arXiv subjects

M. T. Pena

Publications and source records attributed to M. T. Pena.

31 records · Page 2Linked to original sources

arXiv2005

Charged and neutral pion production in the S-matrix approach

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 chiral perturbation theory piN scattering amplitude is used. For the nucleon-nucleon distortions a newly developed realistic potential within the Bonn family of potentials, valid well above the pion production threshold, is considered. In the pi+ production case, the NN potential, the piN relative p-waves and the treatment of the exchanged pion energy build up the observed cross-section strength.

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arXiv2005

Neutral and charged pion production in the S-matrix approach

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 chiral perturbation theory rescattering amplitude is employed. The contributions from the single-nucleon, Z-diagrams and Delta-isobar mechanisms are also included. For the reactions investigated, the convergence of the partial wave series is quite satisfactory and J=0 is enough to describe the data close to threshold.

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arXiv2004

Relativistic NN scattering without partial wave decomposition

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 state components are included in that equation. We develop a method, based on the Padé method, to solve the Spectator equation without partial wave decomposition. The convergence of the partial wave decomposition series is tested as a function of the energy. The on- and off-mass-shell amplitudes are calculated. The NN interaction was fitted to the differential cross section for NN scattering, in the energy range below the pion production threshold.

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arXiv2004

Pion re-scattering operator in the S-matrix approach

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. We show that this construction is equivalent to the on-shell approximation introduced in previous papers. For a realistic NN interaction, the S-matrix approach, and its simplified fixed threshold-kinematics version, work well near threshold.

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arXiv2003

Meson-Meson Interactions and Resonances in the 't Hooft Model

We studied meson-meson interactions using the 't Hooft model, which represents QCD in 1+1 dimensions and assumes a large number of colors ($N_c$). The dominant interactions in this large $N_c$ limit are generated by quark exchange. Our results show that QCD in 1+1 dimensions allows the realization of a constituent-type quark model for the mesons, and generates a scalar ``$σ$''-like meson-meson resonance, whose effective coupling and mass are determined by the underlying QCD dynamics. These results suggest an interpretation of the lightest scalar mesons as $q\bar{q} q \bar{q}$ systems.

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arXiv2003

Pion re-scattering in pi0 production

This work discusses the approximations frequently used to calculate the contribution to pion production in proton-proton scattering near threshold from irreducible pion re-scattering. The reference result is obtained from the quantum mechanical reduction of the corresponding Feynman diagram to Time-Ordered Field Theory diagrams. We conclude that the Distorted Wave Born Approximation is quite adequate at threshold energies and even above. We discuss the choices for the energy of the exchanged pion, both for its propagator and for the piN amplitude describing the re-scattering vertex. The calculation employs a physical model for nucleons and pions -- pseudo-vector coupling for the piNN vertex and realistic amplitudes for the piN re-scattering and for the NN transitions in the initial and final states.

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arXiv2003

Electromagnetic interactions of three-body systems in the covariant spectator theory

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 spectator theory. We show that the equations obtained are fully consistent with bound-state equations and the normalization condition previously derived for the covariant three-body bound state, and that the results conserve current.

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arXiv2003

Short-range three-nucleon forces and low-energy constants

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 cancellation between pi-sigma and pi-omega forces is rather sensitive to the values of the coupling constants and to the form of the piNN vertex. Experimental uncertainties in the parameters of the TM pi-pi potential are assessed. They lead to uncertainties in theoretical predictions of the triton binding energy of about 0.4 MeV. The low-energy limit of pi-sigma and pi-omega potentials is performed. It defines the coupling constants of effective contact piNNNN vertices, which are compared with the corresponding contact vertices of Chiral Perturbation Theory.

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arXiv2003

Pion re-scattering in pi0 production near threshold

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 calculation employs a pseudo-vector coupling for the pi NN vertex and realistic amplitudes for the pi N re-scattering and the NN transitions.

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arXiv2000

Off-Mass-Shell $π$N Scattering and $pp \to pp π^0$

We adapt the off-shell $π$N amplitude of the Tucson-Melbourne three-body force to the half-off-shell amplitude of the pion rescattering contribution to $pp \to pp π^0$ near threshold. This {\em pion} rescattering contribution, together with the impulse term, provides a good description of the data when the half-off-shell amplitude is linked to the phenomenological invariant amplitudes obtained from meson factory $π$N scattering data.

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arXiv2000

eta d scattering in the region of the S11 resonance

We have studied the reaction eta d -> eta d close to threshold within a nonrelativistic three-body formalism. We considered several eta N and NN models, in particular potentials with separable form, fitted to the low-energy eta N and NN data to represent the two-body interactions. We found that with realistic two-body interactions a quasibound state does not exist in this system, although there is an enhancement of the cross section by one order of magnitude, in the region near threshold, which is a genuine three-body effect not predicted within the impulse approximation.

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arXiv1996

The Dynamical Structure of the Delta-Resonance and its Effect on Two- and Three-Nucleon Systems

The pion-nucleon interaction in the P33 partial wave is assumed to proceed simultaneously through the excitation of the Delta-isobar and through a phenomenologically introduced non-resonant background potential. The introduction of the background potential allows a more realistic parameterization of the pion-nucleon-Delta vertex compared with the previously used one without background. It also modifies the propagation of the Delta-isobar in the nuclear medium and gives rise to novel effective nucleon-Delta interactions. Their consequences on predictions for observables in the two-nucleon system at intermediate energies and in the three-nucleon bound state are studied.

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arXiv1993

Momentum and Coordinate Space Three-nucleon Potentials

In this paper we give explicit formulae in momentum and coordinate space for the three-nucleon potentials due to $ρ$ and $π$ meson exchange, derived from off-mass-shell meson-nucleon scattering amplitudes which are constrained by the symmetries of QCD and by the experimental data. Those potentials have already been applied to nuclear matter calculations. Here we display additional terms which appear to be the most important for nuclear structure. The potentials are decomposed in a way that separates the contributions of different physical mechanisms involved in the meson-nucleon amplitudes. The same type of decomposition is presented for the $π- π$ TM force: the $Δ$, the chiral symmetry breaking and the nucleon pair terms are isolated.

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