4,572 research outputs found
Stable divisorial gonality is in NP
Divisorial gonality and stable divisorial gonality are graph parameters,
which have an origin in algebraic geometry. Divisorial gonality of a connected
graph can be defined with help of a chip firing game on . The stable
divisorial gonality of is the minimum divisorial gonality over all
subdivisions of edges of .
In this paper we prove that deciding whether a given connected graph has
stable divisorial gonality at most a given integer belongs to the class NP.
Combined with the result that (stable) divisorial gonality is NP-hard by
Gijswijt, we obtain that stable divisorial gonality is NP-complete. The proof
consist of a partial certificate that can be verified by solving an Integer
Linear Programming instance. As a corollary, we have that the number of
subdivisions needed for minimum stable divisorial gonality of a graph with
vertices is bounded by for a polynomial
Carbon nanotubes grown in situ by a novel catalytic method
Carbon nanotubes can be produced by the catalytic decomposition of hydrocarbons on small metal particles. However, nanotubes are generally produced together with non-tubular filaments and tubes coated by pyrolytic carbon. We propose a novel catalyst method for the in situ production, in a composite powder, of a huge amount
of single- and multiwalled carbon nanotubes, having a diameter between 1.5 and 15 nm and arranged in bundles up to 100 mm long. We anticipate that dense materials prepared from such composite powders could have interesting mechanical and physical properties
Nuclear structure and elastic scattering observables obtained consistently with different NN interactions
Nucleon-nucleon () interactions based on chiral effective theories are
commonly used in ab initio calculations of light nuclei. Here we present a
study based on three different NN interactions (up to next-to-next-to-leading
order) for which structure and elastic proton scattering observables are
consistently calculated for He, C, and O. The interactions
are compared at the two-body level in terms of Wolfenstein amplitudes, and
their predictions for ground state energies, point-proton radii, and charge
form factors, as well as proton elastic scattering observables in the
leading-order spectator expansion in the energy range between 65 and 160 MeV
projectile energy are presented. To gain further insight into differences
visible in elastic scattering observables, we investigate the behavior of the
calculated effective nucleon-nucleus interactions for the C nucleus
based on the different interactions.Comment: 17 pages, 17 figure
Stochastic optimization of a cold atom experiment using a genetic algorithm
We employ an evolutionary algorithm to automatically optimize different
stages of a cold atom experiment without human intervention. This approach
closes the loop between computer based experimental control systems and
automatic real time analysis and can be applied to a wide range of experimental
situations. The genetic algorithm quickly and reliably converges to the most
performing parameter set independent of the starting population. Especially in
many-dimensional or connected parameter spaces the automatic optimization
outperforms a manual search.Comment: 4 pages, 3 figure
Ab initio nucleon-nucleus elastic scattering with chiral effective field theory uncertainties
Background: Effective interactions for nucleon-nucleus () elastic
scattering from first principles require the use of the same nucleon-nucleon
() interaction in the structure and reaction calculations, and a consistent
treatment of the relevant operators at each order.
Purpose: Truncation uncertainties of chiral forces have been studied for
scattering observables in few-body systems and for bound state properties of
light nuclei. We extend this to elastic scattering.
Methods: With the spectator expansion of multiple scattering theory and the
no-core shell model, we use a chiral interaction from the LENPIC collaboration
to consistently calculate the leading order effective interaction up to
third chiral order (N2LO) and extract elastic scattering observables. We
quantify the chiral truncation error using pointwise and correlated methods.
Results: We analyze proton-O and neutron-C elastic scattering
observables between 65 and 185 MeV projectile kinetic energy. We find
qualitatively similar results for the chiral truncation uncertainties as in
few-body systems, which we assess using similar diagnostic tools. The
order-by-order convergence of the scattering observables for O and
C is reasonable near 100 MeV, but for higher energies the expansion
parameter becomes too large to converge. We find a near-perfect correlation
between the neutron differential cross section and the Wolfenstein
amplitudes for small momentum transfers.
Conclusions: The tools used to study the convergence of a chiral
interaction in few-body systems can be applied to scattering with minor
changes. The interaction used here gives a good description of O
and C scattering observables as low as 65 MeV. The very forward
direction of the neutron differential cross section mirrors the behavior of the
interaction amazingly well.Comment: 17 pages, 13 figures, 1 tabl
Prospects for Spin Physics at RHIC
Colliding beams of 70% polarized protons at up to =500 GeV, with
high luminosity, L=2 cmsec, will represent a
new and unique laboratory for studying the proton. RHIC-Spin will be the first
polarized-proton collider and will be capable of copious production of jets,
directly produced photons, and and bosons. Features will include direct
and precise measurements of the polarization of the gluons and of ,
, , and quarks in a polarized proton. Parity violation searches
for physics beyond the standard model will be competitive with unpolarized
searches at the Fermilab Tevatron. Transverse spin will explore transversity
for the first time, as well as quark-gluon correlations in the proton. Spin
dependence of the total cross section and in the Coulomb nuclear interference
region will be measured at collider energies for the first time. These
qualitatively new measurements can be expected to deepen our understanding of
the structure of matter and of the strong interaction.Comment: 51 pages, 22 figures. Scheduled to appear in the Annual Review of
Nuclear and Particle Science Vol. 50, to be published in December 2000 by
Annual Reviews, http://AnnualReviews.or
Ab initio Leading Order Effective Potentials for Elastic Nucleon-Nucleus Scattering
Background: Calculating microscopic effective interactions (optical
potentials) for elastic nucleon-nucleus scattering has already in the past led
to a large body of work. For first-order calculations a nucleon-nucleon
(\textit{NN}) interaction and a one-body density of the nucleus were taken as
input to rigorous calculations of microscopic full-folding calculations.
Purpose: Based on the spectator expansion of the multiple scattering series
we employ a chiral next-to-next-to-leading order (NNLO) nucleon-nucleon
interaction on the same footing in the structure as well as in the reaction
calculation to obtain an in leading-order consistent effective potential for
nucleon-nucleus elastic scattering, which includes the spin of the struck
target nucleon.
Methods: The first order effective folding potential is computed by first
deriving a nonlocal scalar density as well as a spin-projected momentum
distribution. Those are then integrated with the off-shell Wolfenstein
amplitudes , , and . The resulting nonlocal potential serves as input
to a momentum-space Lippmann-Schwinger equation, whose solutions are summed to
obtain the nucleon-nucleus scattering observables.
Results: We calculate elastic scattering observables for He, He,
He, C, and O in the energy regime between 100 and 200 MeV
projectile kinetic energy, and compare to available data. We also explore the
extension down to about 70 MeV, and study the effect of ignoring the spin of
the struck nucleon in the nucleus.
Conclusions: In our calculations we contrast elastic scattering off
closed-shell and open-shell nuclei. We find that for closed-shell nuclei the
approximation of ignoring the spin of the struck target nucleon is excellent.
We only see effects of the spin of the struck target nucleon when considering
He and He, which are nuclei with a ratio larger than 1.Comment: 13 pages, 13 figure
Nuclear spin features relevant to ab initio nucleon-nucleus elastic scattering
Background: Effective interactions for elastic nucleon-nucleus scattering
from first principles require the use of the same nucleon-nucleon interaction
in the structure and reaction calculations, as well as a consistent treatment
of the relevant operators at each order.
Purpose: Previous work using these interactions has shown good agreement with
available data. Here, we study the physical relevance of one of these
operators, which involves the spin of the struck nucleon, and examine the
interpretation of this quantity in a nuclear structure context.
Methods: Using the framework of the spectator expansion and the underlying
framework of the no-core shell model, we calculate and examine spin-projected,
one-body momentum distributions required for effective nucleon-nucleus
interactions in nuclear states.
Results: The calculated spin-projected, one-body momentum distributions for
He, He, and He display characteristic behavior based on the
occupation of protons and neutrons in single particle levels, with more
nucleons of one type yielding momentum distributions with larger values.
Additionally, we find this quantity is strongly correlated to the magnetic
moment of the excited state in the ground state rotational band for each
nucleus considered.
Conclusions: We find that spin-projected, one-body momentum distributions can
probe the spin content of a wave function. This feature may allow future
\textit{ab initio} nucleon-nucleus scattering studies to inform spin properties
of the underlying nucleon-nucleon interactions. The observed correlation to the
magnetic moment of excited states illustrates a previously unknown connection
between reaction observables such as the analyzing power and structure
observables like the magnetic moment.Comment: 13 pages, 7 figures, 1 tabl
An investigation of carbon nanotubes obtained from the decomposition of methane over reduced Mg1− xM xAl2O4 spinel catalysts
Carbon nanotubes produced by the treatment of Mg1−xMxAl2O4 (M = Fe, Co, or Ni; x = 0.1, 0.2, 0.3, or 0.4) spinels with an H2–CH4 mixture at 1070 °C have been investigated systematically. The grains of the oxide-metal composite particles are uniformly covered by a weblike network of carbon nanotube bundles, several tens of micrometers long, made up of single-wall nanotubes with a diameter close to 4 nm. Only the smallest metal particles (<5 nm) are involved in the formation of the nanotubes. A macroscopic characterization method involving surface area measurements and chemical analysis has been developed in order to compare the different nanotube specimens. An increase in the transition metal content of the catalyst yields more carbon nanotubes (up to a metal content of 10.0 wt% or x = 0.3), but causes a decrease in carbon quality. The best compromise is to use 6.7 wt% of metal (x = 0.2) in the catalyst. Co gives superior results with respect to both the quantity and quality of the nanotubes. In the case of Fe, the quality is notably hampered by the formation of Fe3C particles
Nuclear and peroxisomal targeting of catalase
Catalase is a well-known component of the cellular antioxidant network, but there have been conflicting conclusions reached regarding the nature of its peroxisome targeting signal. It has also been reported that catalase can be hijacked to the nucleus by effector proteins of plant pathogens. Using a physiologically relevant system where native untagged catalase variants are expressed in a cat2-1 mutant background, the C terminal most 18 amino acids could be deleted without affecting activity, peroxisomal targeting or ability to complement multiple phenotypes of the cat2-1 mutant. In contrast, converting the native C terminal tripeptide PSI to the canonical PTS1 sequence ARL resulted in lower catalase specific activity. Localisation experiments using split superfolder green fluorescent protein revealed that catalase can be targeted to the nucleus in the absence of any pathogen effectors, and that C terminal tagging in combination with alterations of the native C terminus can interfere with nuclear localisation. These findings provide fundamental new insights into catalase targeting and pave the way for exploration of the mechanism of catalase targeting to the nucleus and its role in non-infected plants
- …