Searcharxiv⌕ Search

arXiv subjects

A. Schwenk

Publications and source records attributed to A. Schwenk.

At least 163 records · Page 9Linked to original sources
arXiv2010

From low-momentum interactions to nuclear structure

We present an overview of low-momentum two-nucleon and many-body interactions and their use in calculations of nuclei and infinite matter. The softening of phenomenological and effective field theory (EFT) potentials by renormalization group (RG) transformations that decouple low and high momenta leads to greatly enhanced convergence in few- and many-body systems while maintaining a decreasing hierarchy of many-body forces. This review surveys the RG-based technology and results, discusses the connections to chiral EFT, and clarifies various misconceptions.

nucl-th↗
arXiv2010

Lowest-order contributions of chiral three-nucleon interactions to pairing properties of nuclear ground states

We perform a systematic study of the odd-even mass staggering generated using a pairing interaction computed at first order in low-momentum interactions. Building on previous work including the (nuclear plus Coulomb) two-nucleon interaction only, we focus here on the first-order contribution from chiral three-nucleon forces. We observe a significant repulsive effect from the three-nucleon interaction. The combined contribution from two- and three-nucleon interactions accounts for approximately 70% of the experimental pairing gaps. This leaves room for higher-order contributions to the pairing kernel and the normal self-energy that need to be computed consistently.

nucl-th↗
arXiv2010

Is chiral symmetry manifested in nuclear structure?

Spontaneously broken chiral symmetry is an established property of low-energy quantum chromodynamics, but finding direct evidence for it from nuclear structure data is a difficult challenge. Indeed, phenomenologically successful energy-density functional approaches do not even have explicit pions. Are there smoking guns for chiral symmetry in nuclei?

nucl-th↗
arXiv2010

How should one formulate, extract, and interpret `non-observables' for nuclei?

Nuclear observables such as binding energies and cross sections can be directly measured. Other physically useful quantities, such as spectroscopic factors, are related to measured quantities by a convolution whose decomposition is not unique. Can a framework for these nuclear structure `non-observables' be formulated systematically so that they can be extracted from experiment with known uncertainties and calculated with consistent theory? Parton distribution functions in hadrons serve as an illustrative example of how this can be done. A systematic framework is also needed to address questions of interpretation, such as whether short-range correlations are important for nuclear structure.

nucl-th↗
arXiv2009

Non-empirical pairing energy functional in nuclear matter and finite nuclei

We study 1S0 pairing gaps in neutron and nuclear matter as well as in finite nuclei on the basis of microscopic two-nucleon interactions. Special attention is paid to the consistency of the pairing interaction and normal self-energy contributions. We find that pairing gaps obtained from low-momentum interactions depend only weakly on approximation schemes for the normal self-energy, required in present energy-density functional calculations, while pairing gaps from hard potentials are very sensitive to the effective-mass approximation scheme.

nucl-th↗
arXiv2009

Isospin-symmetry-breaking corrections to superallowed Fermi beta decay: Radial excitations

Based on an exact formalism, we study the effects of isospin-symmetry-breaking interactions on superallowed 0+ -> 0+ transitions. We calculate the second-order renormalization of the Fermi matrix element due to radial contributions and show that radial excitations neglected in the treatment of Towner and Hardy are significant. These are estimated to decrease the isospin-symmetry-breaking corrections. Our results provide a correction term that can be included in existing approaches.

nucl-th↗
arXiv2009

Chiral effective field theory calculations of neutrino processes in dense matter

We calculate neutrino processes involving two nucleons at subnuclear densities using chiral effective field theory. Shorter-range noncentral forces reduce the neutrino rates significantly compared with the one-pion exchange approximation currently used in supernova simulations. For densities rho < 10^{14} g cm^{-3}, we find that neutrino rates are well constrained by nuclear interactions and nucleon-nucleon scattering data. As an application, we calculate the mean-square energy transfer in scattering of a neutrino from nucleons and find that collision processes and spin-dependent mean-field effects dominate over the energy transfer due to nucleon recoil.

nucl-th↗
arXiv2009

Spin response of a normal Fermi liquid with noncentral interactions

We consider the spin response of a normal Fermi liquid with noncentral interactions under conditions intermediate between the collisionless and hydrodynamic regimes. This problem is of importance for calculations of neutrino properties in dense matter. By expressing the deviation of the quasiparticle distribution function from equilibrium in terms of eigenfunctions of the transport equation under the combined influence of collisions and an external field, we derive a closed expression for the spin-density--spin-density response function and compare its predictions with that of a relaxation time approximation. Our results indicate that the relaxation time approximation is reliable for neutrino properties under astrophysically relevant conditions.

nucl-th↗
arXiv2009

Helium halo nuclei from low-momentum interactions

We present ground-state energies of helium halo nuclei based on chiral low-momentum interactions, using the hyperspherical-harmonics method for 6He and coupled-cluster theory for 8He, with correct asymptotics for the extended halo structure.

nucl-th↗
arXiv2009

Nuclear matter from chiral low-momentum interactions

Nuclear matter calculations based on low-momentum interactions derived from chiral nucleon-nucleon and three-nucleon effective field theory interactions and fit only to few-body data predict realistic saturation properties with controlled uncertainties. This is promising for a unified description of nuclei and to develop a universal density functional based on low-momentum interactions.

nucl-th↗
arXiv2009

Nuclei and chiral dynamics

Nuclear theory has entered an exciting era. This is due to advances on many fronts, including the development of effective field theory and the renormalization group for nuclear forces, advances in ab-initio methods for nuclear structure, an effort to develop a universal density functional based on microscopic interactions, and the application of large-scale computing resources. I discuss their impact, recent highlights and the frontiers in understanding and predicting nuclei and the structure of strongly-interacting matter based on chiral interactions.

nucl-th↗
arXiv2008

Unified approach to structure factors and neutrino processes in nucleon matter

We present a unified approach to neutrino processes in nucleon matter based on Landau's theory of Fermi liquids that includes one- and two-quasiparticle-quasihole pair states as well as mean-field effects. We show how rates of neutrino processes involving two nucleons may be calculated in terms of the collision integral in the Landau transport equation for quasiparticles. Using a relaxation time approximation, we solve the transport equation for density and spin-density fluctuations and derive a general form for the response functions. We apply our approach to neutral-current processes in neutron matter, where the spin response function is crucial for calculations of neutrino elastic and inelastic scattering, neutrino-pair bremsstrahlung and absorption from strongly-interacting nucleons. We calculate the relaxation rates using modern nuclear interactions and including many-body contributions, and find that rates of neutrino processes are reduced compared with estimates based on the one-pion exchange interaction, which is used in current simulations of core-collapse supernovae.

nucl-th↗
arXiv2008

Isospin-symmetry-breaking corrections to superallowed Fermi beta decay: Formalism and schematic models

We study the formalism to include isospin-symmetry-breaking corrections when extracting the up-down Cabibbo-Kobayashi-Maskawa matrix element from superallowed 0+ -> 0+ nuclear beta decay. We show that there are no first order isospin-symmetry-breaking corrections to the relevant nuclear matrix elements. We find corrections to the treatment of Towner and Hardy, and assess these using schematic models of increasing complexity.

nucl-th↗
arXiv2008

Influence of light nuclei on neutrino-driven supernova outflows

We study the composition of the outer layers of a protoneutron star and show that light nuclei are present in substantial amounts. The composition is dominated by nucleons, deuterons, tritons and alpha particles; 3He is present in smaller amounts. This composition can be studied in laboratory experiments with new neutron-rich radioactive beams that can reproduce similar densities and temperatures. After including the corresponding neutrino interactions, we demonstrate that light nuclei have a small impact on the average energy of the emitted electron neutrinos, but are significant for the average energy of antineutrinos. During the early post-explosion phase, the average energy of electron antineutrinos is slightly increased, while at later times during the protoneutron star cooling it is reduced by about 1 MeV. The consequences of these changes for nucleosynthesis in neutrino-driven supernova outflows are discussed.

astro-ph↗
arXiv2008

Three-nucleon interactions: A frontier in nuclear structure

Three-nucleon interactions are a frontier in understanding and predicting the structure of strongly-interacting matter in laboratory nuclei and in the cosmos. We present results and discuss the status of first calculations with microscopic three-nucleon interactions beyond light nuclei. This coherent effort is possible due to advances based on effective field theory and renormalization group methods in nuclear physics.

nucl-th↗
arXiv2008

Block Diagonalization using SRG Flow Equations

By choosing appropriate generators for the Similarity Renormalization Group (SRG) flow equations, different patterns of decoupling in a Hamiltonian can be achieved. Sharp and smooth block-diagonal forms of phase-shift equivalent nucleon-nucleon potentials in momentum space are generated as examples and compared to analogous low-momentum interactions ("v_lowk").

nucl-th↗
arXiv2007

Convergence in the no-core shell model with low-momentum two-nucleon interactions

The convergence of no-core shell model (NCSM) calculations using renormalization group evolved low-momentum two-nucleon interactions is studied for light nuclei up to Li-7. Because no additional transformation was used in applying the NCSM framework, the energy calculations satisfy the variational principle for a given Hamiltonian. Dramatic improvements in convergence are found as the cutoffs are lowered. The renormalization group equations are truncated at two-body interactions, so the evolution is only approximately unitary and converged energies for A > 2 vary with the cutoff. This approximation is systematic, however, and for useful cutoff ranges the energy variation is comparable to natural-size truncation errors inherent from the initial chiral effective field theory potential.

nucl-th↗