Pion Exchange Currents Research Articles

Inclusive electron scattering and pion degrees of freedom in light nuclei.

We demonstrate that realistic models of nuclear interactions and currents quantitatively reproduce the [alpha]-particle longitudinal and transverse responses as measured in inclusive electron scattering at intermediate energies. Pion degrees of freedom in both the nuclear interactions and currents play a crucial role in reproducing the experimental data. The charge-exchange character of the pion-exchange interaction leads to shifts of both longitudinal and transverse strength to higher excitation energies, thus producing a significant quenching of the response in the region of the quasielastic peak. However, in the transverse channel this mechanism is more than offset by the two-body pion-exchange currents required by gauge invariance, and hence the response is enhanced over the entire quasielastic spectrum. We relate these results to responses to idealized single-nucleon couplings and to quasielastic data from hadronic reactions on nuclei.

Multinucleon mechanisms in ( gamma,N) and ( gamma,NN) reactions.

The similarities in the experimental indications for multinucleon mechanisms in (\ensuremath{\gamma},p) and (e,e'p) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive (\ensuremath{\gamma},N) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behavior of the $^{12}\mathrm{C}$(\ensuremath{\gamma},p) and $^{12}\mathrm{C}$(\ensuremath{\gamma},n) data at high excitation energies in the residual nucleus. In the calculations, effects from nonresonant and resonant pion exchange currents are included. Photoabsorption on these currents is predicted to produce the major contributions to the exclusive $^{16}\mathrm{O}$(\ensuremath{\gamma},${\mathit{n}}_{0}$${)}^{15}$O process at photoenergies above the pion threshold. Double differential cross sections for photon induced pp and pn emission from $^{16}\mathrm{O}$ are calculated and compared with the data.

Exchange currents and nucleon magnetic moments

We investigate the effect of gluon and pion exchange currents on the magnetic moments of the nucleon using the constituent quark model. We also study the effect of scalar exchange currents connected with the confinement and sigma exchange potentials. We conclude that although individual exchange currents are quite large, the total exchange current contribution is relatively small, due to a cancellation of gluon, pion, and scalar exchange currents.

Gluon, pion and confinement exchange currents in the nucleon

We calculate the magnetic form factors of the nucleon in the constituent quark model, including gluon and pion exchange currents. Furthermore, we take into account the effect of configuration mixing. We also study the effect of the exchange current connected with the confinement potential.

Pion-exchange currents in photoproduction of baryon resonances

The need for pion-exchange currents consistent with quark models which respect chiral symmetry is presented. Applications to photopion production via the Δ(1232) and several N ∗ resonances indicate significant contribution from exchange currents. Possible improvements to the theory are discussed.

THE QUARK MODEL, DEUTERON FORM FACTORS AND NUCLEAR MAGNETIC MOMENTS

The study of the deuteron electromagnetic form factors based on the quark cluster model is reviewed. The deuteron wave function is derived from a microscopic quark Hamiltonian with the help of the Resonating Group Method. One-pion and one-gluon exchange potentials are included in addition to a quadratic confinement potential. The photon is coupled directly to the quarks. Aside from the one-body impulse current, pion and gluon exchange currents are included on the quark level. Due to the Pauli principle on the quark level, new electromagnetic currents arise which are not present on the nucleon level. These currents, called quark exchange currents, describe processes in which a photon couples to a quark or a pair of quarks interacting via gluon or pion exchange and which are accompanied by a simultaneous quark interchange between the two threequark clusters (nucleons). They are small for low momentum transfers but appreciably influence the electromagnetic structure of the deuteron beyond a momentum transfer of q=5 fm−1. The discussion is extended to the magnetic moments of 15N, 17O and 39K by introducing the quark exchange currents as effective operators on the nucleon level. The quark exchange currents written in terms of nonlocal and spin-isospin dependent nuclear operators are effective only at short distances. They are evaluated with shell-model (harmonic oscillator) wave functions including the (short-range) Brueckner correlations. The Bethe-Goldstone equation is solved with our effective NN potential, which is derived from a microscopic quark Hamiltonian. The quark exchange currents shift the isovector magnetic moment of 39K by −20% from its Schmidt value.

The axial current in the nuclear medium

A method is presented for constructing the part of the axial exchange current density operator that is associated with the nucleon-nucleon interaction directly from a given complete interaction model. The resulting two-body operator is then used to construct the general expression for the effective axial current density operator of a nucleon in the nuclear medium. In this way it is shown that the empirically found large enhancement of the effective axial charge (gACH) of a nucleon in heavy nuclei is due mainly to the contributions of the exchange current that is associated with the effective scalar component of the nucleon-nucleon interaction in addition to that of the pion exchange current. The empirically found substantial quenching of the effective axial current coupling constant (gAGT) is on the other hand mainly due to a combination of Δ33-resonance and second order configuration mixing effects.

Do proton-neutron pairs behave like quasideuterons in the photoabsorption process?

Calculations have been performed for the 16O(γ,pn) reaction in the photon-energy range E γ =50–300MeV. Effects due to both one-pion exchange currents and the Δ have been included. We discriminate between different couplings of the initial proton-neutron (pn) pair involved in the reaction. At lower photon energies where the pion exchange currents dominate the cross section it is found that pn pairs in the finite nucleus do not fully behave like a quasideuteron. Therefore, we feel that the factorized form of the (γ,pn) cross section should be used with reservation.

Gluon and pion exchange currents in the nucleon

We evaluate the nucleon charge form factor in the constituent quark model including gluon and pion exchange currents. In addition, we take into account the effect of configuration mixing. We find a large contribution of exchange currents in the neutron. For a pure (0s) 3 wave function we obtain a new relation which relates the neutron charge radius and the Δ and N masses to the quark core radius b, i.e. 〈r 2〉 n = −b 2(M Δ−M N ) M N .

Muon capture in deuterium and the meson exchange current effect

One-meson exchange current effect in reaction μ − + d → 2 n + v μ is calculated by using the weak axial current operator which satisfies the nuclear continuity equation up to the order 1 M 2 ( M is the nucleon mass). In the vector part of the weak nuclear interaction the vector-isovector pion exchange currents are also included. Their contribution is found to be non-negligible. The nuclear wave functions are generated from the Paris, Reid soft-core and Bonn potential models. Our result is compatible with the previous one, obtained with only the static axial exchange currents included.

Hard pion exchange currents and the backward deuteron disintegration

Relation between exchange currents and a potential is discussed in the framework of the Schroedinger-like scheme. Backward deuteron disintegration is studied using the Paris and Reid soft-core potentials and the static hard pion exchange currents up to the order ▪

Deuteron threshold electrodisintegration in a microscopic meson-quark cluster model

A microscopic meson-quark cluster model is applied to deuteron threshold electrodisintegration at backward angle. Not only gluon exchange between quarks but also pion exchange between quarks is incorporated into this model. In order to leave the equation of motion gauge invariant, electromagnetic two-body quark operators, that is, pion-exchange currents and gluon-exchange currents in addition to electromagnetic one-body quark operators are introduced. The effects of the quark exchange between the two clusters on the electromagnetic one-body current and two-body currents are investigated by putting particular emphasis upon consistency between potentials and currents.

B-->B gamma decays in a bag model for heavy-light-quark states.

The radiative decays of ${B}^{\mathrm{*}}$ mesons are studied using a confined Coulombic model for heavy-light-quark bags, which contains also the effects of pion-exchange currents. The calculated widths of the ${B}^{\mathrm{*}}$\ensuremath{\rightarrow}B\ensuremath{\gamma} decay modes come out smaller than in simple quark models and in other previous calculations. The consequences of a possible anomalous magnetic moment of the b quark are mentioned.

Effects of pion exchange currents and short-range correlation on charge form factor of4He

Effects of short-range correlation (SRC) and meson exchange currents (MEC) on elastic charge form factor of4He have been investigated. The one-pion exchange approximation is employed in the nonrelativistic limit. Form factors are calculated with SRC and MEC for a wide range of momentum transfer (up toq2=100 fm−2). It is noticed that SRC produces the first minimum and second maximum in good agreement with the experimental data. The inclusion of MEC does not improve the fit if not appreciably worsens it. However, the MEC does produce the second minimum and third maximum which SRC alone does not.

A consistent nonrelativistic quark model with dynamical pion exchange

Abstract We investigate the nonrelativistic quark model with a residual pion-quark interaction of the Weinberg-Schwinger type including p-wave and s-wave coupling. The pion-nucleon vertex is renormalised to third order in the pion-quark coupling constant ƒπ and adjusted to reproduce the empirical pion-nucleon coupling strength, leading to an axial charge gA = 1.42 and a value of ƒ π = 98.8 MeV. Including the pion exchange currents dynamically through one-pion loops, we reproduce the nucleon magnetic moments within less than 10% for any quark core radius 0.3 fm ⩽ rq ⩽ 0.7 fm. All these results change by less than 5% if the pion mass is varied from zero to its physical value.

Radiative and pionic decays of the D mesons and the magnetic moment of the charmed quark.

The radiative and pionic decays of the ${D}^{\mathrm{*}}$ mesons are studied using a quark-bag model which includes effects of pion-exchange currents. We find that uncertainties in current experiments are large enough to allow a significant value of a possible charmed-quark anomalous magnetic moment. Therefore, improved experiments would be useful. Other consequences of an anomalous magnetic moment are the following. The ratio of hadrons to ${\ensuremath{\mu}}^{+}$${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$ pairs produced in ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ annihilations should rise significantly as the center-of-mass energy increases from 40 GeV; the decay rate for \ensuremath{\psi}J\ensuremath{\rightarrow}${\ensuremath{\eta}}_{c}$+\ensuremath{\gamma} is reduced; and computed magnetic moments of charmed baryons are modified.

Gauge-invariant nuclear Compton amplitude manifesting low-energy theorems.

The electromagnetic field representation of the nuclear electromagnetic interaction is developed, by extending previous work on the nuclear current (one-photon amplitude) to the nuclear Compton amplitude. The resulting form of the Compton amplitude is gauge invariant and, in transverse gauge, manifests the low-energy theorems for the ${0}^{+}$\ensuremath{\rightarrow}${0}^{+}$ case, which is discussed in detail. Deuteron forward photodisintegration is also studied numerically, and it is shown that the generalized Siegert form of the retarded electric dipole interaction efficiently incorporates pion-exchange currents implicitly, while large explicit forms of these currents are needed in the standard representation of the interaction. Gauge invariance and Reiss's strong-field momentum-translation approximation are also discussed.

Possible role of six-quark clusters in deuteron electrodisintegration

Deuteron electrodisintegration cross sections at large angle are calculated for q 2 ⩽ (2GeV/ c) 2 within the framework of the hybrid quark-hadron model. Although six-quark cluster effects seem to be of the order of the data at q ≈ 1GeV/ c, impulse and pionic processes are dominant. Improved microscopic understanding of pion exchange currents is needed for further progress in using this reaction to study short-range phenomena. A second maximum is predicted for q 2 ≈ 50 fm −2.

Quark interchange effects in deuteron threshold electrodisintegration

The resonating group quark-cluster method is applied to deuteron threshold electrodisintegration at backward angles. We work in the momentum-space representation and restrict ourselves in numerical applications to the dominant two-nucleon cluster component. An analysis is made of the bare quark interchange effects as well as of the associated perturbative gluon exchange effects. We also consider a treatment within the quark-cluster method of the pion exchange currents. In order to describe the virtual gluon and pion effects we introduce an effective field-theory action for the quark-gluon-pion system. The problems of identifying the proper quark effects and of connecting the cluster method and the conventional Schroedinger equation wave functions are addressed phenomenologically. Numerical results are presented for the contributions of the bare quark interchange, of the gluon-recoil exchange corrections as well as of the wave-function normalization correction. We also give results of a numerical application to the pion exchange direct terms, which are the counterpart of the conventional meson exchange currents, and their associated quark interchange terms.

Electromagnetic pion pair current from quark models.

The pionic pair current is calculated from bag and constituent quark models which include a two-quark pion exchange potential. Except for form factors from the large Dirac-quark wave function, the result agrees with conventional nuclear theory. Gauge invariance is maintained. From the pion quark-pair and exchange currents inside a nucleon, a corresponding contribution to the proton and neutron magnetic moments is calculated.