Exchange currents and nucleon magnetic moments (original) (raw)

Nucleon magnetic moments in an extended chiral constituent quark model

2000

We present results for the nucleon magnetic moments in the context of an extended chiral constituent quark model based on the mechanism of the Goldstone boson exchange, as suggested by the spontaneous breaking of chiral symmetry in QCD. The electromagnetic charge-current operator is consistently deduced from the model Hamiltonian, which includes all force components for the pseudoscalar, vector and scalar meson exchanges. Thus, the continuity equation is satisfied for each piece of the interaction, avoiding the introduction of any further parameter. A good agreement with experimental values is found. The role of isoscalar two-body operators, not constrained by the continuity equation, is also investigated.

Note on the Magnetic Moment of the Nucleon

1942

The Goldberger-Treiman relation M = 2π/ √ 3 f cl π where M is the constituent quark mass in the chiral limit (cl) and f cl π the pion decay constant in the chiral limit predicts constituent quark masses of m u = 328.8±1.1 MeV and m d = 332.3±1.1 MeV for the up and down quark, respectively, when f cl π = 89.8 ± 0.3 MeV is adopted. Treating the constituent quarks as bare Dirac particles the following zero order values µ (0) p = 2.850 ± 0.009 and µ (0) n = −1.889 ± 0.006 are obtained for the proton and neutron magnetic moments, leading to deviations from the experimental data of 2.0% and 1.3%, respectively. These unavoidable deviations are discussed in terms of contributions to the magnetic moments proposed in previous work.

Baryon magnetic moments and the quark model reexamined

Physical Review D, 1996

The baryon magnetic moments are considered within the framework of the constituent quark model assuming charge symmetry d ϭϪ u /2. The baryons on the perimeter of the octet have two quarks of the same flavor, and a single quark of different flavor. The effective moments of the single flavor quarks were allowed to vary with I and Y. The resulting formulas predict eight magnetic moments in terms of four parameters. The average discrepancy is ͉͗ fit Ϫ expt ͉͘ϭ0.024 N .

Meson Exchange Current Contribution to the Deuteron Magnetic Moment

Progress of Theoretical Physics, 1984

Meson exchange current (MEC) contribution to the deuteron magnetic moment is reexamined on the basis of nucleon•nucleon (N-N) interaction model dependent effects and P7fY process. MEC contribution is found to be an order of magnitude smaller than the value obtained previously by Adler. It also strongly depends on the choice of the N-N interaction model, in particular on the percentage of D-state and the radius of the repulsive core.

Six-quark bag, exchange currents and trinucleon magnetic moments

Physics Letters B, 1986

The magnetic moments of 3H and 3He are reexamined in the Karl-Miller-Rafelski model of six-quark bag formation. Realistic three-nucleon wavefunctions are taken, and long-range one-pion exchange current corrections are included. It is concluded that the model is compatible with the data.

Nucleon magnetic moments in light-front models with quark mass asymmetries

Brazilian Journal of Physics, 2004

We show that the systematic inconsistence found in the simultaneous fit of the neutron and proton magnetics moments in light-front models, disappears when one allows an asymmetry in the constituent quark masses. The difference between the constituent quarks masses is an effective way to include in the nucleon model the effect of the attractive short ranged interaction in the singlet spin channel.

Baryon magnetic moments in the effective quark Lagrangian approach

Physical Review D, 2002

An effective quark Lagrangian is derived from first principles through bilocal gluon field correlators. It is used to write down equations for baryons, containing both perturbative and nonperturbative fields. As a result one obtains magnetic moments of octet and decuplet baryons without introduction of constituent quark masses and using only string tension as an input. Magnetic moments come out on average in reasonable agreement with experiment, except for nucleons and Σ − . The predictions for the proton and neutron are shown to be in close agreement with the empirical values once we choose the string tension such to yield the proper nucleon mass. Pionic corrections to the nucleon magnetic moments have been estimated. In particular, the total result of the twobody current contributions are found to be small. Inclusion of the anomalous magnetic moment contributions from pion and kaon loops leads to an improvement of the predictions.

Anomalous magnetic moment of quarks

Physical Review C, 1999

In the case of massless current quarks we find that the breaking of chiral symmetry usually triggers the generation of an anomalous magnetic moment for the quarks. We show that the kernel of the Ward identity for the vector vertex yields an important contribution. We compute the anomalous magnetic moment in several quark models. The results show that it is hard to escape a measurable anomalous magnetic moment for the quarks in the case of spontaneous chiral symmetry breaking.

Magnetic moment of hyperons in nuclear matter by using quark–meson coupling models

2009

We calculate the magnetic moments of hyperons in dense nuclear matter by using relativistic quark models. Hyperons are treated as MIT bags, and the interactions are considered to be mediated by the exchange of scalar and vector mesons which are approximated as mean fields. Model dependence is investigated by using the quark-meson coupling model and the modified quark-meson coupling model; in the former the bag constant is independent of density and in the latter it depends on density. Both models give us the magnitudes of the magnetic moments increasing with density for most octet baryons. But there is a considerable model dependence in the values of the magnetic moments in dense medium. The magnetic moments at the nuclear saturation density calculated by the quark-meson coupling model are only a few percents larger than those in free space, but the magnetic moments from the modified quark-meson coupling model increase more than 10% for most hyperons. The correlations between the bag radius of hyperons and the magnetic moments of hyperons in dense matter are discussed.