469 research outputs found
Toward the Ab-initio Description of Medium Mass Nuclei
As ab-initio calculations of atomic nuclei enter the A=40-100 mass range, a
great challenge is how to approach the vast majority of open-shell (degenerate)
isotopes. We add realistic three-nucleon interactions to the state of the art
many-body Green's function theory of closed-shells, and find that physics of
neutron driplines is reproduced with very good quality. Further, we introduce
the Gorkov formalism to extend ab-initio theory to semi-magic, fully
open-shell, isotopes. Proof-of-principle calculations for Ca-44 and Ni-74
confirm that this approach is indeed feasible. Combining these two advances
(open-shells and three-nucleon interactions) requires longer, technical, work
but it is otherwise within reach.Comment: Contribution to Summary Report of EURISOL Topical and Town Meetings,
15-19 October 2012; missing affiliations added and corrected errors in Tab
Ab-initio calculation of the binding energy with the Hybrid Multideterminant scheme
We perform an ab-initio calculation for the binding energy of using
the CD-Bonn 2000 NN potential renormalized with the Lee-Suzuki method. The
many-body approach to the problem is the Hybrid Multideterminant method. The
results indicate a binding energy of about , within a few hundreds KeV
uncertainty. The center of mass diagnostics are also discussed.Comment: 18 pages with 3 figures. More calculations added, to be published in
EPJ
Beyond the Shell Model: The Canonical Nuclear Many-Body Problem as an Effective Theory
We describe a strategy for attacking the canonical nuclear structure problem
---bound-state properties of a system of point nucleons interacting via a
two-body potential---which involves an expansion in the number of particles
scattering at high momenta, but is otherwise exact. The required
self-consistent solutions of the Bloch-Horowitz equation for effective
interactions and operators are obtained by an efficient Green's function method
based on the Lanczos algorithm. We carry out this program for the simplest
nuclei, d and He, to contrast a rigorous effective theory with the shell
model, thereby illustrating several of the uncontrolled approximations in the
latter.Comment: Revtex; two columns; four pages; two figures; submitted to Phys. Rev.
Let
Discrepancy between experimental and theoretical -decay rates resolved from first principles
-decay, a process that changes a neutron into a proton (and vice
versa), is the dominant decay mode of atomic nuclei. This decay offers a unique
window to physics beyond the standard model, and is at the heart of
microphysical processes in stellar explosions and the synthesis of the elements
in the Universe. For 50 years, a central puzzle has been that observed
-decay rates are systematically smaller than theoretical predictions.
This was attributed to an apparent quenching of the fundamental coupling
constant 1.27 in the nucleus by a factor of about 0.75 compared
to the -decay of a free neutron. The origin of this quenching is
controversial and has so far eluded a first-principles theoretical
understanding. Here we address this puzzle and show that this quenching arises
to a large extent from the coupling of the weak force to two nucleons as well
as from strong correlations in the nucleus. We present state-of-the-art
computations of -decays from light to heavy nuclei. Our results are
consistent with experimental data, including the pioneering measurement for
Sn. These theoretical advances are enabled by systematic effective
field theories of the strong and weak interactions combined with powerful
quantum many-body techniques. This work paves the way for systematic
theoretical predictions for fundamental physics problems. These include the
synthesis of heavy elements in neutron star mergers and the search for
neutrino-less double--decay, where an analogous quenching puzzle is a
major source of uncertainty in extracting the neutrino mass scale.Comment: 20 pages, 18 figure
Exact calculation of three-body contact interaction to second order
For a system of fermions with a three-body contact interaction the
second-order contributions to the energy per particle are
calculated exactly. The three-particle scattering amplitude in the medium is
derived in closed analytical form from the corresponding two-loop rescattering
diagram. We compare the (genuine) second-order three-body contribution to with the second-order term due to the density-dependent
effective two-body interaction, and find that the latter term dominates. The
results of the present study are of interest for nuclear many-body calculations
where chiral three-nucleon forces are treated beyond leading order via a
density-dependent effective two-body interaction.Comment: 9 pages, 6 figures, to be published in European Journal
Response of a First-Order Stream in Maine to Short-Term In-Stream Acidification
An experimental short-term acidification with HCl at a first-order stream in central Maine, USA was used to study processes controlling the changes in stream chemistry and to assess the ability of stream substrate to buffer pH. The streambed exerted a strong buffering capacity against pH change by ion exchange during the 6-hour acidification. Streambed substrates had substantial cation and anion exchange capacity in the pH range of 4.1 to 6.5. The ion exchange for cations and SO42- were rapid and reversible. The speed of release of cations from stream substrates was Na1+\u3e Ca2+ \u3e Mg2+ \u3e Aln+ \u3e Be2+, perhaps relating to charge density of these cations. Ca2+ desorption dominated neutralisation of excess H+ for the first 2 hr. As the reservoir of exchangeable Ca diminished, desorption (and possibly dissolution) of Al3+ became the dominant neutralising mechanism. The exchangeable (and possibly soluble) reservoir of Al was not depleted during the 6-hour acidification. Sulphate adsorption during the acidification reduced the concentration of SO42- in stream water by as much as 20 μeq L-1 (from 70 μeq L-1). Desorption of SO42- and adsorption of base cations after the artificial acidification resulted in a prolongation of the pH depression. The streambed had the capacity to buffer stream water chemistry significantly during an acidifying event affecting the entire upstream catchment
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