103 research outputs found
Quasi-free Photoproduction from the Bound Nucleon
Differential cross-sections for quasi-free photoproduction from the
proton and neutron bound in the deuteron have been measured for MeV at usind the Glasgow photon
tagger at MAMI, the Mainz 48 cm 64 cm NaI(Tl) photon
detector and the G\"ottingen SENECA recoil detector. For the proton
measurements made with both liquid deuterium and liquid hydrogen targets allow
direct comparison of "free" photoproduction cross-sections as extracted
from the bound proton data with experimental free cross sections which are
found to be in reasonable agreement below 320 MeV. At higher energies the
"free" cross sections extracted from quasifree data are significantly smaller
than the experimental free cross sections and theoretical predictions based on
multipole analysis. For the first time, "free" neutron cross sections have been
extracted in the -region. They are also in agreement with the
predictions from multipole analysis up to 320 MeV and significantly smaller at
higher photon energies
Quasi-free Compton Scattering and the Polarizabilities of the Neutron
Differential cross sections for quasi-free Compton scattering from the proton
and neutron bound in the deuteron have been measured using the Glasgow/Mainz
tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48
cm 64 cm NaI(Tl) photon detector and the G\"ottingen SENECA
recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV
at . Liquid deuterium and hydrogen targets
allowed direct comparison of free and quasi-free scattering from the proton.
The neutron detection efficiency of the SENECA detector was measured via the
reaction . The "free" proton Compton scattering cross
sections extracted from the bound proton data are in reasonable agreement with
those for the free proton which gives confidence in the method to extract the
differential cross section for free scattering from quasi-free data.
Differential cross sections on the free neutron have been extracted and the
difference of the electromagnetic polarizabilities of the neutron have been
obtained to be
in units . In combination with the polarizability sum deduced from photoabsorption data, the neutron electric and
magnetic polarizabilities, and
are obtained. The backward spin polarizability of the neutron was determined to
be
Compton Scattering by the Proton using a Large-Acceptance Arrangement
Compton scattering by the proton has been measured using the tagged-photon
facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The new
data are interpreted in terms of dispersion theory based on the SAID-SM99K
parameterization of photo-meson amplitudes. It is found that two-pion exchange
in the t-channel is needed for a description of the data in the second
resonance region. The data are well represented if this channel is modeled by a
single pole with mass parameter m(sigma)=600 MeV. The asymptotic part of the
spin dependent amplitude is found to be well represented by pi-0-exchange in
the t-channel. A backward spin-polarizability of
gamma(pi)=(-37.1+-0.6(stat+syst)+-3.0(model))x10^{-4}fm^4 has been determined
from data of the first resonance region below 455 MeV. This value is in a good
agreement with predictions of dispersion relations and chiral pertubation
theory. From a subset of data between 280 and 360 MeV the resonance
pion-photoproduction amplitudes were evaluated leading to a E2/M1 multipole
ratio of the p-to-Delta radiative transition of EMR(340
MeV)=(-1.7+-0.4(stat+syst)+-0.2(model))%. It was found that this number is
dependent on the parameterization of photo-meson amplitudes. With the MAID2K
parameterization an E2/M1 multipole ratio of EMR(340
MeV)=(-2.0+-0.4(stat+syst)+-0.2(model))% is obtained
Compton Scattering from the Deuteron and Extracted Neutron Polarizabilities
Differential cross sections for Compton scattering from the deuteron were
measured at MAX-lab for incident photon energies of 55 MeV and 66 MeV at
nominal laboratory angles of , , and . Tagged
photons were scattered from liquid deuterium and detected in three NaI
spectrometers. By comparing the data with theoretical calculations in the
framework of a one-boson-exchange potential model, the sum and difference of
the isospin-averaged nucleon polarizabilities, and (in units of fm),
have been determined. By combining the latter with the global-averaged value
for and using the predictions of the Baldin sum rule for
the sum of the nucleon polarizabilities, we have obtained values for the
neutron electric and magnetic polarizabilities of (total) (model) and (total) (model), respectively.Comment: 4 pages, 2 figures, revtex. The text is substantially revised. The
cross sections are slightly different due to improvements in the analysi
Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron
Measuring Compton scattered photons and recoil neutrons in coincidence,
quasi-free Compton scattering by the neutron has been investigated at MAMI
(Mainz) at in an energy range from 200 to 400 MeV.
From the data a polarizability difference of in units of has been
determined. In combination with the polarizability sum deduced from photo absorption data, the neutron electric and
magnetic polarizabilities, and ,
are obtained
The helicity amplitudes A and A for the D resonance obtained from the reaction}
The helicity dependence of the reaction
has been measured for the first time in the photon energy range from 550 to 790
MeV. The experiment, performed at the Mainz microtron MAMI, used a
4-detector system, a circularly polarized, tagged photon beam, and a
longitudinally polarized frozen-spin target. These data are predominantly
sensitive to the resonance and are used to determine its
parameters.Comment: 5 pages, 4 figure
Effective theory of the Delta(1232) in Compton scattering off the nucleon
We formulate a new power-counting scheme for a chiral effective field theory
of nucleons, pions, and Deltas. This extends chiral perturbation theory into
the Delta-resonance region. We calculate nucleon Compton scattering up to
next-to-leading order in this theory. The resultant description of existing
p cross section data is very good for photon energies up to about 300
MeV. We also find reasonable numbers for the spin-independent polarizabilities
and .Comment: 29 pp, 9 figs. Minor revisions. To be published in PR
Acer rubrum Wats.
https://thekeep.eiu.edu/herbarium_specimens_byname/21742/thumbnail.jp
First measurement of the Gerasimov-Drell-Hearn integral for Hydrogen from 200 to 800 MeV
A direct measurement of the helicity dependence of the total photoabsorption
cross section on the proton was carried out at MAMI (Mainz) in the energy range
200 < E_gamma < 800 MeV. The experiment used a 4 detection system, a
circularly polarized tagged photon beam and a frozen spin target.
The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward
spin polarizability determined from the data are 226 \pm 5 (stat)\pm
12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for
200 < E_\gamma < 800 MeV.Comment: 6 pages, 3 figures, 3 table
Nucleon Polarizabilities from Deuteron Compton Scattering within a Green's-Function Hybrid Approach
We examine elastic Compton scattering from the deuteron for photon energies
ranging from zero to 100 MeV, using state-of-the-art deuteron wave functions
and NN-potentials. Nucleon-nucleon rescattering between emission and absorption
of the two photons is treated by Green's functions in order to ensure gauge
invariance and the correct Thomson limit. With this Green's-function hybrid
approach, we fulfill the low-energy theorem of deuteron Compton scattering and
there is no significant dependence on the deuteron wave function used.
Concerning the nucleon structure, we use Chiral Effective Field Theory with
explicit \Delta(1232) degrees of freedom within the Small Scale Expansion up to
leading-one-loop order. Agreement with available data is good at all energies.
Our 2-parameter fit to all elastic data leads to values for the
static isoscalar dipole polarizabilities which are in excellent agreement with
the isoscalar Baldin sum rule. Taking this value as additional input, we find
\alpha_E^s= (11.3+-0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and \beta_M^s =
(3.2-+0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and conclude by comparison to the
proton numbers that neutron and proton polarizabilities are essentially the
same.Comment: 47 pages LaTeX2e with 20 figures in 59 .eps files, using graphicx.
Minor modifications; extended discussion of theoretical uncertainties of
polarisabilities extraction. Version accepted for publication in EPJ
- …