Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W = 1748 MeV to W = 2045 MeV). The data ...cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the pη′ threshold Eγ = 1447 MeV (W = 1896 MeV). The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N (1895)1/2− nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.
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.
Abstract
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
$$\pi ^0$$
π
0
and
$$\eta $$
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
$$\gamma p \rightarrow \pi ^0 p$$
γ
p
→
π
0
p
and
$$\gamma p \rightarrow \eta p$$
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
$$\eta n$$
η
n
system at
$$W = 1.68\ \textrm{GeV}$$
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
$$S_{11}(1535)$$
S
11
(
1535
)
and
$$S_{11}(1650)$$
S
11
(
1650
)
resonances within the
$$S_{11}$$
S
11
-partial wave.
The target asymmetry
T
, recoil asymmetry
P
, and beam-target double polarization observable
H
were determined in exclusive
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\begin{document}$$\pi ^0$$\end{document}
π
0
and
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\begin{document}$$\eta $$\end{document}
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
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\begin{document}$$\gamma p \rightarrow \pi ^0 p$$\end{document}
γ
p
→
π
0
p
and
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\begin{document}$$\gamma p \rightarrow \eta p$$\end{document}
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
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\begin{document}$$\eta n$$\end{document}
η
n
system at
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\begin{document}$$W = 1.68\ \textrm{GeV}$$\end{document}
W
=
1.68
GeV
. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the
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\begin{document}$$S_{11}(1535)$$\end{document}
S
11
(
1535
)
and
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\begin{document}$$S_{11}(1650)$$\end{document}
S
11
(
1650
)
resonances within the
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\begin{document}$$S_{11}$$\end{document}
S
11
-partial wave.
The reaction γp→pπ0η has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been ...motivated by the recently claimed observation of a narrow structure in the MNη invariant mass distribution at a mass of 1678MeV/c2. The existence of this structure cannot be confirmed in the present work. Instead, for Eγ = 1400–1500 MeV and the cut Mpπ0≤1190MeV/c2 a statistically significant structure in the Mpη invariant mass distribution near 1700 MeV/c2 is observed with a width of Γ≈35MeV/c2 while the mass resolution is σres = 5 MeV/c2. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from ≈ 1700 MeV/c2 to ≈ 1725 MeV/c2; the width increases to about 50 MeV/c2 and decreases thereafter. The cross section associated with this structure reaches a maximum of ≈ 100 nb around Eγ≈ 1490 MeV (W ≈ 1920 MeV), which coincides with the pa0 threshold. Three scenarios are discussed which might be the origin of this structure in the Mpη invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the γp→pa0→pπ0η reaction.
The reaction
γ
p
→
p
π
0
η
has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has ...been motivated by the recently claimed observation of a narrow structure in the
M
N
η
invariant mass distribution at a mass of
1678
MeV
/
c
2
. The existence of this structure cannot be confirmed in the present work. Instead, for
E
γ
= 1400–1500 MeV and the cut
M
p
π
0
≤
1190
MeV
/
c
2
a statistically significant structure in the
M
p
η
invariant mass distribution near 1700
MeV
/
c
2
is observed with a width of
Γ
≈
35
MeV
/
c
2
while the mass resolution is
σ
res
= 5
MeV
/
c
2
. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from
≈
1700
MeV
/
c
2
to
≈
1725
MeV
/
c
2
; the width increases to about 50
MeV
/
c
2
and decreases thereafter. The cross section associated with this structure reaches a maximum of
≈
100 nb around
E
γ
≈
1490 MeV (W
≈
1920 MeV), which coincides with the
p
a
0
threshold. Three scenarios are discussed which might be the origin of this structure in the
M
p
η
invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the
γ
p
→
p
a
0
→
p
π
0
η
reaction.
The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from ...photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final states. The new readout uses avalanche photodiodes in the front-end and a dual back-end with branches optimized for energy and time measurement, respectively. An FPGA-based cluster finder processes the whole hit pattern within less than 100 ns. The important downside of APDs -- the temperature dependence of their gain -- is handled with a temperature stabilization and a compensating bias voltage supply. Additionally, a light pulser system allows the APDs' gains to be measured during beamtimes.
Measurements of target asymmetries and double-polarization observables for the reaction \(\gamma p\to p\pi^0\pi^0\) are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA ...facility (Bonn University) using the Bonn frozen-spin butanol (C\(_4\)H\(_9\)OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The data cover the photon energy range from \(E_{\gamma}\)=650 MeV to \(E_{\gamma}\)=2600 MeV and nearly the complete angular range. The results have been included in the BnGa partial wave analysis. Experimental results and the fit agree very well. Observed systematic differences in the branching ratios for decays of \(N^*\) and \(\Delta^*\) resonances are attributed to the internal structure of these excited nucleon states. Resonances which can be assigned to SU(6)\(\times\)O(3) two-oscillator configurations show larger branching ratios to intermediate states with non-zero intrinsic orbital angular momenta than resonances assigned to one-oscillator configurations.
The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the
CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for
incident photon energies from threshold up to ...3.1 GeV. This paper has been
motivated by the recently claimed observation of a narrow structure in the
M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. The
existence of this structure cannot be confirmed in the present work. Instead,
for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV/$c^2$
a statistically significant structure in the M$_{p\eta}$ invariant mass
distribution near 1700 MeV/$c^2$ is observed with a width of $\Gamma\approx 35$
MeV/$c^2$ while the mass resolution is $\sigma_{res}$ = 5 MeV/$c^2$. Increasing
the incident photon energy from 1420 to 1540 MeV this structure shifts in mass
from $\approx$ 1700MeV/c$^2$ to $\approx$ 1725 MeV/$c^2$; the width increases
to about 50 MeV/$c^2$ and decreases thereafter. The cross section associated
with this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma}
\approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$
threshold. Three scenarios are discussed which might be the origin of this
structure in the M$_{p\eta}$ invariant mass distribution. The most likely
interpretation is that it is due to a triangular singularity in the $\gamma p
\rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction
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