10,253 research outputs found
Semilocal momentum-space regularized chiral two-nucleon potentials up to fifth order
We introduce new semilocal two-nucleon potentials up to fifth order in the
chiral expansion. We employ a simple regularization approach for the
pion-exchange contributions which (i) maintains the long-range part of the
interaction, (ii) is implemented in momentum space and (iii) can be
straightforwardly applied to regularize many-body forces and current operators.
We discuss in detail the two-nucleon contact interactions at fourth order and
demonstrate that three terms out of fifteen used in previous calculations can
be eliminated via suitably chosen unitary transformations. The removal of the
redundant contact terms results in a drastic simplification of the fits to
scattering data and leads to interactions which are much softer (i.e. more
perturbative) than our recent semilocal coordinate-space regularized
potentials. Using the pion-nucleon low-energy constants from matching
pion-nucleon Roy-Steiner equations to chiral perturbation theory, we perform a
comprehensive analysis of nucleon-nucleon scattering and the deuteron
properties up to fifth chiral order and study the impact of the leading F-wave
two-nucleon contact interactions which appear at sixth order. The resulting
chiral potentials lead to an outstanding description of the proton-proton and
neutron-proton scattering data from the self-consistent Granada-2013 database
below the pion production threshold, which is significantly better than for any
other chiral potential. For the first time, the chiral potentials match in
precision and even outperform the available high-precision phenomenological
potentials, while the number of adjustable parameters is, at the same time,
reduced by about ~40%. Last but not least, we perform a detailed error analysis
and, in particular, quantify for the first time the statistical uncertainties
of the fourth- and the considered sixth-order contact interactions.Comment: 57 pages, 17 figures, 19 table
Precision nucleon-nucleon potential at fifth order in the chiral expansion
We present a nucleon-nucleon potential at fifth order in chiral effective
field theory. We find a substantial improvement in the description of
nucleon-nucleon phase shifts as compared to the fourth-order results of Ref.
[E. Epelbaum, H. Krebs, U.-G. Mei{\ss}ner, arXiv:1412.0142 [nucl-th]]. This
provides clear evidence of the corresponding two-pion exchange contributions
with all low-energy constants being determined from pion-nucleon scattering.
The fifth-order corrections to nucleon-nucleon observables appear to be of a
natural size which confirms the good convergence of the chiral expansion for
nuclear forces. Furthermore, the obtained results provide strong support for
the novel way of quantifying the theoretical uncertainty due to the truncation
of the chiral expansion proposed in Ref. [E. Epelbaum, H. Krebs, U.-G.
Mei{\ss}ner, arXiv:1412.0142 [nucl-th]]. Our work opens up new perspectives for
precision ab initio calculations in few- and many-nucleon systems and is
especially relevant for ongoing efforts towards a quantitative understanding
the structure of the three-nucleon force in the framework of chiral effective
field theory.Comment: 5 pages, 4 figures, 3 table
Nuclear axial current operators to fourth order in chiral effective field theory
We present the complete derivation of the nuclear axial charge and current
operators as well as the pseudoscalar operators to fourth order in the chiral
expansion relative to the dominant one-body contribution using the method of
unitary transformation. We demonstrate that the unitary ambiguity in the
resulting operators can be eliminated by the requirement of renormalizability
and by matching of the pion-pole contributions to the nuclear forces. We give
expressions for the renormalized single-, two- and three-nucleon contributions
to the charge and current operators and pseudoscalar operators including the
relevant relativistic corrections. We also verify explicitly the validity of
the continuity equation.Comment: 72 pages, 21 figures, 3 table
Improved chiral nucleon-nucleon potential up to next-to-next-to-next-to-leading order
We present improved nucleon-nucleon potentials derived in chiral effective
field theory up to next-to-next-to-next-to-leading order. We argue that the
nonlocal momentum-space regulator employed in the two-nucleon potentials of
Refs. [E. Epelbaum, W. Gloeckle, U.-G. Mei{\ss}ner, Nucl. Phys. A747 (2005)
362], [D.R. Entem, R. Machleidt, Phys. Rev. C68 (2003) 041001] is not the most
efficient choice, in particular since it affects the long-range part of the
interaction. We are able to significantly reduce finite-cutoff artefacts by
using an appropriate regularization in coordinate space which maintains the
analytic structure of the amplitude. The new potentials do not require the
additional spectral function regularization employed in Ref. [E. Epelbaum, W.
Gloeckle, U.-G. Mei{\ss}ner, Nucl. Phys. A747 (2005) 362] to cut off the
short-range components of the two-pion exchange and make use of the low-energy
constants c_i and d_i determined from pion-nucleon scattering without any fine
tuning. We discuss in detail the construction of the new potentials and
convergence of the chiral expansion for two-nucleon observables. We also
introduce a new procedure for estimating the theoretical uncertainty from the
truncation of the chiral expansion that replaces previous reliance on cutoff
variation.Comment: 34 pages, 13 figures, 7 table
Near threshold neutral pion electroproduction on deuterium in chiral perturbation theory
Near threshold neutral pion electroproduction on the deuteron is studied in
the framework of baryon chiral perturbation theory at next-to-leading order in
the chiral expansion. We develop the multipole decomposition for pion
production off spin-1 particles appropriate for the threshold region. The
existing data at photon virtuality k^2 = -0.1 GeV^2 can be described
satisfactorily. Furthermore, the prediction for the S-wave multipole E_d at the
photon point is in good agreement with the data.Comment: 27 pp, 15 fig
Test instrumentation evaluates electrostatic hazards in fluid system
RJ-1 fuel surface potential is measured with a probe to determine the degree of hazard originating from static electricity buildup in the hydraulic fluid. The probe is mounted in contact with the fluid surface and connected to an electrostatic voltmeter
Complex Behavior in Simple Models of Biological Coevolution
We explore the complex dynamical behavior of simple predator-prey models of
biological coevolution that account for interspecific and intraspecific
competition for resources, as well as adaptive foraging behavior. In long
kinetic Monte Carlo simulations of these models we find quite robust 1/f-like
noise in species diversity and population sizes, as well as power-law
distributions for the lifetimes of individual species and the durations of
quiet periods of relative evolutionary stasis. In one model, based on the
Holling Type II functional response, adaptive foraging produces a metastable
low-diversity phase and a stable high-diversity phase.Comment: 8 pages, 5 figure
ENSO suppression due to weakening of the North Atlantic thermohaline circulation
Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections
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