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A. Arriaga

Publications and source records attributed to A. Arriaga.

12 recordsLinked to original sources
arXiv2023

Heavy baryon spectroscopy in a quark-diquark approach

We report progress on calculations of the heavy-light baryons $Σ_c$ and $Λ_c$ and their excitations with $J^P = 1/2^+$ using functional methods. We employ a covariant quark-diquark approach, where the interaction amounts to a quark exchange between quarks and diquarks and the ingredients are determined from the quark level. A partial-wave analysis reveals the presence of orbital angular momentum components in terms of p waves, which are non-relativistically suppressed.

hep-ph↗
arXiv2019

Many-body effects in (p,pN) reactions within a unified approach

We study knockout reactions with proton probes within a theoretical framework where {\it ab initio} Quantum Monte Carlo wave functions are combined with the Faddeev/Alt-Grassberger-Sandhas few-body reaction formalism. New Quantum Monte Carlo wave functions are used to describe $^{12}$C, yielding, for the first time, results consistent with the experimental point rms radii, electron scattering data and (p,2p) total cross sections data. Our results for $\mathrm{A}\leq 12$ and $(N-Z) \leq 3$ nuclei show that the theoretical ratios between the (i) {\it ab initio} and Mean Field Approximation theoretical cross sections, $\mathcal{R}_σ$, (ii) corresponding ratios between the spectroscopic factors, $\mathcal{R}_Σ$, summed over states below particle emission, depend moderately on the nucleon separation energy S$_{\rm N}$. These ratios are determined by a delicate interplay between the radii of the parent and the residual nuclei and the nucleon separation energy, and were found to be always smaller for the knockout of the more correlated deficient species nucleon. In the case of the symmetric $^{12}$C nucleus, the theoretical ratios still appear to indicate that protons are more correlated than neutrons.

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arXiv2007

A relativistic calculation of the deuteron threshold electrodisintegration at backward angles

The threshold electrodisintegration of the deuteron at backward angles is studied with a relativistic Hamiltonian, including a relativistic one-pion-exchange potential (OPEP) with off-shell terms as predicted by pseudovector coupling of pions to nucleons. The bound and scattering states are obtained in the center-of-mass frame, and then boosted from it to the Breit frame, where the evaluation of the relevant matrix elements of the electromagnetic current operator is carried out. The latter includes, in addition to one-body, also two-body terms due to pion exchange, as obtained, consistently with the OPEP, in pseudovector pion-nucleon coupling theory. The full Lorentz structure of these currents is retained. In order to estimate the magnitude of the relativistic effects we perform, for comparison, the calculation with a non-relativistic phase-equivalent Hamiltonian and the standard non-relativistic expressions for the one-body and two-body pion-exchange currents. Our results for the electrodisintegration cross section show that, in the calculations using one-body currents, relativistic corrections become significant ({\it i.e.}, larger than 10%) only at high momentum transfer $Q$ ($Q^2 \simeq 40$fm$^{-2}$ and beyond). However, the inclusion of two-body currents makes the relativistic predictions considerably smaller than the corresponding non-relativistic results in the $Q^2$ region (18--40) fm$^{-2}$. The calculations based on the relativistic model also confirm the inadequacy, already established in a non-relativistic context, of the present electromagnetic current model to reproduce accurately the experimental data at intermediate values of momentum transfers.

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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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arXiv2003

Quadratic momentum dependence in the nucleon-nucleon interaction

We investigate different choices for the quadratic momentum dependence required in nucleon-nucleon potentials to fit phase shifts in high partial-waves. In the Argonne v18 potential L**2 and (L.S)**2 operators are used to represent this dependence. The v18 potential is simple to use in many-body calculations since it has no quadratic momentum-dependent terms in S-waves. However, p**2 rather than L**2 dependence occurs naturally in meson-exchange models of nuclear forces. We construct an alternate version of the Argonne potential, designated Argonne v18pq, in which the L**2 and (L.S)**2 operators are replaced by p**2 and Qij operators, respectively. The quadratic momentum-dependent terms are smaller in the v18pq than in the v18 interaction. Results for the ground state binding energies of 3H, 3He, and 4He, obtained with the variational Monte Carlo method, are presented for both the models with and without three-nucleon interactions. We find that the nuclear wave functions obtained with the v18pq are slightly larger than those with v18 at interparticle distances < 1 fm. The two models provide essentially the same binding in the light nuclei, although the v18pq gains less attraction when a fixed three-nucleon potential is added.

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arXiv2001

Relativistic effects and quasipotential equations

We compare the scattering amplitude resulting from the several quasipotential equations for scalar particles. We consider the Blankenbecler-Sugar, Spectator, Thompson, Erkelenz-Holinde and Equal-Time equations, which were solved numerically without decomposition into partial waves. We analyze both negative-energy state components of the propagators and retardation effects. We found that the scattering solutions of the Spectator and the Equal-Time equations are very close to the nonrelativistic solution even at high energies. The overall relativistic effect increases with the energy. The width of the band for the relative uncertainty in the real part of the scattering $T$ matrix, due to different dynamical equations, is largest for backward-scattering angles where it can be as large as 40%.

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arXiv1998

Two-pion exchange and strong form-factors in covariant field theories

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-box diagram, using models developed within the spectator-on-mass-shell covariant formalism. We found that the form factors used in these models induce spurious contributions that violate the unitary cut requirement. We tested then some alternative form-factors in order to preserve the unitarity condition. With this new choice, the difference between the exact and the spectator-on-mass-shell amplitudes is of the order of the one boson scalar exchange, supporting the idea that this difference may be parameterized by this type of terms.

nucl-th↗
arXiv1998

Quantum Monte Carlo Studies of Relativistic Effects in Light Nuclei

Relativistic Hamiltonians are defined as the sum of relativistic one-body kinetic energy, two- and three-body potentials and their boost corrections. In this work we use the variational Monte Carlo method to study two kinds of relativistic effects in the binding energy of 3H and 4He. The first is due to the nonlocalities in the relativistic kinetic energy and relativistic one-pion exchange potential (OPEP), and the second is from boost interaction. The OPEP contribution is reduced by about 15% by the relativistic nonlocality, which may also have significant effects on pion exchange currents. However, almost all of this reduction is canceled by changes in the kinetic energy and other interaction terms, and the total effect of the nonlocalities on the binding energy is very small. The boost interactions, on the other hand, give repulsive contributions of 0.4 (1.9) MeV in 3H (4He) and account for 37% of the phenomenological part of the three-nucleon interaction needed in the nonrelativistic Hamiltonians.

nucl-th↗
arXiv1996

Femtometer Toroidal Structures in Nuclei

The two-nucleon density distributions in states with isospin $T=0$, spin $S$=1 and projection $M_S$=0 and $\pm$1 are studied in $^2$H, $^{3,4}$He, $^{6,7}$Li and $^{16}$O. The equidensity surfaces for $M_S$=0 distributions are found to be toroidal in shape, while those of $M_S$=$\pm$1 have dumbbell shapes at large density. The dumbbell shapes are generated by rotating tori. The toroidal shapes indicate that the tensor correlations have near maximal strength at $r<2$ fm in all these nuclei. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron, the quasi-deuteron model, and the $dp$, $dd$ and $αd$ $L$=2 ($D$-wave) components in $^3$He, $^4$He and $^6$Li. The toroidal distribution has a maximum-density diameter of $\sim$1 fm and a half-maximum density thickness of $\sim$0.9 fm. Many realistic models of nuclear forces predict these values, which are supported by the observed electromagnetic form factors of the deuteron, and also predicted by classical Skyrme effective Lagrangians, related to QCD in the limit of infinite colors. Due to the rather small size of this structure, it could have a revealing relation to certain aspects of QCD.

nucl-th↗
arXiv1995

Three-body correlations in few-body nuclei

A detailed comparison of Faddeev and variational wave functions for $^3$H, calculated with realistic nuclear forces, has been made to study the form of three-body correlations in few-body nuclei. Three new three-body correlations for use in variational wave functions have been identified, which substantially reduce the difference with the Faddeev wave function. The difference between the variational upper bound and the Faddeev binding energy is reduced by half, to typically $<2\%$. These three-body correlations also produce a significant lowering of the variational binding energy for $^4$He and larger nuclei.

nucl-th↗
arXiv1994

Full determination of transition matrix elements in the $\vec{d} (\vec{p},pp)n$ reaction

It is shown that in the $d(p,pp)n$ reaction with a colinear geometry and the two outgoing protons in a singlet state, the only two independent T-matrix elements can be determined from spin correlation experiments. These matrix elements are strongly dependent on the deuteron helicity amplitudes, which are expected to vanish for $1.4fm^{-1}<p<1.7fm^{-1}$.\vspace{10 mm}\\

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