.
Results from measurements of the photoproduction of
η
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
η
→
3
π
0
→
6
γ
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
γ
n
→
n
η
. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow
P
11
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
E
. Both data sets together were also used to extract the helicity-dependent cross sections
σ
1
/
2
and
σ
3
/
2
. The narrow structure in the excitation function of
γ
n
→
n
η
appears associated with the helicity-1/2 component of the reaction.
.
This paper reports on a measurement of the double-polarization observable
G
in
π
0
photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The observable
G
...is determined from reactions of linearly polarized photons with longitudinally polarized protons. The polarized photons are produced by bremsstrahlung off a diamond radiator of well-defined orientation. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for
G
are compared to predictions by the Bonn-Gatchina (BnGa), Jülich-Bonn (JüBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.
A measurement of the double-polarization observable
for the reaction
is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol ...(C
H
OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from
to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.
A measurement of the double-polarization observable E for the reaction γp→π0p is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin ...butanol (C4H9OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from Eγ=600 to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.
Photoproduction of \(\eta\) mesons from neutrons} \abstract{Results from measurements of the photoproduction of \(\eta\) 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 \(\eta\to3\pi^{0}\to6\gamma\) 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 \(\gamma n\rightarrow n\eta\). The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow \(P_{11}\) 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 \(E\). Both data sets together were also used to extract the helicity dependent cross sections \(\sigma_{1/2}\) and \(\sigma_{3/2}\). The narrow structure in the excitation function of \(\gamma n\rightarrow n\eta\) appears associated with the helicity-1/2 component of the reaction.
New data on the polarization observables T, P, and H for the reaction \(\gamma p \to p\pi^0\) 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 excitation function and momentum distribution of \(\eta^\prime\) mesons have been measured in photon induced reactions on \(^{12}{}\)C in the energy range of 1250-2600 MeV. The experiment was ...performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the \(\eta^\prime\)-nucleus potential. Within the model, the comparison indicates an attractive potential of -(\(37 \pm 10(stat)\pm10(syst)\)) MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the \(\eta^\prime\)-nucleus potential of -(\(10\pm2.5\)) MeV, determined by transparency ratio measurements, a search for resolved \(\eta^\prime\)-bound states appears promising.
The \(\gamma + p \rightarrow K^0 + \Sigma^+\) photoproduction reaction is investigated in the energy region from threshold to \(E_\gamma = 2250\)\,MeV. The differential cross section exhibits ...increasing forward-peaking with energy, but only up to the \(K^*\) threshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the \(K^*\Lambda\) and \(K^*\Sigma\) thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the \(K^*\) production threshold to an s-channel mechanism associated with the formation of a dynamically generated \(K^*\)-hyperon intermediate state.
At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be ...deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of P = 40% are typically achieved at half of the primary electron energy. The determination of P is confirmed by measuring the beam asymmetry, \Sigma, in pi^0 photoproduction and a comparison of the results to independent measurements using laser backscattering.
A measurement of the double-polarization observable \(E\) for the reaction \(\gamma p\to \pi^0 p\) is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using ...the Bonn frozen-spin butanol (C\(_4\)H\(_9\)OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from \(E_\gamma =600\)~MeV to \(E_\gamma =2310\)~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.
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