Influence of core excitations on one-particle and particle-hole Green's functions

An approximation method for the coupling of collective states with particle and hole states is derived by a partial summation of the perturbation series. The approximation is applied to low-lying, normal parity states of¹⁷O and¹⁵O as well as to the giant dipole resonance of¹⁶O.     

Theory of deuteron stripping on doubly even nuclei

Using the method of quantum Green functions we derive a rigorous formula for the deuteron stripping on a doubly even nucleus. Starting from this formula we discuss the distorted-wave Born approximation, some additional assumptions usually made in numerical calculations and the high-energy case. In addition to this we show that the formfactor can be calculated using a model for the nucleon self energy.     

The break-up of the deuteron and stripping to unbound states

We treat the break-up of the deuteron in the field of a heavy nucleaus. Our theory is based on the post formalism of the DWBA, which is discussed in detail. In particular we consider the case where the energy of the incoming deuteron is below the Coulomb barrier. This sub-Coulomb break-up permits a test of the DWBA by comparing experimental data with the theory. Small effects are investigated in this context, and other treatments of this process are reviewed critically. To give an intuitive picture of the break-up process we introduce a semiclassical model.In the second part the theory is extended to stripping reactions to unbound (resonant) states. We show that this reaction which interferes with the direct break-up process, can again be well described in the post-form of the DWBA. Using simple models we discuss the most important features of such reactions. Particular attention is paid to the “l-enhancement”, “parallelism” and the influence of the specific wave functions on resonance line shapes. A survey and comparison of various theories of stripping reactions to unbound states is given and finally we discuss possible future developments.     

Pole expansion of the deuteron vertex functions revisited

New fits to the deuteronS andD state vertex functions are presented using a standard relativistic pole expansion. The new data from electron scattering admit two solutions which should be distinguishable by measuring the tensor polarization ined scattering.This note has been stimulated by discussions with Barbara Badelek. We thank S. Platchkov for providing the Saclay data in numerical form.     

Structure functions of the deuteron and meson exchange currents

The deuteron structure functionsA(q ²)B(q ²) and polarization tensorT(q ²) are studied within the Bonn model with meson exchange currents taken into account. It is shown that it is necessary to take all these currents into consideration including the retardation effects. ForA(q ²) the contribution of retardation effects does not depend on the choice of vertex form factors; whereas forB(q ²) it essentially depends on the choice of strong vertices. The best agreement with experiment is achieved for the Bonn relativistic potential.Dedicated to the memory of M. Gmitro.     

Generalization of the stabilization method for green functions and quantum resonances

The stabilization method provides an efficient approach to many problems in atomic and molecular dynamics. Real avoided crossings and smoothing techniques provide the relevant information to compute real density of states. The aim of this letter is to present an extension of the stabilization method that allows for a direct determination of full Green functions and resonance energies. The method is based on the use of optical potentials and perturbation theory. Real avoided crossings of the original stabilization method become complex and resonance energies appear to stabilize in the complex-energy plane. A numerical illustration is presented for a simple model of shape resonance. Accurate results are obtained with a small number of real square-integrable functions as in the original stabilization method. The computational efficiency of the approach and its generality are emphasized.     

On the method of double-time green functions in the theory of spin systems

The thermodynamics of the Ising model in a transverse field is considered within RPA. Contrary to the conventional approach, the order parameter is determined by the free energy while the sum rules are used to check the accuracy for the computed correlation functions.     

Alleviation of the Fermion-sign problem by optimization of many-body wave functions

We present a simple, robust, and highly efficient method for optimizing all parameters of many-body wave functions in quantum Monte Carlo calculations, applicable to continuum systems and lattice models. Based on a strong zero-variance principle, diagonalization of the Hamiltonian matrix in the space spanned by the wave function and its derivatives determines the optimal parameters. It systematically reduces the fixed-node error, as demonstrated by the calculation of the binding energy of the small but challenging C(2) molecule to the experimental accuracy of 0.02 eV.     

Influence of broken-pair excitations on the exact pair wavefunction

Doubly occupied configuration interaction (DOCI), the exact diagonalisation of the Hamiltonian in the paired (seniority zero) sector of the Hilbert space, is a combinatorial cost wave function that can be very efficiently approximated by pair coupled cluster doubles (pCCD) at mean-field computational cost. As such, it is a very interesting candidate as a starting point for building the full configuration interaction (FCI) ground state eigenfunction belonging to all (not just paired) seniority sectors. The true seniority zero sector of FCI (referred to here as FCI₀) includes the effect of coupling between all seniority sectors rather than just seniority zero, and is, in principle, different from DOCI. We here study the accuracy with which DOCI approximates FCI₀. Using a set of small Hubbard lattices, where FCI is possible, we show that DOCI ～ FCI₀ under weak correlation. However, in the strong correlation regime, the nature of the FCI₀ wavefunction can change significantly, rendering DOCI and pCCD a less than ideal starting point for approximating FCI.     

Relations between the differential cross sections for diffractive processes of nucleon stripping and halo-nucleus core stripping

A comparative analysis of the nucleon stripping and core stripping reactions induced by a halo nucleus is presented on the basis of certain models for diffractive processes. It follows from calculations for the halo nucleus 11Be that the transverse momentum distribution of nucleons for core stripping (as the longitudinal momentum distribution) is less distorted by absorption in a target than the distribution of core fragments for nucleon stripping. The obtained relations between the cross sections for the stripping reactions simplify interpretation of the calculation results.     

A new method in the theory of many-body scattering

New coupled equations for the transition (T) and reactance (K) operators for N-channel, many-body, rearrangement scattering are derived. The key to the new method is the channel coupling array W, which links the various rearrangement channels together. By specializing W to the class of channel permuting arrays, it is shown that the (N−1)st iterate of the kernel of the coupled equations is connected.     

Ab initio calculation of optical spectra of liquids: many-body effects in the electronic excitations of water

We present ab initio calculations of the excited state properties of liquid water in the framework of many-body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. The insensitivity of screening effects to a particular configuration make these calculations feasible. The resulting spectra, which are strongly modified by many-body effects, are in good agreement with experiments.     

Influence of nuclear excitations on absorption cross sections

From the mean free path in excited nuclear matter a temperature dependent optical model is derived. With its help the approximation of inverse cross sections by ground state absorption cross sections is studied.     

Probing the origin of the EMC effect via tagged structure functions of the deuteron

We demonstrate that measurement of tagged structure functions of the deuteron in (e, e′N) semi-inclusive reactions can discriminate between different hypotheses on the origin of the nuclear EMC effect. By choosing extreme backward kinematics for the spectator nucleon to minimize effects from the deuteron wave function and final state interactions, one can isolate the modifications in the structure of the bound nucleon within the impulse approximation. The same reaction can be used to extract the large-x neutron to proton structure function ratio.     

A comparison of high statistics measurements of the proton and deuteron structure functions

We present a detailed comparison of high statistics measurements of the structure functionF ₂(x, Q ²) from deep inelastic scattering of charged leptons on hydrogen and deuterium targets. It is found that the CERN results from the EMC and BCDMS Collaborations are both compatible with the lowQ ² electron scattering results from SLAC after an adjustment of the relative normalizations. We discuss the residual discrepancy between EMC and BCDMS which persists after this renormalization and find that the SLAC data are in better agreement with the steeperx dependence of the BCDMS results. A phenomenological parametrization ofF ₂(x, Q ²) is given.     

Influence of final state interaction on incoherent η-photoproduction on the deuteron near threshold*

Incoherent η-photoproduction on the deuteron is studied for photon energies from threshold up to 720 MeV. The elementary γN → ghN amplitude is constructed in the frame of a conventional isobar model. Effects of final state interaction are investigated and their role in the cross section as well as in polarization observables is found to be important. Inclusion of such effects leads to a satisfactory agreement with experimental data in the near threshold region. In addition, recent experimental results on the coherent reaction d(γ, gh)d are discussed.     

Measurement of the neutron-induced deuteron breakup reaction cross-section between 5 and 25 MeV

High-spin states in ¹⁸⁷Pt have been studied by means of ??-ray spectroscopy techniques. Known bands have been significantly extended and new bands have been found. The band structures are discussed in the framework of the cranking model and negative-parity states are compared with calculations performed with a semi-microscopic axial-rotor plus one-quasiparticle coupling model. Shape coexistence is observed from low excitation energy.