Participant-spectator model for fragmentation reactions with halo nuclei

We present the theoretical framework of the participant-spectator model for fragmentation reactions with weakly bound (halo) projectiles. The model allows calculations of differential cross sections for different fragmentation processes. Both nuclear and Coulomb interactions are included and the model can therefore be used for all targets, light and heavy.     

Angular correlations in peripheral fragmentation of halo nuclei

The angular correlation between momenta of ⁵He and its decay products has been calculated for the peripheral fragmentation of ⁶He using a two-step model in a sudden approximation: (1) One neutron knock-out leading to the unbound nucleus ⁵He. (2) Subsequent decay of ⁵He undisturbed by the target in an α-particle and a neutron. Angular correlations measured in a recent experiment dealing with the peripheral fragmentation of 240 MeV/u ⁶He on a carbon target can be well described by a dominant ⁵He 3/2^? ground state configuration plus a small admixture of ≈ 7% of the first excited 1/2^? state. The influence of an s-state admixture as well as a contribution of the momentum transfer to the correlation function are investigated. The results obtained for ⁶He are then used as a touch stone for a discussion of the methods to investigate peripheral fragmentation of other Borromean halo nuclei like ¹¹Li and ¹⁴Be. It is shown that the evidence on the possible existence of low-lying s-states in ¹⁰Li and ¹³Be can be derived from the corresponding angular correlations.     

Angular momentum in peripheral fragmentation reactions

The angular momentum induced in peripheral fragmentation reactions at relativistic energies is described on the basis of shell-model considerations. An analytical expression for the spincutoff parameter of prefragments is obtained, and numerical calculations in the framework of the abrasion-ablation model are performed in order to estimate the angular-momentum distribution of prefragments and final fragments. Additionally, an approximative formula is given to calculate the angular-momentum distribution of final fragments.     

Possible approach to the halo size determination from fragmentation reactions

It is shown that the distribution of the sum of the observed transverse momenta q of the core fragment and of the halo nucleons in the case of “elastic” breakup of exotic halo nuclei should have a dip at q=0, the width of which may be a measure of the halo size. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 296–301 (10 March 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

On some approximations in the theory of halo nucleus reactions

The halo nucleus nucleon stripping reaction on a light target is studied using the diffraction theory of reactions with weakly bound nuclei. An improved version of the approximation of a small target’s radius (compared to the size of a halo nucleus) is formulated. Simple analytical expressions for the differential cross section and the longitudinal momentum distribution of observed particles that allow us to calculate them with good accuracy are obtained.     

Compact reformulation of distorted-wave and coupled-channel born approximations for transfer reactions between nuclei

DWBA and CCBA calculations are reformulated in a form which is compact, and which is symmetric with respect to the projected-ejectile system and to the target-residual nucleus system, so that the formulae can be used conveniently for heavy-ion as well as light-ion induced reactions. Clarification is also made of the relation between the exact finite-range calculations on the one hand and the no-recoil and zero-range approximations on the other. Some detailed information is given to show how to carry out efficiently the exact finite-range calculations.     

Dynamical recoil effects in a distorted-wave treatment of heavy-ion transfer reactions

The classical conditions of optimum particle transfer in heavy-ion reactions are shown to follow from a quantal distorted wave theory. To that purpose the method of three dimensional time dependent JWKB approximation is used.     

Semiclassical S-matrix theory for atomic fragmentation

A semiclassical scattering approach is developed which can handle long-range (Coulomb) forces without the knowledge of the asymptotic wave function for multiple charged fragments in the continuum. The classical cross section for potential and inelastic scattering including fragmentation (ionization) is derived from first principles in a form which allows for a simple extension to semiclassical scattering amplitudes as a sum over classical orbits and their associated actions. The object of primary importance is the classical deflection function which can show regular and chaotic behavior. Applications to electron impact ionization of hydrogen and electron–atom scattering in general are discussed in a reduced phase space, motivated by partial fixed points of the respective scattering systems. Special emphasis, also in connection with chaotic scattering, is put on threshold ionization. Finally, motivated by the reflection principle for molecules, a semiclassical hybrid approach is introduced for photoabsorption cross sections of atoms where the time-dependent propagator is approximated semiclassically in a short-time limit with the Baker–Hausdorff formula. Applications to one- and two-electron atoms are followed by a presentation of double photoionization of helium, treated in combination with the semiclassical S-matrix for scattering.     

Angular momentum population in fragmentation reactions

Using a relativistic transport model followed by a statistical sequential binary emission model, the population of metastable high-spin isomeric states are studied in relativistic projectile fragmentation reactions. The initial angular momentum distribution are generated from hole excitations. We find that the angular momentum distribution of the excited prefragments are considerably broadened due to light particle evaporation. The model reproduces the experimentally measured population of relatively low-lying states and underpredicts states with high angular momentum I?17?$I?17?$. We propose that coupling the spin of the excited and hole states in the prefragment will give a better understanding of the data.     

Role of higher multipole excitations in the electromagnetic dissociation of one-neutron halo nuclei

We investigate the role of higher multipole excitations in the electromagnetic dissociation of one-neutron halo nuclei within two different theoretical models --a finite-range distorted-wave Born approximation and another in a more analytical method with a finite-range potential. We also show, within a simple picture, how the presence of a weakly bound state affects the breakup cross-section.     

Longitudinal momentum distributions in stripping reactions with halo nuclei

Differential cross sections and longitudinal momentum distributions of observed particles are calculated for stripping reactions that result from diffractive interaction between halo nuclei and targets. The applicability of different analytic methods of calculation is considered. The advantages of an improved approximation of small target radius are demonstrated for valent halo nucleon absorption radii of 2–4 fm in describing momentum distributions in particular.     

Quasiparticle excitations in relativistic quantum field theory

We analyze the particle-like excitations arising in relativistic field theories in states different than the vacuum. The basic properties characterizing the quasiparticle propagation are studied using two different complementary methods. First we introduce a frequency-based approach, wherein the quasiparticle properties are deduced from the spectral analysis of the two-point propagators. Second, we put forward a real-time approach, wherein the quantum state corresponding to the quasiparticle excitation is explicitly constructed, and the time-evolution is followed. Both methods lead to the same result: the energy and decay rate of the quasiparticles are determined by the real and imaginary parts of the retarded self-energy, respectively. Both approaches are compared, on the one hand, with the standard field-theoretic analysis of particles in the vacuum and, on the other hand, with the mean-field-based techniques in general backgrounds.     

One dimensional three-body model for low energy reactions of halo nuclei

One dimensional three-body model which simulates the low energy reactions of the nuclei with halo structure, is investigated by solving exactly the three-body Schrödinger equation. The dynamical roles of the halo neutron during the reaction are studied in detail. The decrease of the fusion probability, as well as the large transfer and break-up probabilities, are found for halo nuclei.     

Effective theory of excitations in a Feshbach-resonant superfluid

A strongly interacting Fermi gas, such as that of cold atoms operative near a Feshbach resonance, is difficult to study by perturbative many-body theory to go beyond mean-field approximation. Here I develop an effective field theory for the resonant superfluid based on broken symmetry. The theory retains both fermionic quasiparticles and superfluid phonons, the interaction between them being derived nonperturbatively. The theory converges and can be improved order by order, in a manner governed by a low energy expansion rather than by a coupling constant. I apply the effective theory to calculate the specific heat and discuss the theory with a recent heat capacity experiment.     

Entanglement of low-energy excitations in conformal field theory

In a quantum critical chain, the scaling regime of the energy and momentum of the ground state and low-lying excitations are described by conformal field theory (CFT). The same holds true for the von Neumann and Rényi entropies of the ground state, which display a universal logarithmic behavior depending on the central charge. In this Letter we generalize this result to those excited states of the chain that correspond to primary fields in CFT. It is shown that the nth Rényi entropy is related to a 2n-point correlator of primary fields. We verify this statement for the critical XX and XXZ chains. This result uncovers a new link between quantum information theory and CFT.