arXiv subjects

A. De Pace

Publications and source records attributed to A. De Pace.

69 records · Page 4Linked to original sources

arXiv1997

Delta excitation in K^+-nucleus collisions

We present calculations for Δexcitation in the (K^+,K^+) reaction in nuclei. The background from quasielastic K^+ scattering in the Δregion is also evaluated and shown to be quite small in some kinematical regions, so as to allow for a clean identification of the Δexcitation strength. Nuclear effects tied to the Δrenormalization in the nucleus are considered and the reaction is shown to provide new elements to enrich our knowledge of the Δproperties in a nuclear medium.

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arXiv1997

A statistical approach to the theory of the mean field

We develop a statistical theory of the mean field. It is based on the proposition that the mean field can be obtained as an energy average. Moreover, it is assumed that the matrix elements of the residual interaction are random with the average value of zero. Explicit expressions for the mean field and the fluctuation away from the average are obtained. The fluctuation is expanded in terms of more and more complex excitations. Using the randomness of the matrix elements one can then obtain formulas for the contribution to the error from each class of complex excitations and a general condition for the convergence of the expansion is derived. Making some simplifying assumptions a schematic model is developed and applied to the problem of nuclear matter. It yields a measure of the strength of the effective interaction. The latter turns out to be three orders of magnitude less than that calculated using a potential which gives a binding energy of about -7 MeV/nucleon demonstrating the strong damping of the interaction strength induced by the averaging process.

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arXiv1997

A statistical theory of the mean field

A statistical theory of the mean field is developed. It is based on the proposition that the mean field can be obtained as an energy average. Moreover, it is assumed that the matrix elements of the residual interaction, obtained after the average interaction is removed, are random with the average value of zero. With these two assumptions one obtains explicit expressions for the mean field and the fluctuation away from the average. The fluctuation is expanded in terms of more and more complex excitations. Using the randomness of the matrix elements one can then obtain formulas for the contribution to the error from each class of complex excitations and a general condition for the convergence of the expansion is derived. It is to be emphasized that no conditions on the nature of the system being studied are made. Making some simplifying assumptions a schematic model is developed. This model is applied to the problem of nuclear matter. The model yields a measure of the strength of the effective interaction. It turns out to be three orders of magnitude less than that calculated using a potential which gives a binding energy of about -7 MeV/nucleon demonstrating the strong damping of the interaction strength induced by the averaging process.

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arXiv1997

Quasielastic K-nucleus scattering

Quasielastic K^+ - nucleus scattering data at q=290, 390 and 480 MeV/c are analyzed in a finite nucleus continuum random phase approximation framework, using a density-dependent particle-hole interaction. The reaction mechanism is consistently treated according to Glauber theory, keeping up to two-step inelastic processes. A good description of the data is achieved, also providing a useful constraint on the strength of the effective particle-hole interaction in the scalar-isoscalar channel at intermediate momentum transfers. We find no evidence for the increase in the effective number of nucleons participating in the reaction which has been reported in the literature.

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arXiv1996

Probing Nucleon Strangeness with Neutrinos: Nuclear Model Dependences

The extraction of the nucleon's strangeness axial charge, Delta_s, from inclusive, quasielastic neutral current neutrino cross sections is studied within the framework of the plane-wave impulse approximation. We find that the value of Delta_s can depend significantly on the choice of nuclear model used in analyzing the quasielastic cross section. This model-dependence may be reduced by one order of magnitude when Delta_s is extracted from the ratio of total proton to neutron yields. We apply this analysis to the interpretation of low-energy neutrino cross sections and arrive at a nuclear theory uncertainty of plus/minus 0.03 on the value of Delta_s expected to be determined from the ratio of proton and neutron yields measured by the LSND collaboration. This error compares favorably with estimates of the SU(3)-breaking uncertainty in the value of Delta_s extracted from inclusive, polarized deep-inelastic structure function measurements. We also point out several general features of the quasielastic neutral current neutrino cross section and compare them with the analogous features in inclusive, quasielastic electron scattering.

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arXiv1995

Relativistic Hamiltonians in many-body theories

We discuss the description of a many-body nuclear system using Hamiltonians that contain the nucleon relativistic kinetic energy and potentials with relativistic corrections. Through the Foldy-Wouthuysen transformation, the field theoretical problem of interacting nucleons and mesons is mapped to an equivalent one in terms of relativistic potentials, which are then expanded at some order in 1/m_N. The formalism is applied to the Hartree problem in nuclear matter, showing how the results of the relativistic mean field theory can be recovered over a wide range of densities.

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arXiv1995

Parity-violating longitudinal response

The parity-violating quasielastic electron scattering response is explored within the context of a model that builds antisymmetrized random phase approximation and Hartree-Fock correlations on a relativistic Fermi gas basis. Particular emphasis is put on the weak-neutral longitudinal response function, since this observable displays a strong sensitivity to isospin correlations: specifically, it is shown how, through a diagrammatic cancellation/filtration mechanism, this response acts as a magnifier of pionic correlations in the nuclear medium. The parity-violating longitudinal response function also displays appreciable sensitivity to the electric strangeness content of the nucleon, thus making quasielastic electron scattering a possible candidate to measure the nucleon electric strange form factor at relatively high momentum transfers. Finally, we discuss how observables, related to the asymmetry, can be constructed to disentangle the nuclear and nucleonic effects.

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arXiv1995

Parity-violating longitudinal response

The longitudinal quasielastic parity-violating electron scattering response is explored within the context of a model that builds antisymmetrized RPA-HF correlations on a relativistic Fermi gas basis. The large sensitivity to nuclear dynamics of this observable, found in previous studies where only pionic correlations were included, is shown to survive in the present model where the effects from pion, rho, sigma and omega exchange in a version of the Bonn potential are incorporated. Through an intricate diagrammatic cancellation/filtration mechanism the longitudinal parity-violating response turns out to be close to the one obtained in first-order perturbation theory with the pion alone. Finally, in accord with our previous work, the parity-violating response is seen to display appreciable sensitivity to the electric strangeness content of the nucleon, especially at high momentum transfer.

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arXiv1995

Glauber isovector spin responses for (p,n) reactions at 494 MeV

The separated isovector spin-longitudinal and spin-transverse responses from a recent (p,n) quasifree experiment at 494 MeV are analyzed in a Glauber theory framework up to two-step contributions. Nuclear correlations are treated in the continuum random phase approximation, accounting for the spreading width of the particle-hole states. A good description of the spin-longitudinal response is achieved, hinting at some evidence of pionic effects in the quasielastic region. The spin-transverse response is underestimated and we show that multistep contributions, while sizable, are not sufficient to provide an explanation of the discrepancy.

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arXiv1995

The Charge Response of a Meson-Correlated Relativistic Fermi Gas

The quasielastic longitudinal electromagnetic response function R_L is studied within the context of a model that extends our previous investigations of pionic correlations and currents. Four mesons are now employed (pion, rho, sigma and omega, via the Bonn potential) and the many-body dynamics are extended to the full random-phase approximation built upon a Hartree-Fock basis. Wherever possible the Lorentz covariance of the problem is respected. The first three energy-weighted moments of the reduced response are computed, namely, the zeroth moment (Coulomb sum rule), the first moment (related to the position of the quasielastic peak) and the second moment (related to the peak width or variance). We discuss how with a modest downward adjustment of the Fermi momentum it is possible to obtain the expected zeroth and second moments; this implies that the nuclear system has a lower density than that required when using the free relativistic Fermi gas and accordingly that the interaction effects are weaker than one might initially find.

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arXiv1993

Hadron and Quark Form Factors in the Relativistic Harmonic Oscillator Model

Nucleon, pion and quark form factors are studied within the relativistic harmonic oscillator model including the quark spin. It is shown that the nucleon charge, magnetic and axial form factors and the pion charge form factor can be explained with one oscillator parameter if one accounts for the scaling rule and the size of the constituent quarks.

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arXiv1993

Parity-Violating Electron Scattering from the Pion-Correlated Relativistic Fermi Gas

Parity-violating quasielastic electron scattering is studied within the context of the relativistic Fermi gas and its extensions to include the effects of pionic correlations and meson-exchange currents. The work builds on previous studies using the same model; here the part of the parity-violating asymmetry that contains axial-vector hadronic currents is developed in detail using those previous studies and a link is provided to the transverse vector-isovector response. Various integrated observables are constructed from the differential asymmetry. These include an asymmetry averaged over the quasielastic peak, as well as the difference of the asymmetry integrated to the left and right of the peak -- the latter is shown to be optimal for bringing out the nature of the pionic correlations. Special weighted integrals involving the differential asymmetry and electromagnetic cross section, based on the concepts of y-scaling and sum rules, are constructed and shown to be suited to studies of the single-nucleon form factor content in the problem, in particular, to determinations of the isovector/axial-vector and electric strangeness form factors. Comparisons are also made with recent predictions made on the basis of relativistic mean-field theory.

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arXiv1993

Collective Modes in a Slab of Interacting Nuclear Matter: The effects of finite range interactions

We consider a slab of nuclear matter and investigate the collective excitations, which develop in the response function of the system. We introduce a finite-range realistic interaction among the nucleons, which reproduces the full G-matrix by a linear combination of gaussian potentials in the various spin-isospin channels. We then analyze the collective modes of the slab in the S=T=1 channel: for moderate momenta hard and soft zero-sound modes are found, which exhaust most of the excitation strength. At variance with the results obtained with a zero range force, new "massive" excitations are found for the vector-isovector channel .

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arXiv1993

The Pion in Electromagnetic and Weak Neutral Current Nuclear Response Functions

The impact of pionic correlations and meson--exchange currents in determining the (vector) response functions for electroweak quasielastic lepton scattering from nuclei is discussed. The approach taken builds on previous work where the Fermi gas model is used to maintain consistency in treating forces and currents (gauge invariance) and to provide a Lorentz covariant framework. Results obtained in first-order perturbation theory are compared with infinite-order summation schemes (HF and RPA) and found to provide quite successful approximations for the quasielastic response functions. The role of pionic correlations in hardening the responses R_L and R_T is investigated in some detail, including studies of the relative importance of central and tensor pieces of the force and of exchange and self-energy diagrams; in addition, their role in significantly modifying the longitudinal parity-violating response R_{AV}^L is explored. The MEC are shown to provide a small, but non-negligible, contribution in determining the vector responses.

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arXiv1993

The Quasi-Elastic Nuclear Response

We explore the nuclear responses at intermediate energies, particularly in the spin longitudinal and spin transverse isovector channels, within the continuum random phase approximation framework. We also employ an extension of the standard random phase approximation to account for the spreading width of the single particle states through the inclusion of a complex and energy-dependent nucleon self-energy. The nuclear responses are then used as the basic ingredient to calculate hadronic reactions in the Glauber theory framework. Here both one and two-step contributions to the multiple scattering series in the quasi-elastic peak region are taken into account. We find evidence for shell effects in the one-step response and a strong dependence on the momentum regime of the two-step contribution.

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