225 research outputs found
Efficient calculation of chiral three-nucleon forces up to N3LO for ab initio studies
We present a novel framework to decompose three-nucleon forces in a momentum
space partial-wave basis. The new approach is computationally much more
efficient than previous methods and opens the way to ab initio studies of
few-nucleon scattering processes, nuclei and nuclear matter based on
higher-order chiral 3N forces. We use the new framework to calculate matrix
elements of chiral three-nucleon forces at N2LO and N3LO in large basis spaces
and carry out benchmark calculations for neutron matter and symmetric nuclear
matter. We also study the size of the individual three-nucleon force
contributions for H. For nonlocal regulators, we find that the sub-leading
terms, which have been neglected in most calculations so far, provide important
contributions. All matrix elements are calculated and stored in a user-friendly
way, such that values of low-energy constants as well as the form of regulator
functions can be chosen freely.Comment: 10 pages, 4 figure
Saturation with chiral interactions and consequences for finite nuclei
We explore the impact of nuclear matter saturation on the properties and
systematics of finite nuclei across the nuclear chart. Using the ab initio
in-medium similarity renormalization group (IM-SRG), we study ground-state
energies and charge radii of closed-shell nuclei from He to Ni,
based on a set of low-resolution two- and three-nucleon interactions that
predict realistic saturation properties. We first investigate in detail the
convergence properties of these Hamiltonians with respect to model-space
truncations for both two- and three-body interactions. We find one particular
interaction that reproduces well the ground-state energies of all closed-shell
nuclei studied. As expected from their saturation points relative to this
interaction, the other Hamiltonians underbind nuclei, but lead to a remarkably
similar systematics of ground-state energies. Extending our calculations to
complete isotopic chains in the and shells with the valence-space
IM-SRG, the same interaction reproduces not only experimental ground states but
two-neutron-separation energies and first excited states. We also
calculate radii with the valence-space IM-SRG for the first time. Since this
particular interaction saturates at too high density, charge radii are still
too small compared with experiment. Except for this underprediction, the radii
systematics is, however, well reproduced. Our results highlight the importance
of nuclear matter as a theoretical benchmark for the development of
next-generation chiral interactions.Comment: 11 pages, 15 figures, 1 tabl
Role of the total isospin 3/2 component in three-nucleon reactions
We discuss the role of the three-nucleon isospin T=3/2 amplitude in elastic
neutron-deuteron scattering and in the deuteron breakup reaction. The
contribution of this amplitude originates from charge-independence breaking of
the nucleon-nucleon potential and is driven by the difference between
neutron-neutron (proton-proton) and neutron-proton forces. We study the
magnitude of that contribution to the elastic scattering and breakup
observables, taking the locally regularized chiral N4LO nucleon-nucleon
potential supplemented by the chiral N2LO three-nucleon force. For comparison
we employ also the Av18 nucleon-nucleon potential combined with the Urbana IX
three-nucleon force. We find that the isospin T=3/2 component is important for
the breakup reaction and the proper treatment of charge-independence breaking
in this case requires the inclusion of the 1S0 state with isospin T=3/2. For
neutron-deuteron elastic scattering the T=3/2 contributions are insignificant
and charge-independence breaking can be accounted for by using the effective
t-matrix generated with the so-called "2/3-1/3" rule.Comment: 24 pages, 8 figures, 3 Table
Converged ab initio calculations of heavy nuclei
We propose a novel storage scheme for three-nucleon (3N) interaction matrix
elements relevant for the normal-ordered two-body approximation used
extensively in ab initio calculations of atomic nuclei. This scheme reduces the
required memory by approximately two orders of magnitude, which allows the
generation of 3N interaction matrix elements with the standard truncation of
, well beyond the previous limit of 18. We demonstrate that this
is sufficient to obtain ground-state energies in Sn converged to within
a few MeV with respect to the truncation. In addition, we study the
asymptotic convergence behavior and perform extrapolations to the un-truncated
limit. Finally, we investigate the impact of truncations made when evolving
free-space 3N interactions with the similarity renormalization group. We find
that the contribution of blocks with angular momentum is
dominated by a basis-truncation artifact which vanishes in the large-space
limit, so these computationally expensive components can be neglected. For the
two sets of nuclear interactions employed in this work, the resulting binding
energy of Sn agrees with the experimental value within theoretical
uncertainties. This work enables converged ab initio calculations of heavy
nuclei.Comment: 13 pages, 10 figure
Do we understand the incompressibility of neutron-rich matter?
The ``breathing mode'' of neutron-rich nuclei is our window into the
incompressibility of neutron-rich matter. After much confusion on the
interpretation of the experimental data, consistency was finally reached
between different models that predicted both the distribution of isoscalar
monopole strength in finite nuclei and the compression modulus of infinite
matter. However, a very recent experiment on the Tin isotopes at the Research
Center for Nuclear Physics(RCNP) in Japan has again muddled the waters.
Self-consistent models that were successful in reproducing the energy of the
giant monopole resonance (GMR) in nuclei with various nucleon asymmetries (such
as 90Zr, 144Sm, and 208Pb) overestimate the GMR energies in the Tin isotopes.
As important, the discrepancy between theory and experiment appears to grow
with neutron excess. This is particularly problematic as models artificially
tuned to reproduce the rapid softening of the GMR in the Tin isotopes become
inconsistent with the behavior of dilute neutron matter. Thus, we regard the
question of ``why is Tin so soft?'' as an important open problem in nuclear
structure.Comment: 12 pages, 3 figures, and 1 table. Submitted to the "Focus issue on
Open Problems in Nuclear Structure", Journal of Physics
Improved nuclear matter calculations from chiral low-momentum interactions
We present new nuclear matter calculations based on low-momentum interactions
derived from chiral effective field theory potentials. The current calculations
use an improved treatment of the three-nucleon force contribution that includes
a corrected combinatorial factor beyond Hartree-Fock that was omitted in
previous nuclear matter calculations. We find realistic saturation properties
using parameters fit only to few-body data, but with larger uncertainty
estimates from cutoff dependence and the 3NF parametrization than in previous
calculations.Comment: 5 pages, 4 figure
Few-nucleon systems with state-of-the-art chiral nucleon-nucleon forces
We apply improved nucleon-nucleon potentials up to fifth order in chiral
effective field theory, along with a new analysis of the theoretical truncation
errors, to study nucleon-deuteron (Nd) scattering and selected low-energy
observables in 3H, 4He, and 6Li. Calculations beyond second order differ from
experiment well outside the range of quantified uncertainties, providing truly
unambiguous evidence for missing three-nucleon forces within the employed
framework. The sizes of the required three-nucleon force contributions agree
well with expectations based on Weinberg's power counting. We identify the
energy range in elastic Nd scattering best suited to study three-nucleon force
effects and estimate the achievable accuracy of theoretical predictions for
various observables.Comment: 5 pages, 5 figure
Low-energy neutron-deuteron reactions with N3LO chiral forces
We solve three-nucleon Faddeev equations with nucleon-nucleon and
three-nucleon forces derived consistently in the framework of chiral
perturbation theory at next-to-next-to-next-to-leading order in the chiral
expansion. In this first investigation we include only matrix elements of the
three-nucleon force for partial waves with the total two-nucleon
(three-nucleon) angular momenta up to 3 (5/2). Low-energy neutron-deuteron
elastic scattering and deuteron breakup reaction are studied. Emphasis is put
on Ay puzzle in elastic scattering and cross sections in symmetric-space-star
and neutron-neutron quasi-free-scattering breakup configurations, for which
large discrepancies between data and theory have been reported.Comment: 22 pages, 7 figure
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