81 research outputs found
Two-Pion Exchange Currents in Photodisintegration of the Deuteron
Chiral effective field theory (ChEFT) is a modern framework to analyze the
properties of few-nucleon systems at low energies. It is based on the most
general effective Lagrangian for pions and nucleons consistent with the chiral
symmetry of QCD. For energies below the pion-production threshold it is
possible to eliminate the pionic degrees of freedom and derive nuclear
potentials and nuclear current operators solely in terms of the nucleonic
degrees of freedom. This is very important because, despite a lot of experience
gained in the past, the consistency between two-nucleon forces, many-nucleon
forces and the corresponding current operators has not been achieved yet. In
this presentation we consider the recently derived long-range two-pion exchange
(TPE) contributions to the nuclear current operator which appear at next-to
leading order of the chiral expansion. These operators do not contain any free
parameters. We study their role in the deuteron photodisintegration reaction
and compare our predictions with experimental data. The bound and scattering
states are calculated using five different chiral N2LO nucleon-nucleon (NN)
potentials which allows to estimate the theoretical uncertainty at a given
order in the chiral expansion. For some observables the results are very close
to the reference predictions based on the AV18 NN potential and the current
operator (partly) consistent with this force.Comment: Contribution to the 12th International Conference on Meson-Nucleon
Physics and the Structure of the Nucleon (MENU2010), Williamsburg, USA, May
31-June 4, 201
Evolution of Nuclear Many-Body Forces with the Similarity Renormalization Group
The first practical method to evolve many-body nuclear forces to softened
form using the Similarity Renormalization Group (SRG) in a harmonic oscillator
basis is demonstrated. When applied to He4 calculations, the two- and
three-body oscillator matrix elements yield rapid convergence of the
ground-state energy with a small net contribution of the induced four-body
force.Comment: 4 pages, 5 figures, PRL published versio
Evolving Nuclear Many-Body Forces with the Similarity Renormalization Group
In recent years, the Similarity Renormalization Group has provided a powerful
and versatile means to soften interactions for ab initio nuclear calculations.
The substantial contribution of both induced and initial three-body forces to
the nuclear interaction has required the consistent evolution of free-space
Hamiltonians in the three-particle space. We present the most recent progress
on this work, extending the calculational capability to the p-shell nuclei and
showing that the hierarchy of induced many-body forces is consistent with
previous estimates. Calculations over a range of the flow parameter for 6Li,
including fully evolved NN+3N interactions, show moderate contributions due to
induced four-body forces and display the same improved convergence properties
as in lighter nuclei. A systematic analysis provides further evidence that the
hierarchy of many-body forces is preserved.Comment: 26 pages, 15 figures, and 5 table
The chiral long-range two-pion exchange electromagnetic currents in radiative nucleon-deuteron capture
The nucleon-deuteron radiative capture process is investigated using the
chiral nuclear potentials and the electromagnetic currents developed by the
Bochum-Bonn group. While the strong interaction is taken up to the
next-to-next-to-leading order, the electromagnetic current consists of a single
nucleon current, the leading one-pion exchange one and is supplemented by
contributions from the long-range two-pion exchange current at
next-to-leading-order. The theoretical predictions for the cross sections as
well as analyzing powers show strong dependence on the values of regularization
parameters. Only small effects of the three-nucleon force and the long-range
two-pion exchange current are observed. The dependence on the choice of
regularization parameters results in a big theoretical uncertainty and clearly
points to the necessity to include corrections from higher orders of the chiral
expansion both for the nuclear forces and currents.Comment: 10 pages, 5 figure
Effective Field Theory and Finite Density Systems
This review gives an overview of effective field theory (EFT) as applied at
finite density, with a focus on nuclear many-body systems. Uniform systems with
short-range interactions illustrate the ingredients and virtues of many-body
EFT and then the varied frontiers of EFT for finite nuclei and nuclear matter
are surveyed.Comment: 27 pages, 5 figure
A new way to perform partial wave decompositions of few-nucleon forces
We formulate a general and exact method of partial wave decomposition (PWD)
of any nucleon-nucleon (NN) potential and any three-nucleon (3N) force. The
approach allows one to efficiently use symbolic algebra software to generate
the interaction dependent part of the program code calculating the interaction.
We demonstrate the feasibility of this approach for the one-boson exchange
BonnB potential, a recent nucleon-nucleon chiral force and the chiral
two-pion-exchange three-nucleon force. In all cases very good agreement between
the new and the traditional PWD is found. The automated PWD offered by the new
approach is of the utmost importance in view of future applications of numerous
chiral N3LO contributions to the 3N force in three nucleon calculations.Comment: 10 pages, 6 figures (24 eps files
Gas electron tracking detector for beta decay experiments
For identification and 3D-tracking of low-energy electrons a new type of
gas-based detector was designed that minimizes scattering and energy loss. The
current version of the detector is a combination of a plastic scintillator,
serving as a trigger source and energy detector, and a hexagonally structured
multi-wire drift chamber (MWDC), filled with a mixture of helium and isobutane
gas. The drift time information is used to track particles in the plane
perpendicular to the wires, while a charge division technique provides spatial
information along the wires. The gas tracker was successfully used in the
miniBETA project as a beta spectrometer for a measurement of the weak magnetism
form factor in nuclear beta decay. The precision of the three-dimensional
electron tracking, in combination with low-mass, low-Z materials and
identification of backscattering from scintillator, facilitated a reduction of
the main systematics effects. At certain conditions, a spatial resolution
better than 0.5 mm was obtained in the plane perpendicular to the wires, while
resolutions of about 6 mm were achieved along wires. Thanks to precise tracking
information, it is possible to eliminate electrons and other particles not
originating from the desired decay with high efficiency. Additionally, using
the coincidence between MWDC and scintillator, background from gamma emission
typically accompanying radioactive decays, was highly suppressed. An overview
of different event topologies is presented together with the tracker's ability
to correctly recognize them. The analysis is supported by Monte Carlo
simulations using Geant4 and Garfield++ packages. Finally, the preliminary
results from the 114In spectrum study are presented.Comment: 5 pages, 3 figures, accepted for publication in JINST - PSD12(2021
Deuteron-deuteron collision at 160 MeV
The experiment was carried out using BINA detector at KVI in Groningen. For
the first time an extensive data analysis of the data collected in back part of
the detector is presented, where a clusterization method is utilized for
angular and energy information. We also present differential cross-sections for
the (dddpn) breakup reaction within \textit{dp} quasi-free
scattering limit and their comparison with first calculations based on Single
Scattering Approximation (SSA) approach.Comment: 6 pages, 4 figures, presented at Jagiellonian Symposium 2015 in
Krakow, PhD wor
Nuclear forces from chiral EFT: The unfinished business
In spite of the great progress we have seen in recent years in the derivation
of nuclear forces from chiral effective field theory (EFT), some important
issues are still unresolved. In this contribution, we discuss the open problems
which have particular relevance for microscopic nuclear structure, namely, the
proper renormalization of chiral nuclear potentials and sub-leading many-body
forces.Comment: 16 pages, 3 figures; contribution to J. Phys. G, Special Issue, Focus
Section: Open Problems in Nuclear Structur
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