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 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.
.
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.
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 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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