Improved calculation of some meson exchange current contributions to the magnetic form factor of medium-heavy nuclei

The contribution of some meson exchange currents to the magnetic form factor of ⁴⁹Ti, ⁵¹V, ⁸⁷Sr and ⁹³Nb has been claimed to be important. We have reexamined those claims and conclude that previous models would not explain the experimentally large form factors near q = 3.0 fm^?1. Inclusion of short range NN correlations enhances these meson exchange current contribution by factors of 2.5 to 4.0, but further inclusion of meson nucleon vertex form factors goes in the wrong direction.     

Parity violation in helium and cesium and the effect of electron-electron interaction

We investigate the parity violating effect in helium and cesium atoms due to the exchange of a Z⁰-meson between atomic electrons. It is found that this effect is negligible for both atoms in comparison with the contribution from the electron-nucleus interactions, if we take the neutral-current coupling between electrons as predicted by the SU (2) × U (1) theory and sin²θ_W ≈ 0.3.     

n + p → d + γ in effective field theory

The radiative capture process n + p → d + γ provides clear evidence for meson exchange currents in nuclear physics. We compute this process at low energies using a recently developed power counting for the effective field theory that describes nucleon-nucleon interactions. The leading order contribution to this process comes from the photon coupling to the nucleon magnetic moments. At subleading order there are other contributions. Among these are graphs where the photon couples directly to pions, i.e. meson exchange currents. These diagrams are divergent and require the presence of a local four-nucleon-one-photon counterterm. The coefficient of this operator is determined by the measured cross section, σ^expt = 334.2±0.5 mb, for incident neutrons with speed |v| = 2200 m/s.     

The three-body force,off-shell πN scattering and binding energies in nuclear matter

We re-examine the off-shell πN amplitude occurring in the two-pion exchange three-body force, subject to all of the constraints of current algebra. This amplitude is not dominated by the Δ(1231) isobar; instead, if the σ-term is known, it can be determined from on-shell scattering. The resulting contribution to the binding energy of nuclear matter is small but attractive, varying from 0.2 MeV to 1.5 MeV, corresponding to πN σ-terms of 70 MeV to 40 MeV.     

Uniaxial magnetic anisotropy of rhombohedral CoCO3 crystals at T = 0 K

A method for calculating the contribution of exchange interaction to uniaxial anisotropy with the use of g’ factors has been worked out using CoCO₃ crystals as an example. The calculated contribution of dipole-dipole interactions to the anisotropy of CoCO₃ is 0.93 cm^?1. The sum of the contributions to the anisotropy constant of CoCO₃ with the inclusion of the dipole-dipole interactions is 36.1 cm^?1.     

Effect of meson exchange current and three-body force on the charge form factor of3He

We report a calculation of meson exchange current contribution to the charge form factor of³He by the hyper spherical harmonic expansion method with the inclusion of two-pion exchange three nucleon force. Results indicate that the charge form factor is changed appreciably in the right direction at high momentum transfer, due to the inclusion of the meson exchange current.     

The spectrum of theP-wave baryons and hadronic loops

In current QCD inspired analyses of the hadron spectrum nonperturbative quark loops are generally neglected. We study the validity of this assumption by estimating mass splittings and mixings induced by hadronic mass renormalization to the (70,1^?) baryon multiplet. All intermediate states in the loop composed of a ground state meson (0^?+, 1^??) and a ground state baryon (1/2⁺, 3/2⁺) are included. We use a³ P ₀ type model for the partial decay widths and spectral functions. Using unitarity and analyticity one finds a definite prediction for the masses and mixings, in good agreement with experiment, even when the perturbative one-gluon exchange is completely ignored. Our result thus allows for a smaller one-gluon exchange overall contribution, which would resolve the difficulty of the apparent smallness of the observed spin-orbit coupling. In particular if one adds the (nonperturbative) unitarity corrections to the one-gluon exchange contribution the gluon coupling constant can be reduced by a factor of 3 giving a reasonable value α_s≈0.3.     

Hard pions and axial meson exchange current effects in negative muon capture in deuterium

The contribution of the axial meson exchange current effects to the doublet transition rate in the reaction μ^? +d → 2n+ ν_μ is calculated by using the minimal, chiral and approximately gauge invariant Lagrangian model for the A₁ρπ system. The contribution from the ρ-π weak decay process current usually considered is found to be nearly cancelled by that from the A₁ pole graph which is prescribed by the underlying invariance principles. Correct treatment of the N¹ propagator in the N¹ excitation current of the pion range leads to ≈ 30 % suppression of the N¹ effect.     

Spin-orbit effects on Knight shift - a new contribution

We derive an expression for the Knight shift (K) including spin-orbit and many-body effects using fite temperature Green function method. We obtain a new contribution to K which is non-zero only when spin-orbit interaction is taken into account. For p-type PbTe, we find that this contribution is of the same order of magnitude as other contributions which have been considered in the past. The effect of exchange and correlation on this new contribution is discussed briefly.     

Asymptotic freedom,light quarks and the origin of the ΔT = 12 rule in the non-leptonic decays of strange particles

A dynamical mechanism for the $\text{ΔT =}\text{1}\text{2}$ rule in the non-leptonic decays of strange particles is considered. The weak interactions are described within the Weinberg-Salam model while the strong interactions are assumed to be mediated by the exchange of an octet of colour vector gluons. It is shown that the effect of the strong interactions gives rise to new operators in the effective Hamiltonian of weak interactions which contain both left- and right-handed fermions. These operators satisfy the $\text{ΔT =}\text{1}\text{2}$ rule, and estimates within the relativistic quark model indicate that their contribution dominates the physical amplitudes of the K → 2π, 3π decays.     

Two-body photodisintegration of 3He with realistic bound-state wave functions

Total and differential cross sections for two-body photodisintegration of ³He have been recalculated using realistic bound-state wave functions. Contrary to previous calculations we find neither any unusual structure in the energy distributions nor any distinct signature of D-state components in the bound-state wave functions for photon energies ω ≦ 35 MeV. Final-state interactions and, possibly, meson exchange currents increase the total cross section substantially, but decrease the E2 contribution from 1 % to 0.5 %. After subtraction of spurious non-orthogonal components in the wave function, the M1 contribution decreases by about one order of magnitude to a value of 1.5 % in agreement with the experimental data.     

π−A1 interference in πN → ϱN

An estimate of the A₁NN coupling is made using axial vector meson dominance of weak form factors, PCAC and current algebra. We use this to predict a vanishingly small polarisation due to unnatural parity exchanges in πN → ?N near the forward direction. In contrast, the contribution to πN → ωN of the Z (the J^PC = 2^?? exchange degenerate partner of the A₁) is found to be significant as indeed indicated by experiment.     

More α61gα terms in positronium ground state splitting

We reconsider the calculation of the contribution to the positronium ground state splitting from the iteration of one-photon direct and exchange diagrams. New α⁶1gα terms are found besides the previously known vacuum polarization contribution. The present situation in the comparison of theory and experiment is summarized.     

Quantum state transfer via a two-qubit Heisenberg XXZ spin model

Transfer of quantum states through a two-qubit Heisenberg XXZ spin model with a nonuniform magnetic field b is investigated by means of quantum theory. The influences of b, the spin exchange coupling J and the effective transfer time T=Jt on the fidelity have been studied for some different initial states. Results show that fidelity of the transferred state is determined not only by J, T and b but also by the initial state of this quantum system. Ideal information transfer can be realized for some kinds of initial states. We also found that the interactions of the z-component Jz and uniform magnetic field B do not have any contribution to the fidelity. These results may be useful for quantum information processing.     

Quark exchange effects on 3He and 3H charge densities

The exchange of quarks between nucleons bound in a nucleus leads to a definite and evaluable contribution to the nuclear charge density distribution. Although this exchange contribution is a consequence of finite nucleon size, it cannot be included by the conventional procedure of folding nucleon charge distributions with the density of point nucleons. For the A = 3 nuclei investigated here, the effects of quark exchange were found to be of significance for small distances away from the nuclear centre-of-mass.     

Electron paramagnetic resonance of the two-dimensional system Mn(CH3COO)2·4H2O

A single crystal Electron Paramagnetic Resonance (EPR) study of manganese acetate tetrahydrate was carried out at Q-band and room temperature. The EPR spectrum always reduces to a Lorentzian singlet with g=2.008. The maximum of linewidth observed along a¹ reveals the spin diffusion effects in this two-dimensional system. The maximum of linewidth observed along b reveals the exchange narrowed dipolar interactions in the linear trimers, as well as the contribution of the fine structure terms.     

Hara’s theorem and W—exchange in hyperon weak radiative decays

We reconsider Hara’s theorem and relate it to the well-known properties of β-decay. All assumptions necessary for the theorem to be true are explicitly formulated. Further, we study the W—exchange contribution to weak radiative decays and show that it does not violate Hara’s theorem. However, this contribution reveals the essential role of particle mixing in symmetry considerations and some peculiar features of gauge-invariant amplitudes under perturbative expansion. Together they explain an effect, which was treated as contradicting Hara’s theorem, without any violation. The properties of W—exchange we describe here may have more general importance and should be taken into account in further detailed calculations of weak processes.     

Eikonal description of nucleon-nucleon polarizations

Nucleon-nucleon elastic polarization data are analyzed within an eikonal framework in the range 6?plab?45 GeV/c and |t| 2.5 GeV/V². The isovector component is found to be dominated by a nearly exchange degenerate ρ-A₂ contribution while the isoscalar part requires both a lower lying Regge-pole exchange and an asymptotic pomeron contribution.