arXiv subjects

M. Fiolhais

Publications and source records attributed to M. Fiolhais.

28 records · Page 2Linked to original sources

arXiv2002

Roper Electroproduction Amplitudes in a Chiral Confinement Model

A description of the Roper using the chiral chromodielectric model is presented and the transverse $A_{1/2}$ and the scalar $S_{1/2}$ helicity amplitudes for the electromagnetic Nucleon--Roper transition are obtained for small and moderate $Q^2$. The sign of the amplitudes is correct but the model predictions underestimate the data at the photon point. Our results do not indicate a change of sign in any amplitudes up to $Q^2\sim1$ GeV$^2$. The contribution of the scalar meson excitations to the Roper electroproduction is taken into account but it turns out to be small in comparison with the quark contribution. However, it is argued that mesonic excitations may play a more prominent role in higher excited states.

hep-ph↗

arXiv2001

Excitation of non-quark degrees of freedom in N*

We study the contribution of glueball and sigma-meson degrees of freedom to nucleon excited states in the framework of a chiral version of the chromodielectric model. We have found that these degrees of freedom considerably lower the energy of the Roper and may substantially weaken the electroexcitation amplitudes for the N(1440) and in particular for the N(1710).

hep-ph↗

arXiv2001

N^* electroproduction amplitudes in a model with dynamical confinement

The Roper resonance is described in a chiral version of the chromodielectric model as a cluster of three quarks in radial-orbital configuration (1s)$^2$(2s)$^1$, surrounded by $π$ and $σ$-meson clouds and by a chromodielectric field which assures quark dynamical confinement. Radial profiles for all fields are determined self-consistently for each baryon. Transverse $A_{1/2}$ and scalar $S_{1/2}$ helicity amplitudes for the nucleon-Roper transition are calculated. The contribution of glueball and $σ$-meson vibrations is estimated; although small for N(1440), the $σ$ contribution can be large for N(1710).

hep-ph↗

arXiv2001

Pseudospin symmetry as a relativistic dynamical symmetry in the nucleus

Pseudospin symmetry in nuclei is investigated by solving the Dirac equation with Woods-Saxon scalar and vector radial potentials, and studying the correlation of the energy splittings of pseudospin partners with the nuclear potential parameters. The pseudospin interaction is related to a pseudospin-orbit term that arises in a Schroedinger-like equation for the lower component of the Dirac spinor. We show that the contribution from this term to the energy splittings of pseudospin partners is large. The near pseudospin degeneracy results from a significant cancelation among the different terms in that equation, manifesting the dynamical character of this symmetry in the nucleus. We analyze the isospin dependence of the pseudospin symmetry and find that its dynamical character is behind the different pseudospin splittings observed in neutron and proton spectra of nuclei.

nucl-th↗

arXiv2001

The Isospin Asymmetry in the Pseudo-spin Dynamical Symmetry

Pseudo-spin symmetry in nuclei is investigated considering the Dirac equation with a Lorentz structured Woods-Saxon potential. The isospin correlation of the energy splittings of pseudo-spin partners with the nuclear potential parameters is studied. We show that in an isotopic chain, the pseudo-spin symmetry is better realized for neutrons than for protons. This behavior comes from balance effects amongst the central nuclear potential parameters. In general, we found an isospin asymmetry of the nuclear pseudo-spin interaction, opposed to the nuclear spin-orbit interaction which is quasi isospin symmetric.

nucl-th↗

arXiv2000

The electroproduction of the $Δ$(1232) in the chiral quark-soliton model

We calculate the ratios E2/M1 and C2/M1 for the electroproduction of the $Δ$(1232) in the region of photon virtuality $0<-q^2<1$ GeV$^2$. The magnetic dipole amplitude M1 is also presented. The theory used is the chiral quark-soliton model, which is based in the instanton vaccum of the QCD. The calculations are performed in flavor SU(2) and SU(3) taking rotational ($1/N_c$) corrections into account. The results for the ratios agree qualitatively with the available data, although the magnitude of both ratios seems to underestimate the latest experimental results.

hep-ph↗

arXiv1998

On the relativistic L-S coupling

The fact that the Dirac equation is linear in the space and time derivatives leads to the coupling of spin and orbital angular momenta that is of a pure relativistic nature. We illustrate this fact by computing the solutions of the Dirac equation in an infinite spherical well, which allows to go from the relativistic to the non-relativistic limit by just varying the radius of the well.

physics.ed-ph↗

arXiv1996

The role of the pion cloud in electroproduction of the $Δ$(1232)

We calculate the ratios $E2/M1$ and $C2/M1$ of the multipole amplitudes for electroproduction of the $Δ$(1232) in the range of photon virtuality $0<-K^2<1$~GeV$^2$ in a chiral chromodielectric model and a linear $σ$-model. We find that relatively large experimental values can be explained in terms of the pion contribution alone; the contribution arising from d-state quark admixture remains below 10\%. We describe the pion cloud as a coherent state and use spin and isospin projection to obtain the physical nucleon and the $Δ$. The $A_{1/2}$ and $A_{3/2}$ amplitudes are reasonably well reproduced in the $σ$-model; in the chromodielectric model, however, they are a factor of two too small.

hep-ph↗

arXiv1995

E2 and C2 amplitudes for electroproduction of $Δ$

We compute the amplitudes for the electromagnetic $γ_v N \to Δ$ transition in the framework of soliton models with quarks and mesons. The ratios E2/M1 and C2/M1 as functions of the photon four-momentum squared are dominated by the contribution of the pion cloud and are in reasonable agreement with the available experimental data.

hep-ph↗

arXiv1995

Counting pions in the nucleon

The number of pions in the nucleon is a theoretical concept and its value is model dependent. It is, however, a useful tool to understand the pionic contribution to several nucleon observables such as electric charge radii, magnetic moments, axial charge, pion-nucleon coupling constant, amplitudes for electroexcitation of baryon resonances and electroproduction of pions above such resonances.

hep-ph↗