Results are presented for the first measurement of the double-polarization helicity asymmetry E for the η photoproduction reaction γp→ηp. Data were obtained using the FROzen Spin Target (FROST) with ...the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Jülich–Bonn model to examine the case for the existence of a narrow N⁎ resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable from that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.
Branching ratios for the decay of hadrons with large width or near thresholds depend on their definition. We test different definitions and show that rather different branching ratios can be ...obtained. For wide resonances and for sequential decays with wide intermediate resonances, integration over the spectral functions is mandatory. The tests are performed exploiting the latest solution of the Bonn-Gatchina multi-channel analysis and published values for residues of light scalar mesons. For a resonance overlapping with a threshold, in case its pole lies in a non-adjacent sheet, we show how the total width, needed for the branching ratios, does not correspond to the imaginary part of the pole position. We use the Madrid-Krakow dispersive parameterizations to illustrate this situation with the f0(980).
Data on the reaction γp→ωp with ω→π0γ, taken with unpolarized or polarized beams in combination with an unpolarized or polarized proton-target, were analyzed within the Bonn–Gatchina (BnGa) partial ...wave analysis. Differential cross sections, several spin density matrix elements, the beam asymmetry Σ, the normalized helicity difference E, and the correlation G between linear photon and longitudinal target polarization were included in a large data base on pion and photo-induced reactions. The data on ω photoproduction are used to determine twelve N⁎→Nω branching ratios; most of these are determined for the first time.
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
π
0
and
η
photoproduction off quasi-free protons and, for the first time, ...off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the
γ
p
→
π
0
p
and
γ
p
→
η
p
reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the
η
n
system at
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
S
11
(
1535
)
and
S
11
(
1650
)
resonances within the
S
11
-partial wave.
.
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.
The η-carbon potential at low meson momenta Nanova, M.; Friedrich, S.; Metag, V. ...
The European physical journal. A, Hadrons and nuclei,
2018/10, Letnik:
54, Številka:
10
Journal Article
Recenzirano
Odprti dostop
.
The production of
η
mesons in coincidence with forward-going protons has been studied in photon-induced reactions on
12
C and on a liquid hydrogen (LH
2
) target for incoming photon energies of ...1.3-2.6 GeV at the electron accelerator ELSA. The
η
mesons have been identified via the
η
→
π
0
π
0
η
→
6
γ
decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF
2
array at polar angles of
2
∘
≤
θ
p
≤
11
∘
. Under these kinematic constraints the
η
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
η
-carbon potential at low meson momenta by comparing with collision model calculations of the
η
kinetic energy distribution and excitation function. Fitting the latter data for
η
mesons going backwards in the center-of-mass system yields a potential depth of
V
=
-
(
44
±
16
(
s
t
a
t
)
±
15
(
s
y
s
t
)
)
MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average
η
momenta of
≈
1
.
1
GeV/
c
. Within the experimental uncertainties, there is no indication of a momentum dependence of the
η
-carbon potential. The LH
2
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
η
photoproduction off the free proton.
In a search for ω mesic states, the production of ω-mesons in coincidence with forward going protons has been studied in photon induced reactions on 12C for incident photon energies of 1250–3100 MeV. ...The π0γ pairs from decays of bound or quasi-free ω-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the π0γ pairs, which would indicate the population and decay of bound ω11B states, are not observed. The π0γ cross section of 0.3 nb MeV−1 sr−1 observed in the bound state energy regime between −100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the ω-meson. A comparison of the measured total energy distribution with calculations suggests the real part V0 of the ω11B potential to be small and only weakly attractive with V0(ρ=ρ0)=−15±35(stat)±20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100–150 MeV.
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