arXiv subjects

G. Ramalho

Publications and source records attributed to G. Ramalho.

At least 55 records · Page 3Linked to original sources

arXiv2015

Rôle of the pion electromagnetic form factor in the $Δ(1232) \to γ^\ast N$ timelike transition

The $Δ(1232) \to γ^\ast N$ magnetic dipole form factor ($G_M^\ast$) is described here within a new covariant model that combines the valence quark core together with the pion cloud contributions. The pion cloud term is parameterized by two terms: one connected to the pion electromagnetic form factor, the other to the photon interaction with intermediate baryon states. The model can be used in studies of pp and heavy ion collisions. In the timelike region this new model improves the results obtained with a constant form factor model fixed at its value at zero momentum transfer. At the same time, and in contrast to the Iachello model, this new model predicts a peak for the transition form factor at the expected position, i.e. at the $ρ$ mass pole. We calculate the decay of the $Δ\to γN$ transition, the Dalitz decay ($Δ\to e^+ e^- N$), and the $Δ$ mass distribution function. The impact of the model on dilepton spectra in pp collisions is also discussed.

hep-ph↗

arXiv2015

A covariant model for the negative parity resonances of the nucleon

We present a model for the $γ^\ast N \to N^\ast$ helicity amplitudes, where $N$ is the nucleon and $N^\ast$ is a negative parity nucleon excitation, member of the $SU(6)$-multiplet $[70,1^-]$. The model combines the results from the single quark transition model for the helicity amplitudes with the results of the covariant spectator quark model for the $γ^\ast N \to N^\ast(1535)$ and $γ^\ast N \to N^\ast(1520)$ transitions. With the knowledge of the amplitudes $A_{1/2}$ and $A_{3/2}$ for those transitions we calculate three independent coefficients defined by the single quark transition model and make predictions for the helicity amplitudes associated with the $γ^\ast N \to N^\ast(1650)$, $γ^\ast N \to N^\ast(1700)$, $γ^\ast N \to Δ(1620)$, and $γ^\ast N \to Δ(1700)$ transitions. In order to facilitate the comparison with future experimental data at high $Q^2$, we provide also simple parametrizations for the amplitudes, compatible with the expected falloff at high $Q^2$.

hep-ph↗

arXiv2015

A covariant model for the nucleon spin structure

We present the results of the covariant spectator quark model applied to the nucleon structure function $f(x)$ measured in unpolarized deep inelastic scattering, and the structure functions $g_1(x)$ and $g_2(x)$ measured in deep inelastic scattering using polarized beams and targets ($x$ is the Bjorken scaling variable). The nucleon is modeled by a valence quark-diquark structure with $S,P$ and $D$ components. The shape of the wave functions and the relative strength of each component are fixed by making fits to the deep inelastic scattering data for the structure functions $f(x)$ and $g_1(x)$. The model is then used to make predictions on the function $g_2(x)$ for the proton and neutron.

hep-ph↗

arXiv2014

Using the Single Quark Transition Model to predict nucleon resonance amplitudes

We present predictions for the $γ^\ast N \to N^\ast$ helicity amplitudes, where $N^\ast$ is a member of the $[70,1^-]$ supermultiplet. We combine the results from the single quark transition model for the helicity amplitudes with the results of the covariant spectator quark model for the $γ^\ast N \to N^\ast(1535)$ and $γ^\ast N \to N^\ast(1520)$ transitions. The theoretical estimations from the covariant spectator quark model are used to calculate three independent functions $A,B$, and $C$ of $Q^2$, where $Q^2=-q^2$ and $q$ is the momentum transfer. With the knowledge of the functions $A,B$, and $C$ we estimate the helicity amplitudes for the transitions $γ^\ast N \to N^\ast(1650)$, $γ^\ast N \to N^\ast(1700)$, $γ^\ast N \to Δ(1620)$, and $γ^\ast N \to Δ(1700)$. The analysis is restricted to reactions with proton targets. The predictions for the transition amplitudes are valid for $Q^2 > 2$ GeV$^2$.

hep-ph↗

arXiv2014

Valence quark and meson cloud contributions to the $γ^\ast N \to N^\ast(1520)$ form factors

The covariant spectator quark model is applied to the $γ^\ast N \to N^\ast(1520)$ reaction. The valence quark contributions to the electromagnetic form factors and helicity transition amplitudes are estimated from the covariant structure of the $N^\ast(1520)$ wave function calibrated by the experimental data for large squared momentum transfer $Q^2$. The difference between the model results and the experimental data is then used to parameterize the low $Q^2$ behavior, where meson cloud effects are assumed to dominate. This parametrization can be very useful for future studies of the reaction, as well as for the extension of the transition form factors to the timelike region.

hep-ph↗

arXiv2014

$γ^\ast N \to N(1710)$ transition at high momentum transfer

In a relativistic quark model we study the structure of the $N(1710)$ resonance, and the $γ^\ast N \to N(1710)$ reaction focusing on the high momentum transfer region, where the valence quark degrees of freedom are expected to be dominant. The $N(1710)$ resonance, a state with spin 1/2 and positive parity ($J^P = \frac{1}{2}^+$), can possibly be interpreted as the second radial excitation of the nucleon, after the Roper, $N(1440)$. We calculate the $γ^\ast N \to N(1710)$ helicity amplitudes, and predict that they are almost identical to those of the $γ^\ast N \to N(1440)$ reaction in the high momentum transfer region. Thus, future measurement of the helicity amplitudes for the $γ^\ast N \to N(1710)$ reaction can give a significant hint on the internal structure of the $N(1710)$ state.

hep-ph↗

arXiv2014

A covariant model for the $γ^\ast N \to N^\ast(1520)$ reaction

We apply the covariant spectator quark model to the study of the electromagnetic structure of the $N^\ast(1520)$ state ($J^{P}= \frac{3}{2}^-$), an important resonance from the second resonance region in both spacelike and timelike regimes. The contributions from the valence quark effects are calculated for the $γ^\ast N \to N^\ast(1520)$ helicity amplitudes. The results are used to parametrize the meson cloud dominant at low $Q^2$.

hep-ph↗

arXiv2013

What is the role of the meson cloud in the $Σ^{*0} \to γΛ$ and $Σ^\ast \to γΣ$ decays?

We study the effect of the meson cloud dressing in the octet baryon to decuplet baryon electromagnetic transitions. Combining the valence quark contributions from the covariant spectator quark model with those of the meson cloud estimated based on the flavor SU(3) cloudy bag model, we calculate the transition magnetic form factors at $Q^2=0$ ($Q^2=-q^2$ and $q$ the four-momentum transfer), and also the decuplet baryon electromagnetic decay widths. The result for the $γ^\ast Λ\to Σ^{\ast 0}$ decay width is in complete agreement with the data, while that for the $γ^\ast Σ^+ \to Σ^{\ast +}$ is underestimated the data by 1.4 standard deviations. This achievement may be regarded as a significant advance in the present theoretical situation.

hep-ph↗

arXiv2013

Octet to decuplet electromagnetic transition in a relativistic quark model

We study the octet to decuplet baryon electromagnetic transitions using the covariant spectator quark model, and predict the transition magnetic dipole form factors for those involving the strange baryons. Utilizing SU(3) symmetry, the valence quark contributions are supplemented by the pion cloud dressing based on the one estimated in the $γ^\ast N \to Δ$ reaction. Although the valence quark contributions are dominant in general, the pion cloud effects turn out to be very important to describe the experimental data. We also show that, other mesons besides the pion in particular the kaon, may be relevant for some reactions such as $γ^\ast Σ^+ \to Σ^{*+}$, based on our analysis for the radiative decay widths of the strange decuplet baryons.

hep-ph↗

arXiv2013

Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction

Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCD-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the non-perturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

nucl-th↗

arXiv2012

Covariant spectator quark model description of the $γ^\ast Λ\to Σ^0$ transition

We study the $γ^\ast Λ\to Σ^0$ transition form factors by applying the covariant spectator quark model. Using the parametrization for the baryon core wave functions as well as for the pion cloud dressing obtained in a previous work, we calculate the dependence on the momentum transfer squared, $Q^2$, of the electromagnetic transition form factors. The magnetic form factor is dominated by the valence quark contributions. The final result for the transition magnetic moment, a combination of the quark core and pion cloud effects, turns out to give a value very close to the data. The pion cloud contribution, although small, pulls the final result towards the experimental value The final result, $μ_{ΛΣ^0}= -1.486 μ_N$, is about one and a half standard deviations from the central value in PDG, $μ_{ΛΣ^0}= -1.61 \pm 0.08 μ_N$. Thus, a modest improvement in the statistics of the experiment would permit the confirmation or rejection of the present result. It is also predicted that small but nonzero values for the electric form factor in the finite $Q^2$ region, as a consequence of the pion cloud dressing.

hep-ph↗

arXiv2012

Octet baryon electromagnetic form factors in nuclear medium

We study the octet baryon electromagnetic form factors in nuclear matter using the covariant spectator quark model extended to the nuclear matter regime. The parameters of the model in vacuum are fixed by the study of the octet baryon electromagnetic form factors. In nuclear matter the changes in hadron properties are calculated by including the relevant hadron masses and the modification of the pion-baryon coupling constants calculated in the quark-meson coupling model. In nuclear matter the magnetic form factors of the octet baryons are enhanced in the low $Q^2$ region, while the electric form factors show a more rapid variation with $Q^2$. The results are compared with the modification of the bound proton electromagnetic form factors observed at Jefferson Lab. In addition, the corresponding changes for the bound neutron are predicted.

hep-ph↗

arXiv2012

The shape of the $Δ$ baryon in a covariant spectator quark model

Using a covariant spectator quark model that describes the recent lattice QCD data for the $Δ$ electromagnetic form factors and all available experimental data on $γN \to Δ$ transitions, we analyze the charge and magnetic dipole distributions of the $Δ$ baryon and discuss its shape. We conclude that the quadrupole moment of the $Δ$ is a good indicator of the deformation and that the $Δ^+$ charge distribution has an oblate shape. We also calculate transverse moments and find that they do not lead to unambiguous conclusions about the underlying shape.

nucl-th↗

arXiv2012

Timelike gamma* N -> Delta form factors and Delta Dalitz decay

We extend a covariant model, tested before in the spacelike region for the physical and lattice QCD regimes, to a calculation of the gamma* N -> Delta reaction in the timelike region, where the square of the transfered momentum, q^2, is positive (q^2>0). We estimate the Dalitz decay Delta -> Ne+e- and the Delta distribution mass distribution function. The results presented here can be used to simulate the NN -> NNe+e- reactions at moderate beam kinetic energies.

hep-ph↗

arXiv2012

Proceedings of the second International PrimeNet Workshop

These are the proceedings of the second PrimeNet Workshop, held in September 26-28, 2011, at the campus of Forschungszentrum Jülich, Germany. This workshop is part of the activities in the project "Study of Strongly Interacting Matter" (acronym HadronPhysics2), which is an integrating activity of the Seventh Framework Program of EU. This HP2 project contains several activities, one of them being the network PrimeNet having the focus on Meson Physics in Low-Energy QCD. This network is created to exchange information on experimental and theoretical ongoing activities on mainly eta and eta-prime physics at different European accelerator facilities and institutes.

nucl-ex↗

arXiv2012

Valence quark and meson cloud contributions for the gamma* Lambda -> Lambda* and gamma* Sigma0 -> Lambda* reactions

We estimate the valence quark contributions for the gamma* Y -> Lambda* (Y=Lambda, Sigma0) electromagnetic transition form factors. We focus particularly on the case Lambda*=Lambda(1670) as an analog reaction with gamma* N -> N(1535). The results are compared with those obtained from chiral unitary model, where the Lambda* resonance is dynamically generated and thus the electromagnetic structure comes directly from the meson cloud excitation of the baryon ground states. The form factors for the case Y=Sigma0 in particular, depend crucially on the two real phase (sign) combination, a phase between the Lambda and Lambda* states, and the other, the phase between the Lambda and Sigma0 radial wave functions. Depending on the combination of these two phases, the form factors for the gamma* Sigma0 -> Lambda* reaction can be enhanced or suppressed. Therefore, there is a possibility to determine the phase combination by experiments.

hep-ph↗

arXiv2012

Covariant nucleon wave function with S, D, and P-state components

Expressions for the nucleon wave functions in the covariant spectator theory (CST) are derived. The nucleon is described as a system with a off-mass-shell constituent quark, free to interact with an external probe, and two spectator constituent quarks on their mass shell. Integrating over the internal momentum of the on-mass-shell quark pair allows us to derive an effective nucleon wave function that can be written only in terms of the quark and diquark (quark-pair) variables. The derived nucleon wave function includes contributions from S, P and D-waves.

hep-ph↗

arXiv2012

Spin and angular momentum in the nucleon

Using the covariant spectator theory (CST), we present the results of a valence quark-diquark model calculation of the nucleon structure function f(x) measured in unpolarized deep inelastic scattering (DIS), and the structure functions g1(x) and g2(x) measured in DIS using polarized beams and targets. Parameters of the wave functions are adjusted to fit all the data. The fit fixes both the shape of the wave functions and the relative strength of each component. Two solutions are found that fit f(x) and g1(x), but only one of these gives a good description of g2(x). This fit requires the nucleon CST wave functions contain a large D-wave component (about 35%) and a small P-wave component (about 0.6%). The significance of these results is discussed.

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