of $SU_4(N)$. The Dirac's method is employed to show that this theory is subject to both first and second class constraints.

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arXiv2010

The decay $b\to sγ$ in the presence of a constant antisymmetric tensor field

A constant antisymmetric 2-tensor can arise in general relativity with spontaneous symmetry breaking or in field theories formulated in a noncommutative space-time. In this work, the one-loop contribution of a nonstandard $WWγ$ vertex on the flavor violating quark transition $q_i\to q_jγ$ is studied in the context of the electroweak Yang-Mills sector extended with a Lorentz-violating constant 2-tensor. An exact analytical expression for the on shell case is presented. It is found that the loop amplitude is gauge independent, electromagnetic gauge invariant, and free of ultraviolet divergences. The dipolar contribution to the $b\to sγ$ transition together with the experimental data on the $B\to X_sγ$ decay is used to derive the constraint $Λ_{LV}>1.96$ TeV on the Lorentz-violating scale.

hep-ph↗

arXiv2010

Higgs mediated flavor violating top quark decays t --> u_i H, u_i gamma, u_i gamma gamma, and the process gamma gamma --> t c in effective theories

The rare top quark couplings $tu_iγ$ and $tu_iγγ$ ($u_i=u,c$) induced at the one-loop level by a flavor violating $tu_iH$ vertex are studied within the context of an effective Yukawa sector that incorporates $SU_L(2)\times U_Y(1)$-invariant operators of up to dimension six. Data on the recently observed $D^0-\overline{D^0}$ mixing are employed to constrain the $tu_iH$ vertex, which is then used to predict the $t\to u_iH$, $t\to u_iγ$, and $t\to u_iγγ$ decays, as well as the $γγ\to \bar{t}u_i+t\bar{u}_i$ reaction in the context of the ILC. It is found that the $t\to cH$ and $t\to cγγ$ decays can reach sizable branching ratios as high as $5\times 10^{-3}$ and $10^{-4}$, respectively. As for the $t\to cγ$ decay, it can have a branching ratio of $5\times 10^{-8}$ that is about 6 orders of magnitude larger than the standard model prediction, which, however, is still very small to be detected. As for $tc$ production, it is found that, due to the presence of a resonant effect in the convoluted cross section $σ(e^+e^-\to γγ\to t\bar{c}+\bar{t}c)$, about $(0.5 - 2.7)\times 10^{3}$ $tc$ events may be produced at the ILC for a value of the Higgs mass near to the top mass.

hep-ph↗

arXiv2010

Bounding the B_s -> gamma gamma decay from Higgs mediated FCNC transitions

The Higgs mediated flavor violating bottom-strange quarks transitions induced at the one-loop level by a nondiagonal $Hbs$ coupling are studied within the context of an effective Yukawa sector that comprises $SU_L(2)\times U_Y(1)$-invariant operators of up to dimension-six. The most recent experimental result on $B\to X_sγ$ with hard photons is employed to constraint the $Hbs$ vertex, which is used to estimate the branching ratio for the $B_s\to γγ$ decay. It is found that the $B_s\to γγ$ decay can reach a branching ratio of the order of $4\times 10^{-8}$, which is 2 orders of magnitude more stringent than the current experimental limit.

hep-ph↗

arXiv2010

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.

hep-ph↗

arXiv2009

Higgs mediated Double Flavor Violating top decays in Effective Theories

The possibility of detecting double flavor violating top quark transitions at future colliders is explored in a model-independent manner using the effective Lagrangian approach through the $t \to u_iτμ$ ($u_i=u,c$) decays. A Yukawa sector that contemplates $SU_L(2)\times U_Y(1)$ invariants of up to dimension six is proposed and used to derive the most general flavor violating and CP violating $q_iq_jH$ and $l_il_jH$ vertices of renormalizable type. Low-energy data, on high precision measurements, and experimental limits are used to constraint the $tu_iH$ and $Hτμ$ vertices and then used to predict the branching ratios for the $t \to u_iτμ$ decays. It is found that this branching ratios may be of the order of $ 10^{-4}-10^{-5}$, for a relative light Higgs boson with mass lower than $2m_W$, which could be more important than those typical values found in theories beyond the standard model for the rare top quark decays $t\to u_iV_iV_j$ ($V_i=W,Z,γ, g$) or $t\to u_il^+l^-$. %% LHC experiments, by using a total integrated luminosity of $\rm 3000 fb^{-1}$ of data, will be able to rule out, at 95% C.L., DFV top quark decays up to a Higgs mass of 155 GeV/$c^2$ or discover such a process up to a Higgs mass of 147 GeV/$c^2$.

hep-ph↗

arXiv2009

Decays Z --> g g gamma and Z-prime --> g g gamma in the minimal 331 model

The one-loop induced Z --> g g gamma and Z-prime --> g g gamma decays are studied within the context of the minimal 331 model, which predicts the existence of new gauge bosons and three exotic quarks. It is found that the Z --> g g gamma decay is insensitive to the presence of the exotic quarks, as it is essentially governed by the first two families of known quarks. As to the Z-prime --> g g gamma decay, it is found that the exotic quark contribution dominates and that for a heavy Z-prime boson it leads to a Gamma(Z-prime --> g g gamma) that is more than one order of magnitude larger than that associated with Gamma(Z-prime --> g g g).

hep-ph↗

arXiv2009

Decays Z --> ggg and Z-prime --> ggg in the minimal 331 model

We perform a complete calculation at the one-loop level for the Zggg and Z'ggg couplings in the context of the minimal 331 model, which predicts the existence of a new Z' gauge boson and new exotic quarks. Bose symmetry is exploited to write a compact and manifest SU_C(3)-invariant vertex function for the Vggg (V=Z,Z') coupling. Previous results on the $Z\to ggg$ decay in the standard model are reproduced. It is found that this decay is insensitive to the effects of the new exotic quarks. This in contrast with the $Z'\to ggg$ decay, which is sensitive to both the standard model and exotic quarks, whose branching ratio is larger than that of the $Z\to ggg$ transition by about a factor of 4.

hep-ph↗

arXiv2009

U_e(1)--covariant R_ξ--gauge for the two Higgs doublet model

An $U_e(1)$--covariant $R_ξ$ gauge for the two Higgs doublet model based in the BRST symmetry is introduced. This gauge allows one to remove an important number of nonphysical vertices appearing in conventional linear gauges, which greatly simplifies the loop calculations, since the the resultant theory satisfies QED--like Ward identities. The presence of four ghost interactions in this type of gauges and its connection with the BRST symmetry is stressed. The Feynman rules for those new vertices that arise in this gauge, as well as for those couplings already present in the linear $R_ξ$ gauge but that are modified by this gauge--fixing procedure, are presented.

hep-th↗

arXiv2009

Implications of $D^0-\overline{D^0}$ on the rare top quark decays $t\to uγ$ and $t\to ug$

The recently observed mass difference of the $D^0-\overline{D^0}$ mixing is used to predict the branching ratios of the rare top quark decays $t\to uγ$ and $t\to ug$ in a model independent way using the effective Lagrangian approach. It is found that $Br(t\to uγ)<4\times 10^{-4}$ and $Br(t\to ug)<2\times 10^{-3}$, which still may be within reach of the LHC collider.

hep-ph↗

arXiv2009

Effective Lagrangian description of Higgs mediated flavor violating electromagnetic transitions: implications on lepton flavor violation

Higgs mediated flavor violating electromagnetic interactions, induced at the one--loop level by a nondiagonal $Hf_if_j$ vertex, with $f_i$ and $f_j$ charged leptons or quarks, are studied within the context of a completely general effective Yukawa sector that comprises $SU_L(2)\times U_Y(1)$--invariant operators of up to dimension--six. Exact formulae for the one--loop $γf_if_j$ and $γγf_if_j$ couplings are presented and their related processes used to study the phenomena of Higgs mediated lepton flavor violation. The experimental limit on the $μ\to eγ$ decay is used to derive a bound on the branching ratio of the $μ\to eγγ$ transition, which is 6 orders of magnitude stronger than the current experimental limit. Previous results on the $τ\to μγ$ and $τ\to μγγ$ decays are reproduced. The possibility of detecting signals of lepton flavor violation at $γγ$ colliders is explored through the $γγ\to l_il_j$ reaction, putting special emphasis on the $τμ$ final state. Using the bound imposed on the $Hτμ$ vertex by the current experimental data on the muon anomalous magnetic moment, it is found that about half a hundred events may be produced in the International Linear Collider.

hep-ph↗

arXiv2008

Effects of physics beyond the standard model on the neutrino charge radius: an effective Lagrangian approach

In this work, we look for possible new physics effects on the electromagnetic charge and anapole form factors, $f_Q(q^2)$ and $f_A(q^2)$, for a massless Dirac neutrino, when these quantities are calculated in the context of an effective electroweak Yang-Mills theory, which induces the most general $SU_L(2)$--invariant Lorentz tensor structure of nonrenormalizable type for the $WWγ$ vertex. It is found that in this context, besides the standard model contribution, the additional contribution to $f_{Q}(q^2)$ and $f_{A}(q^2)$ ($f_{Q}^{O_W}(q^2)$ and $f_{A}^{O_W}(q^2)$, respectively) are gauge independent and finite functions of $q^2$ after adopting a renormalization scheme. These form factors, $f_{Q}^{O_W}(q^2)$ and $f_{A}^{O_W}(q^2)$, get contribution at the one loop level only from the proper neutrino electromagnetic vertex. Besides, the relation $f_{Q}^{eff}(q^2)=q^2f_{A}^{eff}(q^2)$ ($f_{Q}^{eff}(q^2)=f_{Q}^{SM}(q^2)+f_{Q}^{O_W}(q^2)$, $f_{A}^{eff}(q^2)=f_{A}^{SM}(q^2)+f_{A}^{O_W}(q^2)$) is still fulfilled and hence the relation $a_ν^{eff} = < r^2_ν > ^{eff} /6$ ($a_ν^{eff} = a_ν^{SM}+ a_ν^{O_W}$, $ ^{eff} = < r^2_ν > ^{SM}+< r^2_ν > ^{O_W}$)is gotten, just as in the SM. Using the experimental constraint on the anomalous $WWγ$ vertex, a value for the additional contribution to the charge radius of $|< r^2_ν >^{O_W}| \lsim 10^{-34} cm^2$ is obtained, which is one order of magnitude lower than the SM value.

hep-ph↗

arXiv2008

Higgs mediated lepton flavor violating tau decays $τ\to μγ$ and $τ\to μγγ$ in effective theories

The size of the branching ratios for the $τ\to μγ$ and $τ\to μγγ$ decays induced by a lepton flavor violating Higgs interaction $Hτμ$ is studied in the frame of effective field theories. The best constraint on the $Hτμ$ vertex, derived from the know measurement on the muon anomalous magnetic moment, is used to impose the upper bounds $Br(τ\to μγ)<2.5\times 10^{-10}$ and $Br(τ\to μγγ)<2.3\times 10^{-12}$, which are more stringent than current experimental limits on this class of transitions.

hep-ph↗

arXiv2008

Double Flavor Violating Top Quark Decays in Effective Theories

The possibility of detecting double flavor violating top quark transitions $t \to u_iτμ$ ($u_i=u,c$) is explored in a model--independent manner, using the effective Lagrangian approach. Low--energy data, on high precision measurements, and current experimental limits are used to constraint the $tu_iH$ and $Hτμ$ vertices and then to calculate the branching ratio BR$(t \to u_iτμ)$. If in the Standard Model BR$(t \to u_iτμ)$ is of the order of $10^{-13}$$-10^{-14}$, higgs--mediated double flavor violating top quark decays can occur with branching ratios ranging from $10^{-3}$ to $10^{-4}$ for 114.4 GeV$/c^2$ $< m_H<$ $2m_W$, that is at the reach of the CERN Large Hadron Collider.

hep-ph↗

arXiv2007

Effective Lagrangian approach to fermion electric dipole moments induced by a CP--violating $WWγ$ vertex

The one--loop contribution of the two CP--violating components of the $WWγ$ vertex, $ \tildeκ_γW^+_μW^-_ν\tilde{F}^{μν}$ and $(\tildeλ_γ/ m^2_W)W^+_{μν}W^{-ν}_{\ ρ}\tilde{F}^{ρμ}$, on the electric dipole moment (EDM) of fermions is calculated using dimensional regularization and its impact at low energies reexamined in the light of the decoupling theorem. The Ward identities satisfied by these couplings are derived by adopting a $SU_L(2)\times U_Y(1)$--invariant approach and their implications in radiative corrections discussed. Previous results on $\tildeκ_γ$, whose bound is updated to $|\tildeκ_γ| <5.2\times 10^{-5}$, are reproduced, but disagreement with those existing for $\tildeλ_γ$ is found. In particular, the upper bound $|\tildeλ_γ|<1.9\times10^{-2}$ is found from the limit on the neutron EDM, which is more than 2 orders of magnitude less stringent than that of previous results. It is argued that this difference between the $\tildeκ_γ$ and $\tildeλ_γ$ bounds is the one that might be expected in accordance with the decoupling theorem. This argument is reinforced by analyzing careful the low--energy behavior of the loop functions. The upper bounds on the $W$ EDM, $|d_W|<6.2\times 10^{-21} e\cdot cm$, and the magnetic quadrupole moment, $|\tilde{Q}_W|<3\times 10^{-36} e\cdot cm^2$, are derived. The EDM of the second and third families of quarks and charged leptons are estimated. In particular, EDM as large as $ 10^{-20} e\cdot cm$ and $10^{-21} e\cdot cm$ are found for the $t$ and $b$ quarks, respectively.

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