224 citations · 1.6k across the 72 of their papers we have counts for
28 papers · 1 filter
form factors in the timelike regime
G. Ramalho, M. T. Peña
The covariant spectator quark model, tested before in a variety of electromagnetic baryon excitations, is applied here to the reaction in the timelike r…
Electromagnetic Structure of Few-Nucleon Ground States
L. E. Marcucci, F. Gross, M. T. Pena +6
Experimental form factors of the hydrogen and helium isotopes, extracted from an up-to-date global analysis of cross sections and polarization observables measured in elastic elect…
Singularity-free two-body equation with confining interactions in momentum space
Alfred Stadler, Sofia Leitão, M. T. Peña +1
We are developing a covariant model for all mesons that can be described as quark-antiquark bound states in the framework of the Covariant Spectator Theory (CST) in Minkowski space…
Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction
I. G. Aznauryan, A. Bashir, V. Braun +24
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 up…
The shape of the baryon in a covariant spectator quark model
G. Ramalho, M. T. Pena, A. Stadler
Using a covariant spectator quark model that describes the recent lattice QCD data for the electromagnetic form factors and all available experimental data on transi…
Fixed-axis polarization states: covariance and comparisons
Franz Gross, G. Ramalho, M. T. Pena
Addressing the recent criticisms of Kvinikhidze and Miller, we prove that the spectator wave functions and currents based on ``fixed-axis'' polarization states (previously introduc…
A pure S-wave covariant model for the nucleon
Franz Gross, G. Ramalho, M. T. Pena
Using the manifestly covariant spectator theory, and modeling the nucleon as a system of three constituent quarks with their own electromagnetic structure, we show that all four nu…
Charged and neutral pion production in the S-matrix approach
V. Malafaia, M. T. Pena, Ch. Elster +1
The S-matrix approach is used to calculate charged as well as neutral pion production reactions from NN scattering, with the same set of underlying processes and interactions. The…
Neutral and charged pion production in the S-matrix approach
V. Malafaia, M. T. Pena, J. Adam
The S-matrix approach is applied to charged and neutral pion production in pp collisions, in which the spin/isospin channels filter differently the rescattering mechanism. The chir…
Pion re-scattering operator in the S-matrix approach
V. Malafaia, J. Adam, M. T. Pena
The pion re-scattering operator for pion production, derived recently in time-ordered perturbation theory, is compared with the one following from the simple S-matrix construction.…
Relativistic NN scattering without partial wave decomposition
G. Ramalho, A. Arriaga, M. T. Pena
We consider the covariant Spectator equation with a One Boson Exchange (OBE) kernel, and apply it to the NN system. Relativistic effects such as retardation and negative-energy sta…
Meson-Meson Interactions and Resonances in the 't Hooft Model
Z. Batiz, M. T. Pena, A. Stadler
We studied meson-meson interactions using the 't Hooft model, which represents QCD in 1+1 dimensions and assumes a large number of colors (). The dominant interactions in this…
Pion re-scattering in pi0 production
V. Malafaia, M. T. Pena
This work discusses the approximations frequently used to calculate the contribution to pion production in proton-proton scattering near threshold from irreducible pion re-scatteri…
Electromagnetic interactions of three-body systems in the covariant spectator theory
Franz Gross, Alfred Stadler, M. T. Pena
We derive a complete Feynman diagram expansion for the elastic form factor and the three-body photo and electrodisintegration of the three-body bound state using the covariant spec…
Short-range three-nucleon forces and low-energy constants
J. Adam, M. T. Pena, A. Stadler
The 3N forces due to pi-rho, pi-sigma and pi-omega exchanges following from the nucleon Born diagrams and diagrams with an intermediate N^{*}(1440) are re-analyzed. The cancellatio…
Pion re-scattering in pi0 production near threshold
V. Malafaia, M. T. Pena
We compare the most used approximations to calculate the contribution from pion re-scattering mechanisms to pion production from proton-proton scattering, near threshold. Our calcu…
Relativistic effects and quasipotential equations
G. Ramalho, A. Arriaga, M. T. Peña
We compare the scattering amplitude resulting from the several quasipotential equations for scalar particles. We consider the Blankenbecler-Sugar, Spectator, Thompson, Erkelenz-Hol…
Off-Mass-Shell N Scattering and
M. T. Pena, S. A. Coon, J. Adam +1
We adapt the off-shell N amplitude of the Tucson-Melbourne three-body force to the half-off-shell amplitude of the pion rescattering contribution to near thresho…
Relativistic NN scattering equations without partial-wave decomposition
G. Ramalho, A. Arriaga, M. T. Peña
Relativistic quasi-potential equations describing NN scattering are compared. Within the spectator formalism a cancellation is seen to occur between retardation and negative-energy…
Multiplicity distributions associated to subthreshold events in heavy-ion collisions
J. Dias de Deus, M. T. Peña, J. C. Seixas
Subthreshold events (pion production, for instance) at energies are treated as rare events. The associated multiplicity distribution and the unconstrained distri…
Nonexistence of a quasibound state
H. Garcilazo, M. T. Peña
We have solved the Faddeev equations for elastic scattering using realistic separable interactions for the and coupled - subsystems. We found that including expli…
The Reaction and the eta-Nucleon and Nucleon-Nucleon Interactions
M. T. Peña, H. Garcilazo, D. O. Riska
The , and exchange contributions to the cross-section for the reaction near threshold are calculated with a phenomenological description of the intermediate…
Nucleon Resonance Effects in near Threshold
M. T. Peña, D. O. Riska, A. Stadler
The role of the low lying nucleon resonances beyond the in the reaction near threshold is shown to be numerically significant by a calculation, which takes…
Two-pion exchange and strong form-factors in covariant field theories
G. Ramalho, A. Arriaga, M. T. Peña
In this work improvements to the application of the Gross equation to nuclear systems are tested. In particular we evaluate the two pion exchange diagrams, including the crossed-bo…