336 research outputs found
The electric form factor of the neutron and its chiral content
Considering the nucleon as a system of confined valence quarks surrounded by
pions we derive a Galster-like parameterization of the neutron electric form
factor . Furthermore, we show that the proposed parameterization can be
linked to properties of the pion cloud. By this, the high quality data for the
pion form factor can be used in predictions of in the low region,
where the direct double polarization measurements are not available.Comment: 11 pages, 3 figure
124-Color Super-resolution Imaging by Engineering DNA-PAINT Blinking Kinetics
Optical super-resolution techniques reach unprecedented spatial resolution down to a few nanometers. However, efficient multiplexing strategies for the simultaneous detection of hundreds of molecular species are still elusive. Here, we introduce an entirely new approach to multiplexed super-resolution microscopy by designing the blinking behavior of targets with engineered binding frequency and duration in DNA-PAINT. We assay this kinetic barcoding approach in silico and in vitro using DNA origami structures, show the applicability for multiplexed RNA and protein detection in cells, and finally experimentally demonstrate 124-plex super-resolution imaging within minutes.We thank Martin Spitaler and the imaging facility of the MPI of Biochemistry for confocal imaging support
Magnetization screening from gluonic currents and scaling law violation in the ratio of magnetic form factors for neutron and proton
The ratio exhibits a decrease for four-momentum transfer
Q^2 increasing beyond 1 GeV^2 indicating different spatial distributions for
charge and for magnetization inside the proton. One-gluon exchange currents can
explain this behaviour. The SU(6) breaking induced by gluonic currents predicts
furthermore that the ratio of neutron to proton magnetic form factors
falls with increasing Q^2. We find that the
experimental data are consistent with our expectations of an almost linear
decrease of the ratio with increasing Q^2,
supporting the statement that the spatial distributions of magnetization are
different for protons and for neutrons.Comment: 10 pages, 3 figure
Three-nucleon mechanisms in photoreactions
The C reaction has been measured for
E=150-800 MeV in the first study of this reaction in a target
heavier than He. The experimental data are compared to a microscopic many
body calculation. The model, which predicts that the largest contribution to
the reaction arises from final state interactions following an initial pion
production process, overestimates the measured cross sections and there are
strong indications that the overestimate arises in this two-step process. The
selection of suitable kinematic conditions strongly suppresses this two-step
contribution leaving cross sections in which up to half the yield is predicted
to arise from the absorption of the photon on three interacting nucleons and
which agree with the model. The results indicate measurements on
nuclei may be a valuable tool for obtaining information on the nuclear
three-body interaction.Comment: 5 pages, 3 figure
Direct Test of the Scalar-Vector Lorentz Structure of the Nucleon- and Antinucleon-Nucleus Potential
Quantum Hadrodynamics in mean field approximation describes the effective
nucleon-nucleus potential (about -50 MeV deep) as resulting from a strong
repulsive vector (about 400 MeV) and a strong attractive scalar (about -450
MeV) contribution. This scalar-vector Lorentz structure implies a significant
lowering of the threshold for photoproduction on a nucleus by about
850 MeV as compared to the free case since charge conjugation reverses the sign
of the vector potential contribution in the equation of motion for the
states. It also implies a certain size of the photon induced
pair creation cross section near threshold which is calculated for a
target nucleus Pb. We also indicate a measurable second signature of
the photoproduction process by estimating the increased cross
section for emission of charged pions as a consequence of
annihilation within the nucleus.Comment: 18 pages latex, 5 PS figure
First measurements of the ^16O(e,e'pn)^14N reaction
This paper reports on the first measurement of the ^16O(e,e'pn)^14N reaction.
Data were measured in kinematics centred on a super-parallel geometry at energy
and momentum transfers of 215 MeV and 316 MeV/c. The experimental resolution
was sufficient to distinguish groups of states in the residual nucleus but not
good enough to separate individual states. The data show a strong dependence on
missing momentum and this dependence appears to be different for two groups of
states in the residual nucleus. Theoretical calculations of the reaction using
the Pavia code do not reproduce the shape or the magnitude of the data.Comment: 10 pages, 11 figures, 2 tables, Accepted for publication in EPJ
Effect of gluon-exchange pair-currents on the ratio G(E(P))/G(M(P))
The effect of one-gluon-exchange (OGE) pair-currents on the ratio for the proton is investigated within a nonrelativistic
constituent quark model (CQM) starting from nucleon wave
functions, but with relativistic corrections. We found that the OGE
pair-currents are important to reproduce well the ratio .
With the assumption that the OGE pair-currents are the driving mechanism for
the violation of the scaling law we give a prediction for the ratio of the neutron.Comment: 5 pages, 4 figure
Isoscalar short-range current in the deuteron induced by an intermediate dibaryon
A new model for short-range isoscalar currents in the deuteron and in the NN
system is developed; it is based on the generation of an intermediate dibaryon
which is the basic ingredient for the medium- and short-range NN interaction
which was proposed recently by the present authors.This new current model can
very well describe the experimental data for the three basic deuteron
observables of isoscalar magnetic type, viz. the magnetic moment, the circular
polarization of the photon in the process at thermal neutron
energies and the structure function B up to Q=60 fm.Comment: LaTex, 22 pages with 8 figure
Shell Structure Studies in the Pb-Region
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Shell Structure Studies in the Pb-Region
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
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