Inclusive charge longitudinal response in finite nuclei

The experimental dynamic and static longitudinal structure factors of ¹²C, ⁴⁰Ca and ⁵⁶Fe are investigated with a semiclassical RPA theory.The ph interaction is linked at zero momentum to the nuclear compression modulus and symmetry energy. Its momentum evolution is handled phenomenologically, as well as the position-dependent nucleon effective mass.Quite reasonable values of these quantities allow a satisfactory account of the experimental data over a wide range of momentum transfers in all the three nuclei.In order to achieve this result the proton root mean square radius has to be increased of about 13% in ¹²C, 23% in ⁴⁰Ca and 21% in ⁵⁶Fe.     

Meson-exchange contributions to the nuclear charge operator

The role of the meson-exchange current correction to the nuclear charge operator is studied in electron scattering processes involving the excitation of medium and heavy nuclei to energies up to the quasi-elastic peak. The effect of these contributions in the quasi-free electron scattering process is a reduction of at most a 3% in the longitudinal response at the energy of the peak, a value which is below the experimental error and must not be taken into account in calculations in this energy region. On the other hand, the excitation of low-lying nuclear levels of neutronic character shows, with respect to the protonic ones, a considerable effect due to the inclusion of the two-body term in the charge operator. More realistic calculations, such as those performed in the random-phase approximation framework, give rise to a mixing of one particle-one hole configurations of both kinds which reduce these effects. However, it has been found that the excitation of some of these levels is sizably affected by the meson-exchange contribution. More precise experimental data concerning some of these states, such as e.g. the high-spin states in ²⁰⁸Pb, could throw some light in the problem of a more feasible determination of these effects and, as a consequence, could provide an alternative procedure to determine the charge neutron form factor.     

On the longitudinal charge response in the quasielastic peak region

The nuclear charge response in the quasielastic peak region is calculated in a semiclassical approach for moderate momentum transfers (q ≅ 1.5 fm⁻¹). Using the Gogny force and taking full account of the antisymmetrisation, reasonable agreement is found with (e, e′) experimental data in the case of ¹²C. This situation is, however, degrading as one goes to heavier nuclei. Arguments that this is due to the increasing influence of 2p-2h states are given.     

Compound nuclear and direct contributions to neutron polarisation

The factors which contribute to neutron scattering and polarisation are discussed in terms of the lifetimes of the compound states formed with the target nucleus. The limitations of the resonance and statistical theories are pointed out with regard to neutron scattering near 1 Mev. The polarisation to be expected from a semi-direct resonance in the presence of an average background of optical scattering is calculated. Attention is drawn to data in which such a phenomenon may occur.     

On the nuclear longitudinal charge response in the quasi-elastic peak region

We calculate in a semiclassical RPA approach the nuclear charge response of ¹²C, ⁴⁰Ca, and ⁵⁶Fe in the quasi-elastic peak region for moderate momentum transfers (q = 1.0–2.0 fm⁻¹). Using the Gogny force and taking full account of antisymmetrization effects we find with no free parameters good agreement with the (e, é) data for ¹²C. However for ⁴⁰Ca and ⁵⁶Fe the missing charge problem persists. Arguments that this may be due to strong influence of 2p-2h states in the isovector channel are advanced.     

The missing charge problem in the longitudinal response and the importance of two-particle-two-holes states

By using semiclassical methods, 2p–2h states are included into the RPA solution of the longitudinal response function in the quasielastic peak region. Consistent use of the finite range Gogny force has been made. An important effect of 2p–2h states is found which quenches and broadens the strength distribution by about 10 to 20% in closer agreement with the experiment.     

Diquark clustering and the neutron charge radius

We show that a quark-diquark model previously introduced to explainSU(6) violations in quasi-two-body reactions is also able to accomodate a (quantitative) interpretation of the negative charge radius of the neutron, provided one uses appropriate hypotheses for the confinements (c.q. sizes) of the quarks and diquarks involved. They effectively imply the existence of a nucleon core (i.e. a massive two-quark state of substantial spatial clustering) with zero spin and zero isospin.     

On neutron charge structure

The aim of the present work is to investigate the charge structure of neutron by calculating its root mean square charge radius r _n ^{2 1/2} and the corresponding charge form factor,G _n(q), within the general framework of quark-parton model. The neutron (the nucleon in general) is considered to be disc-like in conformity with the idea of Lorentz invariance. Using the proton charge radius, r _p ² =0·707 fm² as input, our calculations not only reproduce the observed negative sign correctly but also give reasonable value for the magnitude of r _n ² when compared with the experimental limits. –0·16r _n ² –0·11 (fm²). Predictiona are also made for the proton and the neutron charge form factors and the results compared with the data.One of us (D. P.) would like to express his gratitude to Professor A. N. Mitra for his interest and advice in the work, and to Dr. S. M. Mustafa for the warm hospitality at the University of Mosul.     

Proton and neutron occupation numbers forfp-shell nuclei

Spectroscopic factors for single-particle transfer are calculated from many-particle shell-model wave functions for 23 different transfer reactions onfp-shell nuclei. A comparison of the present theory with the available experimental data is very successful. Subshell occupation numbers of target ground states are studied in detail. Some systematic deficiencies for low seniority shell-model wave functions are revealed. In several cases the experimental information appears to be rather incomplete.     

Pion-exchange contributions to the nuclear charge,current, and Hamiltonian operators

Using a Foldy-Wouthuysen reduction procedure and the time-dependent perturbation theory, we have calculated the one-pion-exchange contribution to the nuclear charge and current densities to order $\text{(}\text{v}\text{c}\text{)}{}^2$. At the same time a renormalization scheme defines the pion-exchange potential including nonstatic and three-body correction terms of order $\text{(}\text{v}\text{c}\text{)}{}^2$. The requirements of relativity are investigated, and it is found that the retardation effect in the potential and charge density plays a critical role. The covariance conditions for the potential, charge, and current densities are shown to be satisfied to the order we have calculated. Finally, the deuteron case is discussed in detail. Throughout the calculation the role of the equivalence theorem is stressed.     

Proton decays and neutron oscillations in the rishon model

We analyze proton decays and neutron oscillations in the rishon model. Proton decay is forbidden in lowest order of the compositeness scale Λ_H. In a one-generation rishon model, the leading proton decays violate B?L (e.g., p→νπ⁺). The lifetime is given by τ_p～Λ_H⁸/(U_R²m_p⁷), where U_R～M(W_R) is the mass scale of the right-handed W boson and m_p is the proton mass. For U_R～500 GeV, the obtained limit for the compositeness scale is Λ_H?10⁸ GeV. Neutron oscillations are also proportional to Λ_H⁸ and cannot be detected in the foreseeable future. All of these predictions may depend strongly on the understanding of higher generations of quarks and leptons within the model.     

Δ-isobar contributions to the charge asymmetry of the three-nucleon bound states

Mechanisms of isospin violation in the nuclear force due to the Δ-isobar are studied. They arise from the coupling of the Δ to the photon and from the charge splitting of the Δ-mass. The charge-asymmetry contribution of the Δ to the ³He-³H binding-energy difference is calculated. Cancellations between different mechanisms are found yielding a small total result. The effect of the photon-induced Δ-excitation, traditionally considered, is negligible. The uncertainty in the Δ-mass splitting shows up in a sizeable theoretical uncertainty for the total result.     

Importance of longitudinal contributions to backward inelastic electron scattering

Coulomb contributions of multipolarity 2 are calculated for backward (θ = 180°) inelastic electron scattering by our improved DWBA code. General features are studied such as nuclear charge (Z) dependence, energy transfer dependence, momentum transfer dependence as well as the angle dependence near θ = 180°, with the help of a simple model for the transition charge density. We compare these features with PWBA and other simple approximations. We show, as a specific example, that the C2 contribution is comparable to the M1 and E2 multipole form factors for inelastic scattering to the rotational excited states of ¹⁸¹Ta.     

Polarization contributions to the spin dependence of the effective interaction in neutron matter

We calculate the modification of the effective interaction of particles on the Fermi surface due to polarization contributions, with particular attention to spin-dependent forces. In addition to the standard spin-spin, tensor, and spin-orbit forces, spin nonconserving effective interactions are induced by screening in the particle-hole channels. Furthermore, a novel long-wavelength tensor force is generated. We compute the polarization contributions to second order in the low-momentum interaction V(low k) and find that the medium-induced spin-orbit interaction leads to a reduction of the 3P2 pairing gap for neutrons in the interior of neutron stars.     

Proton spin-lattice relaxation and charge transfer in quinolinium-tetracyanoquinodimethane

The proton spin-lattice relaxation of selectively deuterated quinolinium-tetracyanoquinodimethan O(D₈) (TCNQ)₂ is reported. Its temperature dependence indicates a metallic state down to at least ～ 130°K. The magnitude of the relaxation rate, when compared to the rate of the non-deuterated compound indicates nearly complete charge transfer.     

Proton and neutron decay rates in conventional and supersymmetric guts

We present a general calculation of the two-body decay rates of the nucleon, for the most general form of four-fermion ΔB = ΔL operators, in the framework of the SU(6) non-relativistic quark model. We have applied our general formulas to Higgs mediated decays in conventional and in supersymmetric SU(5) models. Lower bounds upon the exchanged particles masses are given. We point out that the hierarchies of branching ratios in decays mediated by Higgs bosons are different from those of gauge boson decay modes (in the former case, neutrinos modes are dominant). We give, in conclusion, an experimental way to distinguish non-supersymmetric GUTs from supersymmetric ones, if the nucleon decays via Higgs bosons.