Ernest J. Moniz

Isobar-hole doorway states and π-16O scattering

Abstract We describe and apply the isobar-hole approach to intermediate energy pion-nucleus reactions. Pion propagation, nucleon and isobar binding, Pauli restrictions and Δ propagation are calculated explicitly within a shell model framework. Intermediate coupling to multihole channels, for example through pion absorption, is treated phenomenologically through an isobar spreading potential. We find strongly collective Δ-hole states, leading to a reformulation of the approach in an extended schematic model. This entails systematic construction of a Δ-hole doorway state basis within which the Δ-hole propagator is evaluated. We find that this doorway space can be truncated at very low dimensionality while preserving accuracy, thereby simplifying the calculations appreciably. We make a detailed comparison between the theoretical results and recent data for π+ — 16O scattering in the pion energy range 50–340 MeV. Nonresonant πN interactions and the π-nucleus Coulomb interaction are included in the calculations. The data is reproduced quite well both below and in the resonance region, and we discuss in detail the role of various dynamical mechanisms. Above the resonance, the calculations are far less successful. We discuss possible shortcomings, stressing the role of inclusive pion-nucleus reactions for revealing the important dynamics. As a test of the Δ spreading potential used for describing elastic scattering, we calculate the total cross section for pion absorption. The result agrees reasonably well with the available data.

Nuclear photoabsorption and Compton scattering at intermediate energy

Abstract Intermediate energy nuclear photoabsorption and Compton scattering within the Δ-hole approach are studied. The same Δ-nucleus dynamics used to describe a variety of pion-induced reactions are employed. A dynamical model for the photon-nucleon amplitude is constructed, including both resonant-channel and nonresonant backgrounds in addition to Δ-excitation. Medium corrections to the full amplitude are included for nuclear seattering. A doorway state expansion allows one to discuss quantitatively the role of various contributions to the Δ-hole Hamiltonian, thereby clarifying the level of sensitivity to the structure of the Δ spreading potential. The reactive content of the total cross section is diseussed. The results are compared with available photoabsorption data for 4 He, 12 C and 16 O. We present results for the Δ contribution to the electron seattering transverse response function, for coherent π 0 photoproduction, and for nuclear elastic Compton seattering. The photoabsorption strength is spread over a larger energy range by the Δ dynamics, in agreement with the data, but is centered at too high an energy. The helicity flip Compton cross section is especially sensitive to the Δ-nucleus interaction, such as the spin-orbit potential strength.

Quark confinement and hadronic interactions

Abstract A study of quark models for many-hadron systems is presented. The starting point in the construction of these nonrelativistic models is a proper formal definition of the concept of color singlet and nonsinglet clusters in a multiquark system which respects the exchange symmetry of the quarks. This definition provides a natural way to impose saturation of the confining forces. As a consequence, the resulting models are free from unphysical van der Waals forces whose presence is a characteristic difficulty in the conventional models based on superposition of two-body confining forces. The dynamics of the multiquark system are specified by defining the color projections of the Hamiltonian. These models make explicit the role of the color hidden dynamics in multihadron systems. The properties of these models are investigated in a study of the s-wave q 2 − q 2 system. We develop new analytical methods appropriate for the study of many-body systems in the presence of confining interactions. These methods allow us to extract a variety of analytical results as well as exact numerical solutions, and they are employed to clarify the relation between the simple structure of the multiquark Hamiltonian and the wide variety of the resulting hadronic phenomena. The properties of bound and scattering states are studied for various models of the color hidden dynamics. Although the Hamiltonian contains only the confining force which determines the hadronic scale, phenomena are exhibited at a much lower energy scale. We investigate descriptions of the multiquark system in terms of hadronic degrees of freedom. Ambiguities in the formulation of such effective hadronic theories are discussed as well as their limitations in describing various physical observables. The fine tuning of the hadronic effective forces needed to generate the model phenomena which appear at a low-energy scale is understood from the underlying quark dynamics. Different signatures of quark degrees of freedom are studied, such as the appearance of threshold resonances or color hidden resonances, “EMC”-like modifications of the quark momentum distribution in hadronic bound states, or changes in the asymptotic behaviour of form-factors. The interdependence of these phenomenologically unrelated signatures is established, as well as their relation to properties of the color hidden dynamics.

Off-shell πN amplitudes via a multichannel separable potential

The off-shell pion-nucleon transition matrix is a basic ingredient in theories of pion-nuclear interactions which, in the absence of fundamental theory of πN dynamics, must be obtained by a phenomenological extrapolation from the available on-shell data. As one means of performing such an extrapolation, we explore a multichannel separable potential model with the property that the off-shell elastic scattering amplitude is generated directly from the measured elastic-channel phase shifts. The off-shell πN partial-wave transition amplitudes determined by this procedure are compared with those calculated by Landau and Tabakin using a one-channel absorptive separable potential. We find that the absorptive separable potential approach provides a physically unreasonable off-shell extrapolation at energies where the on-shell amplitude is highly inelastic, and show that the difficulty is a direct consequence of the one-channel nature of that method. The multichannel extrapolation is free of these difficulties.

Motion of extended charges in classical electrodynamics

The Lorentz–Dirac theory of radiation reaction on the motion of point charges is beset by the well known problems of runaway solutions and preacceleration. We examine the classical theory of extended charged particles and obtain a differential‐difference type equation of motion. Analysis of this equation reveals that the theory is internally consistent (i.e., no runaways or acausality) whenever the size of the particle exceeds the classical radius (defined as the radius for which the electrostatic self‐energy equals the mass of the particle). A specific example is presented which explicitly shows the different character of the extended and point charge solutions.

The Lorentz-Lorenz effect in pion-nucleus interactions☆

Abstract The Lorentz-Lorenz effect in pion-nucleus interactions is derived from the Watson expansion of the optical potential. The use of a realistic, finite range π N interaction significantly reduces its importance for nuclear short-range correlations in pionic atoms.

Extended schematic model for pion-nucleus scattering☆

Abstract We present the results of a microscopic isobar-hole calculation of π−16O elastic scattering and find that the transition matrix is dominated by a few collective isobar-hole doorway states. Further, we propose an extended schematic model which provides a rapidly convergent calculational scheme for obtaining these doorway states.

Doorway state approach to optical potential scattering

Abstract We extend and apply the doorway state approach in the context of optical potential scattering. This entalls construction of doorway basis states for resolving the transition operator. We focus on analytic solutions to comparatively simple problems in optical potential scattering. Both low and high energy limits are considered, and absorptive interactions are treated; both on- and off-shell partial wave amplitudes are constructed. Further, the full scattering amplitude in the high energy limit is calculated directly in the doorway expansion. With our analytic results, it is possible to identify the physical parameters controlling convergence of the doorway expansion. The same parameters apply over the entire range of cases studied. These parameters are related simply to the target geometry and to the interaction strength. For interactions appropriate to hadron-nucleus scattering, convergence of both on- and off-shell amplitudes is very rapid.

Coupled channel approach to the off-shell transition matrix☆

We propose a coupled-channel separable potential model with the feature that the off-shell elastic transition matrix can be generated directly from the scattering phase shifts. This should be useful in constructing pion-nucleus optical potentials.

Making the world safe for nuclear energy

Just as nuclear energy seems poised for a global comeback, nuclear weapon threats grow in Iran and North Korea. The security risk from expanding nuclear energy programmes could potentially be contained by the commercial provision of nuclear-fuel services. Countries that already possess uranium enrichment and plutonium reprocessing facilities could guarantee client countries with cradle-to-grave nuclear fuel services. The guarantee, backed by governmental or international entities, could persuade governments to abstain from building their own fuel-cycle facilities. Decisions to reject such an arrangement, which provides direct economic benefit and relieves waste management challenges, would spotlight a governments intentions and invite a coordinated international response. Iran and Brazil present crucial early tests.