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.
The photoproduction of 2
π
0
mesons off protons was studied with the Crystal Barrel/TAPS experiment at the electron accelerator ELSA in Bonn. The energy of photons produced in a radiator was tagged ...in the energy range from 600 MeV to 2.5 GeV. Differential and total cross sections and
pπ
0
π
0
Dalitz plots are presented. Part of the data was taken with a diamond radiator producing linearly polarized photons, and beam asymmetries were derived. Properties of nucleon and
Δ
resonances contributing to the
pπ
0
π
0
final state were determined within the Bonn-Gatchina (BnGa) partial-wave analysis. The data presented here allow us to determine branching ratios of nucleon and
Δ
resonances for their decays into
pπ
0
π
0
via several intermediate states. Most prominent are decays proceeding via
Δ
(1232)
π
,
N
(1440)1/2
+
π
,
N
(1520)3/2
−
π
,
N
(1680)5/2
+
π
, but also
pf
0
(500),
pf
0
(980), and
pf
2
(1270) contribute to the reaction.
.
The photoproduction of
and
mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9GeV at the electron accelerator ELSA. ...The mesons have been identified via the
and
decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium
and
widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium
width and
-N absorption cross sections. The energy dependence of the imaginary part of the
- and
-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the
-nucleus potential is found to be about three times smaller than recently determined values of the real part of the
-nucleus potential, which makes the
meson a suitable candidate for the search for meson-nucleus bound states. For the
meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected, which makes the observation of
mesic states very difficult experimentally.
Photoproduction off protons of the
pπ
0
η
three-body final state was studied with the Crystal Barrel/TAPS detector, at the electron stretcher accelerator ELSA in Bonn, for incident energies from the
...π
0
η
production threshold up to 2.5 GeV. Differential cross sections and the total cross sections are presented. The use of linearly polarized photons gives access to the polarization observables
Σ
,
I
s
, and
I
c
, the latter two characterize beam asymmetries in case of three-body final states.
Δ
(1232)
η
,
N
(1535)1/2
−
π
, and
pa
0
(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and
Δ
resonances, for which so far only fair evidence was reported. A large number of decay modes of known nucleon and
Δ
resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and
Δ
resonances.
The polarization observable I super()s a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. ...Results for the reaction MathML equation are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry I super()c The comparably large asymmetries demonstrate a high sensitivity of I super()sto the dynamics of the reaction. The sensitivity of these new polarization observables to the contributing partial waves is demonstrated by fits using the Bonn-Gatchina partial wave analysis.
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