arXiv subjects

Eduardo Rojas

Publications and source records attributed to Eduardo Rojas.

At least 37 records · Page 2Linked to original sources

arXiv2021

The Podolsky propagator in gap and bound-state equations

Based on the Generalized Quantum Electrodynamics expression for the Podolsky propagator, which preserves gauge invariance for massive photons, we propose a model for the massive gluon propagator that reproduces well-known features of established strong-interaction models in the framework of the Dyson-Schwinger equation. By adjusting the Podolsky mass and the coupling strength we thus construct a model with simple analytical properties known from perturbative theory, yet well suited to describe a confining interaction. We obtain solutions of the Dyson-Schwinger equation for the quark at space-like momenta on the real axis as well as on the complex plane and solving the bound-state problem with the Bethe-Salpeter equation yields masses and weak decay constants of the $π, K$ and $η_c$ in excellent agreement with experimental values, while the $D$ and $D_s$ are reasonably well described. The analytical simplicity of this effective interaction has the potential to be useful for phenomenological applications and may facilitate calculations in Minkowski space.

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arXiv2020

Distribution Amplitudes of Heavy Mesons and Quarkonia on the Light Front

The ladder kernel of the Bethe-Salpeter equation is amended by introducing a different flavor dependence of the dressing functions in the heavy-quark sector. Compared with earlier work this allows for the simultaneous calculation of the mass spectrum and leptonic decay constants of light pseudoscalar mesons, the $D_u$, $D_s$, $B_u$, $B_s$ and $B_c$ mesons and the heavy quarkonia $η_c$ and $η_b$ within the same framework at a physical pion mass. The corresponding Bethe-Salpeter amplitudes are projected onto the light front and we reconstruct the distribution amplitudes of the mesons in the full theory. A comparison with the first inverse moment of the heavy meson distribution amplitude in heavy quark effective theory is made.

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arXiv2020

Analysing the charged scalar boson contribution to the charged-current $B$ meson anomalies

Experimental measurements collected by the BABAR, Belle, and LHCb experiments on different observables associated with the semileptonic transition $b \to c τ\barν_τ$, indicate the existence of disagreement respect with the Standard Model predictions. We analyse the charged scalar boson contributions to these charged-current $B$ meson anomalies within the framework of two Higgs doublet model with the most general Yukawa couplings to quarks and leptons from the third generation, involving left-handed and right-handed (sterile) neutrinos. We perform a phenomenological study of the Yukawa couplings parameter space that accommodates these anomalies. We consider the most recent data from HFLAV world-average and Belle combination, and the upper limits ${\rm BR}(B_c^- \to τ^- \barν_τ) < 30\%$ and $10\%$. In addition, we include in our study the prospect measurements on $R(D^{(\ast)})$ that the Belle II experiment could achieve and explore, for the first time, the future implications for the corresponding charged scalar Yukawa couplings. This analysis updates the existing literature and includes new important observables. Our results show that current experimental $b\rightarrow c τ\barν_τ$ data and Belle II projection favor the interpretation of a charged scalar boson interacting with right-handed neutrinos. Furthermore, as a side analysis regarding the charged scalar boson interpretation, we revisit the relation between $R(D^\ast)$ and ${\rm BR}(B_c^- \to τ^- \barν_τ)$ by investigating whether the claim that pseudoscalar new physics interpretations of $R(D^{\ast})$ are implausible due to the $B_c$ lifetime is still valid, to the light of the recent data and Belle II prospects on $R(D^{\ast})$. Lastly, we reexamine addressing the $R(D^{(\ast)})$ anomalies in the context of the 2HDM of Type II.

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arXiv2020

Minimal $Z'$ models for flavor anomalies

By allowing gauge anomaly cancellation between fermions in different families we find a non-universal solution for a $Z'$ family of models with the same content of fermions of the standard model plus three right-handed neutrinos. We also impose constraints from the Yukawa interaction terms in such a way that at the end we obtain a solution with six free parameters. Our solution contains as particular cases well-known models in the literature. As an application, we report a model that evades LHC constraints, flavor changing neutral currents and low energy constraints. Simultaneously, the model is able to explain the flavor anomalies in the Wilson coefficients $C_9(μ)$ and $C_{10}(μ)$ without modifying the corresponding Wilson coefficients for the first family. In our approach, this procedure is always possible for $Z'$ masses smaller than $\sim 2.5$~TeV.

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arXiv2020

Five Texture Zeros for Dirac Neutrino Mass Matrices

In this work we propose new five textures zeros for the mass matrices in the lepton sector in order to predict values for the neutrino masses. In our approach we go beyond the Standard Model by assuming Dirac masses for the neutrinos, feature which allows us to make a theoretical prediction for the lightest neutrino mass in the normal ordering. The textures analyzed have enough free parameters to adjust the $V_{\text{\tiny PMNS}}$ mixing matrix including the CP-violating phase, the neutrino mass squared differences $δm_{21}^2,\,δm_{31}^2$, and the three charged lepton masses. In order to have reliable results two different approaches are used: the first one is based on a least-squares analysis to fit the lepton masses and the mixing parameters to their corresponding experimental values, for this case the best fit for the lightest neutrino mass is $(3.9\pm^{0.6}_{0.8})\times 10^{-3}$ eV.; the second approach is just algebraic, based on the weak basis transformation method, in this case the lightest neutrino mass consistent with the experimental values and the restrictions coming from the five texture zeros of the mass matrices is equal to $(3.5\pm0.9)\times 10^{-3}$ eV.

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arXiv2019

Charged current $b \to c τ\barν_τ$ anomalies in a general $W^\prime$ boson scenario

Very recent experimental information obtained from the Belle experiment, along with that accumulated by the BABAR and LHCb experiments, has shown the existence of anomalies in the ratios $R(D)$ and $R(D^{*})$ associated with the charged-current transition $b \to c τ\barν_τ$. Although the Belle measurements are in agreement with the standard model (SM) predictions, the new experimental world averages still exhibit a tension. In addition, the $D^\ast$ longitudinal polarization $F_L(D^\ast)$ related with the channel $B \to D^\ast τ\barν_τ$ observed by the Belle Collaboration and the ratio $R(J/ψ)$ measured by the LHCb Collaboration also show discrepancies with their corresponding SM estimations. We present a model-independent study based on the most general effective Lagrangian that yields a tree-level effective contribution to the transition $b \to c τ\barν_τ$ induced by a general $W^\prime$ boson. Instead of considering any specific new physics (NP) realization, we perform an analysis by considering all of the different chiral charges to the charm-bottom and $τ$-$ν_τ$ interaction terms with a charged $W^\prime$ boson that explain the anomalies. We present a phenomenological study of parameter space allowed by the new experimental $b \to c τ\barν_τ$ data and with the mono-tau signature $pp \to τ_h X + \rm{MET}$ at the LHC. For comparison, we include some of the $W^\prime$ boson NP realizations that have already been studied in the literature.

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arXiv2019

Electroweak couplings and LHC constraints on alternative $Z^{\prime}$ models in $E_6$

We report the most general expression for the chiral charges of a $Z'$ gauge boson coming from an $E_6$ unification model, as a function of the electroweak parameters and the charges of the $U(1)$ factors in the chain of subgroups. These charges are valid for an arbitrary Higgs sector and only depend on the branching rules of the $E_6$ fundamental representation and the corresponding rules for the fermionic representations of their subgroups. By assuming $E_6$ unification, the renormalization group equations~(RGE) allow us to calculate the electroweak parameters at low energies for most of the chains of subgroups in $E_6$. From RGE and unitary conditions, we show that at low energies there must be a mixing between the gauge boson of the standard model hypercharge and the $Z'$. From this, it is possible to delimit the preferred region in the parameter space for a breaking pattern in $E_6$. In general, without unification, it is not viable to determine this region; however, for some models and under certain assumptions, it is possible to limit the corresponding parameter space. By using the most recent upper limits on the cross-section of extra gauge vector bosons $Z'$ decaying into dileptons from the ATLAS data at 13~TeV with accumulated luminosities of 36.1~fb$^{-1}$ and 13.3~fb$^{-1}$, we report the 95$\%$ C.L. lower limits on the $Z'$ mass for the typical $E_6$ benchmark models. We also show the contours in the 95\% C.L. of the $Z'$ mass bounds for the entire parameter space of $E_6$.

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arXiv2018

Non-universal electroweak extensions of the standard model and the scotogenic models

In order to analyze some low energy experimental anomalies, we charge with a non-universal $U(1)'$ gauge symmetry the standard model fermions, taking as a starting point the well-known scotogenic model. In order to have non-trivial solutions to the anomalies and the Yukawa constraints, we add three neutral singlet Dirac fermions. We have found two possible non-universal solutions which, as a matter of principle, are suitable to analyze family-dependent experimental anomalies.

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arXiv2018

Five Non-Fritzsch Texture Zeros for Quarks Mass Matrices in the Standard Model

In the Standard Model, we obtain a non-Fritzsch like configuration with five texture zeros for the quark mass matrices. This matrix generates the quark masses, the inner angles of the CKM unitary triangle, and the CP-violating phase in the quark sector. This work can be applied to the PMNS matrix in the lepton sector, by assuming Dirac masses for the neutrinos, where non-trivial predictions for the neutrino masses and mixing angles are expected.

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arXiv2017

Minimal non-universal EW extensions of the Standard Model: a chiral multi-parameter solution

We report the most general expression for the chiral charges of a non-universal $U(1)'$ with identical charges for the first two families but different charges for the third one. The model is minimal in the sense that only standard model fermions plus right-handed neutrinos are required. By imposing anomaly cancellation and constraints coming from Yukawa couplings we obtain two different solutions. In one of these solutions, the anomalies cancel between fermions in different families. These solutions depend on four independent parameters which result very useful for model building. We build different benchmark models in order to show the flexibility of the parameterization. We also report LHC and low energy constraints for these benchmark models.

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arXiv2017

Mass spectrum and decay constants of radially excited vector mesons

We calculate the masses and weak decay constants of flavorless ground and radially excited $J^P=1^-$ mesons and the corresponding quantities for the K^*, within a Poincaré covariant continuum framework based on the Bethe-Salpeter equation. We use in both, the quark's gap equation and the meson bound-state equation, an infrared massive and finite interaction in the leading symmetry-preserving truncation. While our numerical results are in rather good agreement with experimental values where they are available, no single parametrization of the QCD inspired interaction reproduces simultaneously the ground and excited mass spectrum, which confirms earlier work on pseudoscalar mesons. This feature being a consequence of the lowest truncation, we pin down the range and strength of the interaction in both cases to identify common qualitative features that may help to tune future interaction models beyond the rainbow-ladder approximation.

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arXiv2017

Couplings between the $ρ$ and $D$- and $D^\ast$-mesons

We compute couplings between the $ρ$-meson and $D$- and $D^\ast$-mesons - $D^{(\ast)}ρD^{(\ast)}$ - that are relevant to phenomenological meson-exchange models used to analyse nucleon-$D$-meson scattering and explore the possibility of exotic charmed nuclei. Our framework is built from elements constrained by Dyson-Schwinger equation studies in QCD, and therefore expresses a consistent, simultaneous description of light- and heavy-quarks and the states they constitute, We find that all interactions, including the three independent $D^{\ast} ρ\,D^{\ast}$ couplings, differ markedly amongst themselves in strength and also in range, as measured by their evolution with $ρ$-meson virtuality. As a consequence, it appears that no single coupling strength or parametrization can realistically be employed in the study of interactions between $D^{(\ast)}$-mesons and matter.

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arXiv2016

Flipped versions of the universal 3-3-1 and the left-right symmetric models in $[SU(3)]^3$: a comprehensive approach

By considering the 3-3-1 and the left-right symmetric models as low energy effective theories of the $SU(3)_C\otimes SU(3)_L\otimes SU(3)_R$~(for short $[SU(3)]^3$) gauge group, alternative versions of these models are found. The new neutral gauge bosons of the universal 3-3-1 model and its flipped versions are presented; also, the left-right symmetric model and its flipped variants are studied. Our analysis shows that there are two flipped versions of the universal 3-3-1 model, with the particularity that both of them have the same weak charges. For the left-right symmetric model we also found two flipped versions; one of them new in the literature which, unlike those of the 3-3-1, requires a dedicated study of its electroweak properties. For all the models analyzed, the couplings of the $Z'$ bosons to the standard model fermions are reported. The explicit form of the null space of the vector boson mass matrix for an arbitrary Higgs tensor and gauge group is also presented. In the general framework of the $[SU(3)]^3$ gauge group, and by using the LHC experimental results and EW precision data, limits on the $Z'$ mass and the mixing angle between $Z$ and the new gauge bosons $Z'$ are obtained. The general results call for very small mixing angles in the range $10^{-3}$ radians and $M_{Z'}>$ 2.5 TeV.

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arXiv2016

Excited hadrons and the analytical structure of bound-state interaction kernels

We highlight Hermiticity issues in bound-state equations whose kernels are subject to a highly asymmetric mass and momentum distribution and whose eigenvalue spectrum becomes complex for radially excited states. We trace back the presence of imaginary components in the eigenvalues and wave functions to truncation artifacts and suggest how they can be eliminated in the case of charmed mesons. The solutions of the gap equation in the complex plane, which play a crucial role in the analytic structure of the Bethe-Salpeter kernel, are discussed for several interaction models and qualitatively and quantitatively compared to analytic continuations by means of complex-conjugate pole models fitted to real solutions.

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arXiv2016

Contemporary continuum QCD approaches to excited hadrons

Amongst the bound states produced by the strong interaction, radially excited meson and nucleon states offer an important phenomenological window into the long-range behavior of the coupling constant in Quantum Chromodynamics. We here report on some technical details related to the computation of the bound state's eigenvalue spectrum in the framework of Bethe-Salpeter and Faddeev equations.

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arXiv2015

Alternative $Z'$ Bosons in $E_6$

We classify the quantum numbers of the extra $U(1)'$ symmetries contained in $E_6$. In particular, we categorize the cases with rational charges and present the full list of models which arise from the chains of the maximal subgroups of $E_6$. As an application, the classification allows us to determine all embeddings of the Standard Model fermions in all possible decompositions of the fundamental representation of $E_6$ under its maximal subgroups. From this we find alternative chains of subgroups for Grand Unified Theories. We show how many of the known models including some new ones appear in alternative breaking patterns. We also use low energy constraints coming from parity-violating asymmetry measurements and atomic parity non-conservation to set limits on the $E_6$ motivated parameter space for a $Z'$ boson mass of~1.2~TeV. We include projected limits for the present and upcoming QWEAK, MOLLER and SOLID experiments.

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arXiv2015

LHC Constraints on 3-3-1 Models

The ATLAS detector data on di-lepton production is used in order to impose constraints on $Z^\prime$ boson masses associated with a variety of 3-3-1 and $E_6$ motivated $Z'$ models. Lower mass bounds for the different models are established at $95\%$ confidence level. Our numerical analysis is extrapolated up to 14~TeV, and further to 30~TeV and 100~TeV, for a broad range of luminosities. Some of our results can be compared with the ATLAS published bounds, being, for those cases, in fairly good agreement. We also report the vector and axial charges for all the 3-3-1-motivated $Z'$ models without exotic electric charges for leptons, known in the literature. To the best of our knowledge most of this charges were not reported before.

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arXiv2015

Completing the picture of the Roper resonance

We employ a continuum approach to the three valence-quark bound-state problem in relativistic quantum field theory to predict a range of properties of the proton's radial excitation and thereby unify them with those of numerous other hadrons. Our analysis indicates that the nucleon's first radial excitation is the Roper resonance. It consists of a core of three dressed-quarks, which expresses its valence-quark content and whose charge radius is 80% larger than the proton analogue. That core is complemented by a meson cloud, which reduces the observed Roper mass by roughly 20%. The meson cloud materially affects long-wavelength characteristics of the Roper electroproduction amplitudes but the quark core is revealed to probes with $Q^2 \gtrsim 3 m_N^2$.

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