120 research outputs found
The {\eta}'-carbon potential at low meson momenta
The production of mesons in coincidence with forward-going
protons has been studied in photon-induced reactions on C and on a
liquid hydrogen (LH) target for incoming photon energies of 1.3-2.6 GeV at
the electron accelerator ELSA. The mesons have been identified
via the decay
registered with the CBELSA/TAPS detector system. Coincident protons have been
identified in the MiniTAPS BaF array at polar angles of . Under these kinematic constraints the
mesons are produced with relatively low kinetic energy (
150 MeV) since the coincident protons take over most of the momentum of the
incident-photon beam. For the C-target this allows the determination of the
real part of the -carbon potential at low meson momenta by
comparing with collision model calculations of the kinetic energy
distribution and excitation function. Fitting the latter data for
mesons going backwards in the center-of-mass system yields a potential depth of
V = (44 16(stat)15(syst)) MeV, consistent with earlier
determinations of the potential depth in inclusive measurements for average
momenta of 1.1 GeV/. Within the experimental
uncertainties, there is no indication of a momentum dependence of the
-carbon potential. The LH data, taken as a reference to check
the data analysis and the model calculations, provide differential and integral
cross sections in good agreement with previous results for
photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with
arXiv:1608.0607
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
The N(1520) 3/2- helicity amplitudes from an energy-independent multipole analysis based on new polarization data on photoproduction of neutral pions
New data on the polarization observables T, P, and H for the reaction are reported. The results are extracted from azimuthal
asymmetries when a transversely polarized butanol target and a linearly
polarized photon beam are used. The data were taken at the Bonn electron
stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier
data are used to perform a truncated energy-independent partial wave analysis
in sliced-energy bins. This energy-independent analysis is compared to the
results from energy-dependent partial wave analyses
The polarization observables T, P, and H and their impact on multipoles
Data on the polarization observables T, P, and H for the reaction are reported. Compared to earlier data from other experiments, our
data are more precise and extend the covered range in energy and angle
substantially. The results were extracted from azimuthal asymmetries measured
using a transversely polarized target and linearly polarized photons. The data
were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS
detector. Within the Bonn-Gatchina partial wave analysis, the new polarization
data lead to a significant narrowing of the error band for the multipoles for
neutral-pion photoproduction
and photoproduction with fine center-of-mass energy resolution
Measurements of and cross-sections have been obtained with the photon
tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement
uses a novel meson identification technique in which the weak decay
products are characterized using the energy and timing characteristics of the
energy deposit in the calorimeter, a method that has the potential to be
applied at many other facilities. The fine center-of-mass energy ()
resolution and statistical accuracy of the new data results in a significant
impact on partial wave analyses aiming to better establish the excitation
spectrum of the nucleon. The new analyses disfavor a strong role for
quark-diquark dynamics in the nucleon.Comment: 7 pages, 8 figure
Measurement of the beam-helicity asymmetry in the photoproduction of -pairs off protons and off neutrons
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz
for the photoproduction of mixed-charge pion pairs in the reactions
off free protons and
and
off quasi-free nucleons bound
in the deuteron for incident photon energies up to 1.4 GeV. Circularly
polarized photons were produced from bremsstrahlung of longitudinally polarized
electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged
pions, recoil protons, recoil neutrons, and decay photons from mesons
were detected in the 4 electromagnetic calorimeter composed of the Crystal
Ball and TAPS detectors. Using a complete kinematic reconstruction of the final
state, excellent agreement was found between the results for free and
quasi-free protons, suggesting that the quasi-free neutron results are also a
close approximation of the free-neutron asymmetries. A comparison of the
results to the predictions of the Two-Pion-MAID reaction model shows that the
reaction mechanisms are still not well understood, in particular at low
incident photon energies in the second nucleon-resonance region.Comment: accepted for publication in Eur. phys. J.
Quasifree photoproduction of mesons off protons and neutrons
Differential and total cross sections for the quasifree reactions and have been determined at the
MAMI-C electron accelerator using a liquid deuterium target. Photons were
produced via bremsstrahlung from the 1.5 GeV incident electron beam and
energy-tagged with the Glasgow photon tagger. Decay photons of the neutral
decay modes and and coincident recoil nucleons were detected in a combined setup of
the Crystal Ball and the TAPS calorimeters. The -production cross
sections were measured in coincidence with recoil protons, recoil neutrons, and
in an inclusive mode without a condition on recoil nucleons, which allowed a
check of the internal consistency of the data. The effects from nuclear Fermi
motion were removed by a kinematic reconstruction of the final-state invariant
mass and possible nuclear effects on the quasifree cross section were
investigated by a comparison of free and quasifree proton data. The results,
which represent a significant improvement in statistical quality compared to
previous measurements, agree with the known neutron-to-proton cross-section
ratio in the peak of the resonance and confirm a peak in the
neutron cross section, which is absent for the proton, at a center-of-mass
energy MeV with an intrinsic width of MeV
Measurement of the transverse target and beam-target asymmetries in meson photoproduction at MAMI
We present new data for the transverse target asymmetry T and the very first
data for the beam-target asymmetry F in the
reaction up to a center-of-mass energy of W=1.9 GeV. The data were obtained
with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility
of the Mainz Microtron MAMI. All existing model predictions fail to reproduce
the new data indicating a significant impact on our understanding of the
underlying dynamics of meson photoproduction. The peculiar nodal
structure observed in existing T data close to threshold is not confirmed.Comment: 5 pages, 3 figures, accepted for publication in PR
T and F asymmetries in π0 photoproduction on the proton
The γp→π0p reaction was studied at laboratory photon energies from 425 to 1445 MeV with a transversely polarized target and a longitudinally polarized beam. The beam-target asymmetry F was measured for the first time and new high precision data for the target asymmetry T were obtained. The experiment was performed at the photon tagging facility of the Mainz Microtron (MAMI) using the Crystal Ball and TAPS photon spectrometers. The polarized cross sections were expanded in terms of associated Legendre functions and compared to recent predictions from several partial-wave analyses. The impact of the new data on our understanding of the underlying partial-wave amplitudes and baryon resonance contributions is discussed
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