94 research outputs found
Two-photon exchange and normal spin asymmetries in the A4 experiment
The A4 Collaboration at the MAMI facility has measured, at backward angles and at Q2 = 0.23 (GeV/c)2 and Q2 = 0.35 (GeV/c)2, the asymmetry in the elastic and quasielastic scattering of normally polarized electrons on hydrogen and deuterium, respectively. Some preliminary results will be presented
Detection of K+ mesons in segmented electromagnetic calorimeters
The combination of the CrystalBall and TAPS electromagnetic calorimeters were installed in the MAMI A2 hall in 2003. Here they are able to detect the reaction products from photo-induced reactions in combination with the Glasgow photon tagger. In the last two years the MAMI facility was upgraded from 885 MeV to 1.5 GeV, the A2 photon tagger underwent a similar upgrade crossing the threshold for strangeness photoproduction. For the CrystalBall this created a new challenge, to identify K+ mesons above the large background from other charged hadrons, in a situation where the detector setup does not benefit from a magnetic field to help separate particle species. These proceedings outline a novel technique which uses the decay products of the K+ as a strangeness tag
Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E
Photoproduction of mesons from neutrons} \abstract{Results from
measurements of the photoproduction of mesons from quasifree protons and
neutrons are summarized. The experiments were performed with the CBELSA/TAPS
detector at the electron accelerator ELSA in Bonn using the
decay. A liquid deuterium target was used for the
measurement of total cross sections and angular distributions. The results
confirm earlier measurements from Bonn and the MAMI facility in Mainz about the
existence of a narrow structure in the excitation function of . The current angular distributions show a forward-backward
asymmetry, which was previously not seen, but was predicted by model
calculations including an additional narrow state. Furthermore, data
obtained with a longitudinally polarized, deuterated butanol target and a
circularly polarized photon beam were analyzed to determine the double
polarization observable . Both data sets together were also used to extract
the helicity dependent cross sections and . The
narrow structure in the excitation function of
appears associated with the helicity-1/2 component of the reaction
Effects of nuclear correlations on the O reactions to discrete final states
Calculations of the O cross sections to the ground state and
first excited levels of the C and N nuclei are presented.
The effects of nuclear fragmentation have been obtained in a self-consistent
approach and are accounted for in the determination of the two-nucleon removal
amplitudes.
The Hilbert space is partitioned in order to compute the contribution of both
long- and short-range effects in a separate way.
Both the two-proton and the proton-neutron emission cross sections have been
computed within the same models for the reaction mechanism and the contribution
from nuclear structure, with the aim of better comparing the differences
between the two physical processes.
The O reaction is found to be sensitive to short-range
correlations, in agreement with previous results. The O cross
section to final states is dominated by the current and tensor
correlations. For both reactions, the interplay between collective (long-range)
effects and short-range and tensor correlations plays an important role. This
suggests that the selectivity of reactions to the final state can be
used to probe correlations also beyond short-range effects.Comment: 13 pages, 9 figure
Nuclear Density Dependence of In-Medium Polarization
It is shown that polarization transfer measurements on
a specific target nucleus can provide constraints on the ratio of the in-medium
electric to magnetic form factor. Thereby one exploits the fact that proton
knockout from single-particle levels exhibit a specific sensitivity to the
effective nuclear density. It is shown that in C the effective nuclear
density for -shell knockout is about twice as high as for -shell
knockout. With current model predictions for the in-medium form factors, one
obtains measurable modifications of the order of 5% in the ratios of the double
polarization observables between those single-particle levels
A luminosity monitor for the A4 parity violation experiment at MAMI
A water Cherenkov luminosity monitor system with associated electronics has
been developed for the A4 parity violation experiment at MAMI. The detector
system measures the luminosity of the hydrogen target hit by the MAMI electron
beam and monitors the stability of the liquid hydrogen target. Both is required
for the precise study of the count rate asymmetries in the scattering of
longitudinally polarized electrons on unpolarized protons. Any helicity
correlated fluctuation of the target density leads to false asymmetries. The
performance of the luminosity monitor, investigated in about 2000 hours with
electron beam, and the results of its application in the A4 experiment are
presented.Comment: 22 pages, 12 figures, submitted to NIM
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