arXiv subjects

J. P. B. C. de Melo

Publications and source records attributed to J. P. B. C. de Melo.

At least 19 recordsLinked to original sources

arXiv2026

Nuclear matter and proton parton distributions in a light-front Hamiltonian framework

We develop a light-front Hamiltonian formulation of symmetric nuclear matter within the quark-meson coupling model, using Basis Light-Front Quantization to solve the in-medium nucleon eigenvalue problem. The Hamiltonian incorporates confinement in the valence sector and is truncated to include up to one dynamical gluon. Medium effects are introduced via scalar and vector mean fields, yielding a self-consistent, density-dependent effective quark mass and modified nucleon structure. The resulting energy per nucleon, pressure, and incompressibility are consistent with empirical constraints at the saturation point. At nuclear saturation density, the gluon probability in the nucleon wave function increases slightly, while the valence probability and quark momentum fraction decrease. The unpolarized quark and gluon distributions show a noticeable enhancement at large momentum fraction ($x \gtrsim 0.4$), illustrated at an evolved scale of $Q^{2} = 10 \mathrm{GeV}^{2}$.

hep-ph↗

arXiv2026

Baryon Bethe-Salpeter Equation in Minkowski-Space QCD$_2$

We study the three-quark ladder Bethe--Salpeter equation in Minkowski-space QCD$_2$ in the light-cone gauge. Using the quasi-potential expansion, we project the baryon equation onto the light front and show that, at leading order in the valence truncation, the resulting mass-squared eigenvalue equation is equivalent to the Bars--Durgut equation. We also derive the endpoint power-law behavior of the valence wave function in terms of the quark mass and coupling, closely paralleling the original 't Hooft analysis for mesons. The resulting three-quark equation is solved numerically for $N_c=3$, and the ground-state baryon mass is found to be in reasonable agreement with previous light-cone quantization results in QCD$_2$, suggesting that the valence sector provides the dominant contribution to the ground state. The excited-state spectrum further yields a Regge trajectory that captures the overall trend of the experimental nucleon spectrum, and we compute selected structure observables, including parton distribution functions, double distribution amplitudes, and coordinate-space densities. This framework provides a useful confining test bed for Minkowski-space bound-state methods and for future developments toward confining formulations in 3+1 dimensions beyond the valence truncation.

hep-ph↗

arXiv2026

Light-front mass operator with dressed quarks

We construct an effective light-front mass-squared operator for quark-antiquark systems that incorporates quark dressing effects through a running quark mass. Starting from a Minkowski-space quark propagator constrained by lattice-QCD-inspired parametrization, we derive the disconnected light-front resolvent for a light quark-antiquark system using a generalized spectral representation of an individual quark propagator separating out the instantaneous contributions. By projecting the resolvent onto a constituent-quark helicity basis, we obtain an effective dressed mass-squared operator suitable for light-front Hamiltonian approaches. We introduce an effective light-front quark self-energy and analyze its momentum dependence. As an application, we study pion structure using representative light-front wave-function models and compute unpolarized transverse-momentum-dependent distributions, unpolarized parton distribution functions and distribution amplitudes. Our results show that quark dressing induces sizable infrared modifications while preserving controlled ultraviolet behavior, providing a framework to incorporate nonperturbative QCD dynamics into light-front descriptions of hadrons.

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arXiv2026

Ward-Takahashi identity in the light-front formalism for a bound state of fermions

We investigate the Ward-Takahashi identity at one-loop in the light-front (LF) formalism for a bound state of fermions. We consider a spinless bound state made up of two fermions in which the Ward-Takahashi identity is satisfied in the covariant formulation. Considering the same system in the light-front formalism, we investigate the proof of Ward-Takahashi identity by integrating the light-front energy component through the identification of the relevant ranges of the longitudinal LF momentum. We elucidate that the pair production diagram plays a crucial role in establishing the Ward-Takahashi identity. We also point out the necessity of taking into account the corresponding zero modes for truly establishing the Ward-Takahashi identity in the LF formalism.

hep-ph↗

arXiv2026

Covariant form factors for spin-1 particles

Spin-1 particles, is a fundamental bound state for the two quarks, and play a crucial role in elucidating the electromagnetic properties within the realm of hadronic physics. Their intrinsic relativistic nature mandates a quantum field theory (QFT) framework for a comprehensive analysis. In this investigation, we employ both the instant form of QFT and light-front quantum field theory (LFQFT) as our theoretical tools. While conventional LFQFT approaches predominantly focus on the plus component of the electromagnetic current to extract the properties of spin-1 vector particles, our study extends this analysis by systematically incorporating the minus component as well. Our findings demonstrate that achieving a rigorous equivalence between these distinct current components necessitates the inclusion of nonvalence terms within the electromagnetic current operator. This crucial inclusion serves to restore manifest covariance and ensures a robust consistency with the results independently derived from the more conventional instant form of quantum field theory.

hep-ph↗

arXiv2025

The glue that binds us all -- Latin America and the Electron-Ion Collider

The Electron-Ion Collider, a next generation electron-hadron and electron-nuclei scattering facility, will be built at Brookhaven National Laboratory. The wealth of new data will shape research in hadron physics, from nonperturbative QCD techniques to perturbative QCD improvements and global QCD analyses, for the decades to come. With the present proposal, Latin America based physicists, whose expertise lies on the theory and phenomenology side, make the case for the past and future efforts of a growing community, working hand-in-hand towards developing theoretical tools and predictions to analyze, interpret and optimize the results that will be obtained at the EIC, unveiling the role of the glue that binds us all. This effort is along the lines of various initiatives taken in the U.S., and supported by colleagues worldwide, such as the ones by the EIC User Group which were highlighted during the Snowmass Process and the Particle Physics Project Prioritization Panel (P5).

nucl-ex↗

arXiv2024

Kaon Structure in the Nuclear Medium within the Light Front Approach

We study the properties of the charged kaon in symmetric nuclear matter using a Bethe-Salpeter amplitude to model the quark-anti-quark bound state, which is well constrained by previous studies of its vacuum properties. The electromagnetic form factor, charge radius, decay constant and the light-front valence component probability are investigated in symmetric nuclear matter. In order to describe the constituent up and anti-strange quarks in nuclear matter, we adopt the ``quark-meson coupling (QMC) model", which has been widely applied to various hadronic and nuclear phenomena in the nuclear medium.

hep-ph↗

arXiv2023

Pseudoscalar current and covariance with the light-front approach

Quantum Field Theory (QFT) is used to describe the physics of particles in terms of their fundamental constituents. The Light-Front Field Theory~(LFFT), introduced by Paul Dirac in 1949, is an alternative approach to solve some of the problems that arise in quantum field theory. The LFFT is similar to the Equal Time Quantum Field Theory~(EQT), however, some particularities are not, such as the loss of covariance in the light-front. Pion electromagnetic form factor is studied in this work at lower and higher momentum transfer regions to explore the constituent quark models and the differences among these and other models. The electromagnetic current is calculated with both the ``plus'' and ``minus'' components in the light-front approach. The results are compared with other models, as well as with experimental data.

hep-ph↗

arXiv2023

The case for an EIC Theory Alliance: Theoretical Challenges of the EIC

We outline the physics opportunities provided by the Electron Ion Collider (EIC). These include the study of the parton structure of the nucleon and nuclei, the onset of gluon saturation, the production of jets and heavy flavor, hadron spectroscopy and tests of fundamental symmetries. We review the present status and future challenges in EIC theory that have to be addressed in order to realize this ambitious and impactful physics program, including how to engage a diverse and inclusive workforce. In order to address these many-fold challenges, we propose a coordinated effort involving theory groups with differing expertise is needed. We discuss the scientific goals and scope of such an EIC Theory Alliance.

hep-ph↗

arXiv2022

Pion inspired by QCD: Nakanishi and Light-Front Integral Representations

The pion structure in Minkowski space is explored using the Nakanishi integral representation. A general framework is developed for the pion Bethe-Salpeter amplitude based on the Kallen-Lehmann representation of the dressed quarks with an ansatz for the pseudo-scalar $π-q\bar q$ vertex fulfilling the axial Ward-Takahashi identity. The Nakanishi weight functions are derived for the scalar amplitudes associated with the decomposition of the pion Bethe-Salpeter amplitude in different operator bases in the Dirac spinor space in terms of the involved spectral densities. The approach is applied to an analytical model of the pion Bethe-Salpeter amplitude, which is combined with Landau gauge lattice QCD results for the quark running mass at space-like momentum. From the Nakanishi integral representation several pion observables were calculated, such as the decay constant, the spin decomposition of the valence probabilities, longitudinal and transverse momentum distributions from the valence component of the light-front wave function.

hep-ph↗

arXiv2022

Pion model with the Nakanishi Integral Representations

In the present work, we describe a model for the pion based on an analytic expression for the Bethe-Salpeter (BSA) amplitude, combined with some ingredients from Lattice QCD calculations. The running quark mass function $M (p^2)$, used here, reproduces well the results of Lattice QCD calculations. The analytical form of the running quark mass function contains a single time-like pole, which implies in time-like poles of the dressed quark propagator. Such a form allows to build the weight functions, $G_i(γ,z)$, for the Nakanishi integral representation of each scalar function, $χ_{i}(k,p)$, appearing in the decomposition of the Bethe-Salpeter amplitude in terms of Dirac operators, Such scalar amplitudes can also be used to obtain the pion valence light-front wave function.

hep-ph↗

arXiv2022

Nucleon Electromagnetic and Axial Form Factors in a Light-front Constituent Quark Model

In the present work we study the effect of the scalar spin coupling of constituent quarks on the nucleon electroweak properties by introducing a valence light-front wave function with two momentum scales. By comparing the results obtained with the one scale and two scale wave function models, we have found that the last one has shown a reasonable description of the static observables and $μ_pG_{Ep}/G_{Mp}$ ratio in which the position of the zero appears around 10~[GeV/c]$^2$ or for higher squared momentum transfers. We have also shown results for the axial-vector coupling $g_{A}$ and the nucleon axial-vector form factor. The best result for $g_A$ was obtained when the parameters of the nucleon wave function model were such that the experimental value of the neutron magnetic moment was described.

hep-ph↗

arXiv2021

Exploring the flavor content of light and heavy-light pseudoscalars

The electroweak properties of light and charmed D and Ds pseudoscalar mesons are investigated within a unified covariant constituent quark model. The quark-antiquark-meson vertices are assumed to have a symmetric form by the exchange of quark momenta, which is successful in describing the light pseudoscalar meson properties. The flavor decomposition of the elastic electromagnetic form factors, electromagnetic charge radii, and weak decay constants are calculated. Based on the results a discussion on the SU(3) and SU(4) symmetry breaking is made and a comparison with the pion and kaon properties to highlight the Higgs contribution to the structure of these mesons.

hep-ph↗

arXiv2021

Electromagnetic form factors of baryons in nuclear medium

The electromagnetic structure of the baryons is modified in the nuclear medium. The modifications can be inferred from the comparison between the electromagnetic form factors in medium with the respective form factor in vacuum. Of particular interest is the ratio between the electric and magnetic form factors in medium ($G_E^*/G_M^*$) and vacuum ($G_E/G_M$) of the octet baryon. The deviation of the double ratios ($G_E^*/G_M^*)/(G_E/G_M)$ from unity measures the impact of the medium modification of the electromagnetic structure in a nuclear medium. Measurements of the double ratios $(G_E^*/G_M^*)/(G_E/G_M)$ for different nuclear densities may become available in a near future using the polarization-transfer method developed at Jefferson Lab. We present estimates of the double ratios of octet baryons based on a covariant constituent quark model, which takes into account pion cloud excitations of the baryon cores,for different nuclear densities. Our results manifest different features, namely, enhancement or quenching depending on the baryon flavor content.

hep-ph↗

arXiv2019

Pion off-shell electromagnetic form factors: data extraction and model analysis

We investigate the pion electromagnetic half off-shell form factors, which parametrize the matrix element of the charged pion electromagnetic current with one leg off-mass-shell and the other leg on-mass-shell, using an exactly solvable manifestly covariant model of a $(3+1)$ dimensional fermion field theory. The model provides a 3D imaging of the two off-shell pion form factors $F_1$ and $F_2$ as a function of $(Q^2,t)$, which are related to each other satisfying the Ward-Takahashi identity. The normalization of the renormalized charge form factor $F_1$ is fixed by $F_1(Q^2=0, t=m^2_π)=1$ while the other form factor $F_2$ vanishes, i.e. $F_2(Q^2, t=m^2_π)=0$ for any value of $Q^2$ due to the time-reversal invariance of the strong interaction. We define the new form factor $g(Q^2,t)=F_2(Q^2,t)/(t-m^2_π)$ and find that $g(Q^2,t)$ can be measurable in the on-mass-shell limit. In particular, $g(Q^2=0, t=m^2_π)$ is related with the pion charge radius. We also compare our form factors with the data extracted from the pion electroproduction reaction for both the off-shell region ($t<0$) and the on-shell limit ($t \rightarrow m_π^2$).

hep-ph↗

arXiv2019

Pion observables with the Minkowski Space Pion Model

The pion structure in Minkowski space is described in terms of an analytic model of the Bethe-Salpeter amplitude combined with Euclidean Lattice QCD results for the running quark mass. In the present work, a pion model previously proposed, which allows for a Nakanishi integral representation, is studied in order to verify the sensitivity of the pion electromagnetic form factor to small variations of the quark self-energy. In addition, we extend the previous work, providing the Nakanishi integral representation for the invariants associated with a decomposition of the pion Bethe-Salpeter amplitude.

hep-ph↗

arXiv2019

Octet baryon electromagnetic form factor double ratios $(G_E^\ast/G_M^\ast)/(G_E/G_M)$ in a nuclear medium

The ratios of the baryon electric ($G_E$) to magnetic ($G_M$) form factors, $G_E/G_M$, can provide us with important information on the structure of baryons in vacuum as demonstrated in recent studies for the proton and neutron systems. It has been argued that the corresponding in-medium ratios, $G_E^\ast/G_M^\ast$, can also provide useful information on the electromagnetic structure of baryons in a nuclear medium. Although the ratios may not be measured directly in experiments except for the proton at present, the double ratios $(G_E^\ast/G_M^\ast)/(G_E/G_M)$ can be directly related to the polarization-transfer measurements of bound baryons. In the present work we estimate, for the first time, the double ratios $(G_E^\ast/G_M^\ast)/(G_E/G_M)$ for all the members of octet baryons in symmetric nuclear matter in the range $Q^2=0$--3 GeV$^2$, where $Q^2=- q^2$ with $q$ being the four-momentum transfer.

hep-ph↗

arXiv2018

Rho-meson properties in medium

Properties of $ρ$-meson in symmetric nuclear matter are investigated in a light-front constituent quark model (LFCQM), using the in-medium inputs calculated by the quark-meson coupling (QMC) model. The LFCQM used in this study was already applied for the studies of the electromagnetic properties of $ρ$-meson in vacuum, namely, the charge~$G_0$, magnetic~$G_1$, and quadrupole~$G_2$ form factors, electromagnetic charge radius, and electromagnetic decay constant. We predict that the electromagnetic decay constant, charge radius, and quadrupole moment are enhanced as increasing the nuclear matter density, while the magnetic moment is slightly quenched. Furthermore, we predict that the value $Q^2_{\rm zero}$, which crosses zero of the charge form factor, $G_0(Q^2_{\rm zero})=0$ ($Q^2 = -q^2 > 0$ with $q$ being the four-momentum transfer), decreases as increasing the nuclear matter density.

hep-ph↗