A. J. Buchmann

Electromagnetic properties of the Δ(1232)

We calculate the electromagnetic moments and radii of the Delta(1232) in the nonrelativistic quark model, including two-body exchange currents. We show that two-body exchange currents lead to nonvanishing Delta and N-->Delta transition quadrupole moments even if the wave functions have no D-state admixture. The usual explanation based on the single-quark transition model involves D-state admixtures but no exchange currents. We derive a parameter- free relation between the N-->Delta transition quadrupole moment and the neutron charge radius: Q(N-->Delta) = r^2(neutron)/sqrt(2). Furthermore, we calculate the M1 and E2 amplitudes for the process photon + N -->Delta. We find that the E2 amplitude receives sizeable contributions from exchange currents. These are more important than the ones which result from D-state admixtures due to tensor forces between quarks if a reasonable quark core radius of about 0.6 fm is used. We obtain a ratio of E2/M1=-3.5%.

The electromagnetic form factors of the deuteron in the quark cluster model

Abstract We evaluate the charge monopole, quadrupole and magnetic-dipole form factors and the tensorpolarization of the deuteron in a microscopic meson-quark cluster model. The deuteron wave function is derived from a microscopic six-quark hamiltonian which, in addition to a quadratic confinement potential includes the one-pion and one-gluon exchange potential between quarks. The electromagnetic current operators are constructed on the quark level, i.e., the photon is coupled directly to the quarks. Aside from the one-body impulse current, pionic and gluonic exchange current corrections are included. Due to the Pauli principle on the quark level, new quark interchange terms arise in the one-body and two-body current matrix elements that are not present in a nucleon level calculation. While these additional quark exchange currents are small for low momentum transfers, we find that they appreciably influence the electromagnetic structure of the deuteron beyond q = 5 fm−1.

On the flavour structure of the constituent quark

We discuss the dressing of constituent quarks with a pseudoscalar meson cloud within the effective chiral quark model. SU(3) flavor symmetry breaking effects are included explicitly. Our results are compared with those of the traditional meson cloud approach in which pions are coupled to the nucleon. The pionic dressing of the constituent quarks explains the experimentally observed violation of the Gottfried Sum Rule and leads to an enhanced nonperturbative sea of quark-antiquark pairs in the constituent quark and consequently in the nucleon. We find 2.5 times more pions and 10-15 times more kaons in the nucleon than in the traditional picture.

Intrinsic quadrupole moment of the nucleon

We address the question of the intrinsic quadrupole moment

Moscow-type NN potentials and three-nucleon bound states

{Q}_{0}

Nuclear matter with a Bose condensate of dibaryons in relativistic mean-field theory

of the nucleon in various models. All models give a positive intrinsic quadrupole moment for the proton. This corresponds to a prolate deformation. We also calculate the intrinsic quadrupole moment of the

Quark exchange currents in nuclei: Effective operator description

\ensuremath{\Delta}(1232).

Field-theoretical description of electromagnetic Δ resonance production and determination of the magnetic moment of the Δ+ (1232) resonance by the ep → e′N′π′γ′ and γp → N′π′γ′ reactions

All our models lead to a negative intrinsic quadrupole moment of the

Quadrupole moments of N and Delta in the 1 / N(c) expansion

{\ensuremath{\Delta}}^{+}

Electromagnetic properties of decuplet hyperons in a chiral quark model with exchange currents

corresponding to an oblate deformation.