7,809 research outputs found
The two-nucleon electromagnetic charge operator in chiral effective field theory (EFT) up to one loop
The electromagnetic charge operator in a two-nucleon system is derived in
chiral effective field theory (EFT) up to order (or N4LO), where
denotes the low-momentum scale and is the electric charge. The specific
form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and
two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for
the non-static terms in the corresponding potentials. We show that different
prescriptions lead to unitarily equivalent potentials and accompanying charge
operators. Thus, provided a consistent set is adopted, predictions for physical
observables will remain unaffected by the non-uniqueness associated with these
off-the-energy-shell effects.Comment: 16 pages, 10 figure
Electromagnetic Structure and Reactions of Few-Nucleon Systems in EFT
We summarize our recent work dealing with the construction of the
nucleon-nucleon potential and associated electromagnetic currents up to one
loop in chiral effective field theory (EFT). The magnetic dipole
operators derived from these currents are then used in hybrid calculations of
static properties and low-energy radiative capture processes in few-body
nuclei. A preliminary set of results are presented for the magnetic moments of
the deuteron and trinucleons and thermal neutron captures on , , and
He.Comment: Invited talk to the 19th International IUPAP Conference on Few-Body
Problems in Physic
Electromagnetic processes in a EFT framework
Recently, we have derived a two--nucleon potential and consistent nuclear
electromagnetic currents in chiral effective field theory with pions and
nucleons as explicit degrees of freedom. The calculation of the currents has
been carried out to include NLO corrections, consisting of two--pion
exchange and contact contributions. The latter involve unknown low-energy
constants (LECs), some of which have been fixed by fitting the S- and
P-wave phase shifts up to 100 MeV lab energies. The remaining LECs entering the
current operator are determined so as to reproduce the experimental deuteron
and trinucleon magnetic moments, as well as the cross section. This
electromagnetic current operator is utilized to study the and He
radiative captures at thermal neutron energies. Here we discuss our results
stressing on the important role played by the LECs in reproducing the
experimental data.Comment: Invited talk at the 5th International Conference on Quarks and
Nuclear Physics, to appear in Chinese Physics
Thermal neutron captures on and He
We report on a study of the and n\,^3He radiative captures at thermal
neutron energies, using wave functions obtained from either chiral or
conventional two- and three-nucleon realistic potentials with the
hyperspherical harmonics method, and electromagnetic currents derived in chiral
effective field theory up to one loop. The predicted and n\,^3He cross
sections are in good agreement with data, but exhibit a significant dependence
on the input Hamiltonian. A comparison is also made between these and new
results for the and n\,^3He cross sections obtained in the conventional
framework for both potentials and currents.Comment: 4 pages, 4 eps figures; references added; corrections to text and
abstract as suggested by referee adde
Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory
The objectives of the present work are twofold. The first is to address and
resolve some of the differences present in independent,
chiral-effective-field-theory (\chiEFT) derivations up to one loop, recently
appeared in the literature, of the nuclear charge and current operators. The
second objective is to provide a complete set of \chiEFT predictions for the
structure functions and tensor polarization of the deuteron, for the charge and
magnetic form factors of 3He and 3H, and for the charge and magnetic radii of
these few-nucleon systems. The calculations use wave functions derived from
high-order chiral two- and three-nucleon potentials and Monte Carlo methods to
evaluate the relevant matrix elements. Predictions based on conventional
potentials in combination with \chiEFT charge and current operators are also
presented. There is excellent agreement between theory and experiment for all
these observables for momentum transfers up to q< 2.0-2.5 (1/fm); for a subset
of them, this agreement extends to momentum transfers as high as q~5-6 (1/fm).
A complete analysis of the results is provided.Comment: 34 pages, Revte
Electromagnetic two-body currents of one- and two-pion range
Nuclear electromagnetic currents are derived in time-ordered perturbation
theory within an effective-field-theory framework including explicit nucleons,
isobars, and pions up to one loop, or NLO. The currents obtained
at NLO, {\it i.e.} ignoring loop corrections, are used in a study of
neutron radiative captures on protons and deuterons at thermal energies, and of
=2 and 3 nuclei magnetic moments. The wave functions for =2 are derived
from solutions of the Schr\"odinger equation with the Argonne (AV18)
or CD-Bonn (CDB) potentials, while those for =3 are obtained with the
hyperspherical-harmonics-expansion method from a realistic Hamiltonian
including, in addition to the AV18 or CDB two-nucleon, also a three-nucleon
potential. With the strengths of the -excitation currents occurring at
NLO determined to reproduce the - cross section and isovector
combination of the trinucleon magnetic moments, we find that the cross section
and photon circular polarization parameter, measured in - and
- processes, are underpredicted by theory, for example the cross
section by (11--38)% as the cutoff is increased from 500 to 800 MeV. A complete
analysis of the results, in particular their large cutoff dependence, is
presented
Local chiral interactions and magnetic structure of few-nucleon systems
The magnetic form factors of H, H, and He, deuteron
photodisintegration cross sections at low energies, and deuteron threshold
electrodisintegration cross sections at backward angles in a wide range of
momentum transfers, are calculated with the chiral two-nucleon (and
three-nucleon) interactions including intermediate states that have
recently been constructed in configuration space. The =3 wave
functions are obtained from hyperspherical-harmonics solutions of the
Schr\"odinger equation. The electromagnetic current includes one- and two-body
terms, the latter induced by one- and two-pion exchange (OPE and TPE,
respectively) mechanisms and contact interactions. The contributions associated
with intermediate states are only retained at the OPE level, and are
neglected in TPE loop (tree-level) corrections to two-body (three-body) current
operators. Expressions for these currents are derived and regularized in
configuration space for consistency with the interactions. The low-energy
constants that enter the contact few-nucleon systems. The predicted form
factors and deuteron electrodisintegration cross section are in excellent
agreement with experiment for momentum transfers up to 2--3 fm. However,
the experimental values for the deuteron photodisintegration cross section are
consistently underestimated by theory, unless use is made of the Siegert form
of the electric dipole transition operator. A complete analysis of the results
is provided, including the clarification of the origin of the aforementioned
discrepancy.Comment: 24 pages, 13 figure
Electromagnetic Currents and Magnetic Moments in EFT
A two-nucleon potential and consistent electromagnetic currents are derived
in chiral effective field theory (EFT) at, respectively, (or
NLO) and (or NLO), where generically denotes the low-momentum
scale and is the electric charge. Dimensional regularization is used to
renormalize the pion-loop corrections. A simple expression is derived for the
magnetic dipole () operator associated with pion loops, consisting of two
terms, one of which is determined, uniquely, by the isospin-dependent part of
the two-pion-exchange potential. This decomposition is also carried out for the
operator arising from contact currents, in which the unique term is
determined by the contact potential. Finally, the low-energy constants (LEC's)
entering the NLO potential are fixed by fits to the S- and P-wave
phase shifts up to 100 MeV lab energies.Comment: v2: references added; corrections to text and abstract suggested by
referee adde
Discrete surface growth process as a synchronization mechanism for scale free complex networks
We consider the discrete surface growth process with relaxation to the
minimum [F. Family, J. Phys. A {\bf 19} L441, (1986).] as a possible
synchronization mechanism on scale-free networks, characterized by a degree
distribution , where is the degree of a node and
his broadness, and compare it with the usually applied
Edward-Wilkinson process [S. F. Edwards and D. R. Wilkinson, Proc. R. Soc.
London Ser. A {\bf 381},17 (1982) ]. In spite of both processes belong to the
same universality class for Euclidean lattices, in this work we demonstrate
that for scale-free networks with exponents this is not true.
Moreover, we show that for these ubiquitous cases the Edward-Wilkinson process
enhances spontaneously the synchronization when the system size is increased,
which is a non-physical result. Contrarily, the discrete surface growth process
do not present this flaw and is applicable for every .Comment: 8 pages, 4 figure
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