Gluon-exchange currents: The nucleon axial-vector coupling constant and magnetic moments in bag models

Possible numerical importance of the two-body currents arising from exchange of one hard gluon between two constituent quarks is investigated by studying the evaluation of the nucleon axial-vector coupling constantg _A and magnetic moments μ _p ^tot , μ _n ^tot in the context of the MIT bag model. The double counting problem is resolved only to the extent that, in accord with the standard wisdom, hard gluons have nothing to do with the formation of bags (quark confinement) and so are not included in bag-model wave functions. For reasonable choices of the parameters, we find that (1) the contribution tog _A arising from one-gluon-exchange currents (OGEC) could account for the discrepancy between the observed value and the one-body bag-model prediction, and that (2) whereas the OGEC contribution to μ _n ^tot is negligible, the contribution to μ _p ^tot could be important but does not allow further understanding of the observed μ _p /μ _n ratio.     

Exchange magnetic moments

Exchange magnetic moments are investigated by adopting a phenomenological approach and a field theoretical model. Their contributions are estimated for nuclei with a particle or hole outside a closed shell. They are expressed in terms of corrections of gyromagnetic factors.     

Isoscalar currents and nuclear magnetic moments

We investigate meson-exchange current effects on isoscalar nuclear magnetic moments taking a one-boson exchange model. For some cases we find appreciable contributions, though the results are very model dependent. We also discuss the connection between exchange currents and the calculation of magnetic moments in the relativistic σ-ω model.     

Current quark-mass dependence of nucleon magnetic moments and radii

A calculation of the current quark-mass dependence of nucleon static electromagnetic properties is necessary in order to use observational data as a means to place constraints on the variation of Nature’s fundamental parameters. A Poincaré-covariant Faddeev equation, which describes baryons as composites of confined quarks and nonpointlike diquarks, is used to calculate this dependence. The results indicate that, like observables dependent on the nucleons’ magnetic moments, quantities sensitive to their magnetic and charge radii, such as the energy levels and transition frequencies in hydrogen and deuterium, might also provide a tool with which to place limits on the allowed variation in Nature’s constants.     

Ground state baryon magnetic moments and nucleon axial vector coupling

Ground-state baryon magnetic moments and nucleon axial vector coupling are calculated usingqcd inspired configuration mixing and relativistic corrections. Unlike earlier attempts, we incorporate a natural mass scale for quarks, taken as one third the nucleon mass for up and down quarks, and the strange quark mass suggested by the Lipkin’s sum rule. In the parameter-free non-relativistic limit, we find a fairly good fit, which improves upon including relativistic corrections.     

Spherical magnetic multipole moments system of currents

Concepts of spherical magnetic multipoles that represent distributions of electric currents over a spherical surface are introduced. Vector potentials of magnetic multipoles meet solenoidal- and harmonic-field conditions outside of the spherical surface and are continuous on it. Within the sphere, the vector potential of currents flowing outside of it is represented by the sum of vector potentials of basis magnetic multipoles with coefficients expressed by spherical multipole moments of system of currents. This expansion of the vector potential is in many respects analogous to the multipole expansion known from electrostatics. The first three terms of the expansion represent components of the well-known magnetic moment, the next five terms represent components of the magnetic quadrupole moment, etc. Possible applications of the magnetic spherical multipole technique are discussed. Krasnoyarsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 66–72, October, 1999.     

Test of a sum rule for the nucleon magnetic moments

It is shown that the original version of the sum rule relating the isovector part of the nucleon magnetic moment to aπ N scattering amplitude is not consistent with the experimental data. However there is good agreement for the corrected version ofMaiani andPreparata.     

Exchange magnetic moments of some odd-mass nuclei

The exchange magnetic moment has been calculated for the nuclei ⁹¹Zr, ⁸⁹Sr and ⁸⁷Kr in the ground state. By adding the core polarization contribution to the magnetic moments computed earlier, an improved value is obtained for the magnetic moments of these nuclei.     

Pion mass dependence on nucleon magnetic moments in the extended linear sigma model

The dependence of the nucleon magnetic moments on the pion mass is studied in the framework of the extended linear sigma model. The field equations have been solved in the mean-field approximation. A remarkable agreement is observed in comparison with other models.     

Axial exchange currents and nucleon spin

We calculate the axial couplings g_A⁸(0) and g_A⁰(0) related to the spin of the nucleon in a constituent quark model. In addition to the standard one-body axial currents, the model includes two-body axial exchange currents. The latter are necessary to satisfy the Partial Conservation of Axial Current (PCAC) condition. For both axial couplings we find significant corrections to the standard quark model prediction. Exchange currents reduce the valence quark contribution to the nucleon spin and afford an interpretation of the missing nucleon spin as orbital angular momentum carried by nonvalence quark degrees of freedom.     

Weak nucleon currents in beta decay and muon capture

We made extensive investigations on the weak charged nucleon currents and structure of complex nuclei in weak nuclear processes. Using our new formalism, which has almost no approximation as far as the lepton part is concerned, we have studied the beta-ray spectra and beta-ray angular distributions in the¹²B and¹²N beta decays. The weak magnetism predicted by CVC is in excellent agreement with the experimental data on beta-ray spectra given by the Columbia and Heidelberg groups. It is noticed here that the difference _– – ₊ of the coefficients in the beta-ray angular distributions in aligned¹²B and¹²N is free from ambiguity of the nuclear model. We found practically no second-class induced tensor current with the data given by the Osaka, Louvain and Zürich groups. On the other hand, the sum _– + ₊ of the coefficients represents the time component of the main axial vector current, and it is free from weak form factors. With this sum we studied the exchange-current and core-polarization effects. These two effects are large, and they are almost cancelled out by each other in this case. We also found the strength of the induced pseudoscalar form factor which is generally consistent with PCAC, from nuclear polarizations of¹²B in muon capture of¹²C performed by the Zürich and Tokyo groups. These nuclear polarizations are insensitive to the nuclear model, if the exchange-current and corepolarization effects are properly taken into account.This paper was partially delivered at the Annual Meeting of the Korean Physical Society, Seoul, 1983; the Asia Pacific Physics Conference, Singapore, 1983; and the International Symposium on Nuclear Spectroscopy and Nuclear Interactions, Osaka, 1984.The present paper is based on our recent theoretical investigations, which have been performed by the theory group, Department of Physics, Osaka Unicersity. The author would like to express his sincere thanks to Professors H. Ohtsubo, Y. Yokoo, Drs. T. Sato, K. Koshigiri, M. Nishimura, M. Kobayashi, and Mr. M. Fukui for their collaboration, to Professors C.W. Kim, Y.K. Lee and K. Kubodera for their stimulating discussions during his 1983 summer visit to Johns Hopkins University, to Professors K. Sugimoto, T. Minamisono, V.L. Telegdi, L. Grenacs, T. Yamazaki and K. Nagamine for sending him their experimental results before publication. Numerical calculations were performed with the aid of NEAC ACOS 1000, Computor Center, Osaka University. This work is partly supported by Grant-in-Aid for Scientific Research, The Ministry of Education, Science, and Culture.This paper is dedicated to Professor K. Sugimoto in commemoration of his sixtieth birthday.     

Upper bounds on the fine structure constant and on nucleon magnetic moments in terms of Compton scattering cross sections

We derive and compare with experimental data the bound

$\text{α??λm}{}_{\mathrm{<mtext>p</mtext>}}\text{?m}{}_{\mathrm{<mtext>p</mtext>}}\text{ν}{}^2{}_1\text{2π}{}^2\text{∫}\text{ν}{}_0\text{∞}\text{d}\text{ν′σ}{}_{\mathrm{<mtext>tot</mtext>}}\text{(ν′)}\text{(ν′}{}^2\text{+ν}{}^2{}_1\text{)}\text{+2πm}{}_{\mathrm{<mtext>p</mtext>}}\text{∫}\text{ν}{}_0\text{∞}\text{ν′}{}^2\text{d}\text{ν′σ}{}_{\mathrm{<mtext>tot</mtext>}}\text{(ν′)}\text{(ν′}{}^2\text{+ν}{}^2{}_1\text{){ν′}{}^2\text{(}\text{d}\text{σ}\text{d}\text{t)}{}_0\text{+πλ}{}^2\text{+2ν′|λ|}\text{π}\text{(}\text{d}\text{σ}\text{d}\text{t)}{}_0\text{?}\text{σ}{}^2{}_{\mathrm{<mtext>tot</mtext>}}\text{16π}\text{}}{}^{\mathrm{?1}}$

, where α is the fine-structure constant, m_p the proton mass, ν₀ the photo-pion production threshold, σ_tot and $\text{(}\text{d}\text{σ}\text{d}\text{t}\text{)}{}_0$ are the unpolarized total hadronic photo-absorption cross section on protons and the unpolarized forward differential cross section for proton Compton scattering at photon-lab energy ν′, and λ and ν₁ are any real numbers. We derive similar bounds on proton and neutron magnetic moments.     

The second moments of quark and gluon distribution functions in nucleon

The second moments of quark and gluon distribution functions in proton are calculated on the basis of the QCD sum rule approach. The results obtained are in a good agreement with the experimental data.     

Photocouplings of nucleon resonances with gluon and pion exchange currents

Photocouplings for the nucleon resonances most relevant to total photoabsorption are calculated in a constituent quark model including gluon and pion exchange. The sensitivity of the results to the different ingredients of the model is discussed.     

Interaction between the nucleon and muon currents in elastic neutrino-nucleon interactions

The spin relations of neutrino-nucleon interaction with the formation of a muonv(V) N N are investigated within the framework of the two-component and four-component theories of two neutrinos under the assumption of a point neutrino. The difference between the predictions of the two theories is identified. It is shown that the four-component theory of two neutrinos, unlike the two-component theory, accommodates the V + A variant of interaction between the muon current (v) and the barion current.The authors are grateful to Prof. A. A. Sokolov and to R. Sh. Yakh'yaev for evaluating the results.     

Quadrupole moments and nucleon excitation strengths in even-A Sm isotopes

We present a coherent coupled-channel analysis of 7 MeV neutron and 16 MeV proton elastic and inelastic scattering from ^{148, 152, 154}Sm. The optical potential and nuclear deformation parameters are determined so as to fit not only these elastic and inelastic scattering data but also the low-energy neutron scattering properties and the total cross sections over a wide energy range. This analysis provides evidence of the same excitation strengths for both projectiles in the case of ^{152, 154}Sm, and of a smaller excitation strength for the proton than for the neutron in case of ¹⁴⁸Sm. Moreover the quadrupole moments of these deformed optical potentials are in good agreement with those extracted from Coulomb excitation measurements and from nuclear matter distribution calculations.     

Dressed-quark anomalous magnetic moments

Perturbation theory predicts that a massless fermion cannot possess a measurable magnetic moment. We explain, however, that the nonperturbative phenomenon of dynamical chiral symmetry breaking generates a momentum-dependent anomalous chromomagnetic moment for dressed light quarks, which is large at infrared momenta, and demonstrate that consequently these same quarks also possess an anomalous electromagnetic moment with similar magnitude and opposite sign.     

Giant magnetic moments in dilute magnetic semiconductors

The concentration dependence of the specific magnetic moment value at room temperature in dilute semiconductor titanium oxides doped with either Co or Fe has been investigated. This value was found to increase sharply at small concentrations of magnetic impurity. The magnetic moment of 22.9 μ_B per impurity atom has been revealed for TiO₂ doped with 0.15 at% Co, not yet reported in any semiconductor oxide systems. We conclude the observed giant magnetic moments are caused by the crystal lattice polarization at small impurity concentrations. The comparison with published data point to different types of the magnetization concentration dependence for various semiconductor matrixes that is probably related to the dielectric permittivity of the environment.