The excitation function and momentum distribution of η′ mesons have been measured in photon induced reactions on 12C 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 η′-nucleus potential. Within the model, the comparison indicates an attractive potential of −(37±10(stat)±10(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 η′-nucleus potential of −(10±2.5) MeV, determined by transparency ratio measurements, a search for resolved η′-bound states appears promising.
The photoproduction of η′-mesons off different nuclei has been measured with the CBELSA/TAPS detector system for incident photon energies between 1500–2200 MeV. The transparency ratio has been ...deduced and compared to theoretical calculations describing the propagation of η′-mesons in nuclei. The comparison indicates a width of the η′-meson of the order of Γ=15–25 MeV at ρ=ρ0 for an average momentum pη′=1050 MeV/c, at which the η′-meson is produced in the nuclear rest frame. The inelastic η′N cross section is estimated to be 3–10 mb. Parameterizing the photoproduction cross section of η′-mesons by σ(A)=σ0Aα, a value of α=0.84±0.03 has been deduced.
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 first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the ...full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed, which are traced to differences in the helicity amplitudes of well-known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.
.
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.
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 Nπ^{0}π^{0} decays of positive-parity N^{*} and Δ^{*} resonances at about 2 GeV are studied at ELSA by photoproduction of two neutral pions off protons. The data reveal clear evidence for several ...intermediate resonances: Δ(1232), N(1520)3/2^{-}, and N(1680)5/2^{+}, with spin parities J^{P}=3/2^{+}, 3/2^{-}, and 5/2^{+}. The partial wave analysis (within the Bonn-Gatchina approach) identifies N(1440)1/2^{+} and the N(ππ)_{S wave} (abbreviated as Nσ here) as further isobars and assigns the final states to the formation of nucleon and Δ resonances and to nonresonant contributions. We observe the known Δ(1232)π decays of Δ(1910)1/2^{+}, Δ(1920)3/2^{+}, Δ(1905)5/2^{+}, Δ(1950)7/2^{+}, and of the corresponding spin-parity series in the nucleon sector, N(1880)1/2^{+}, N(1900)3/2^{+}, N(2000)5/2^{+}, and N(1990)7/2^{+}. For the nucleon resonances, these decay modes are reported here for the first time. Further new decay modes proceed via N(1440)1/2^{+}π, N(1520)3/2^{-}π, N(1680)5/2^{+}π, and Nσ. The latter decay modes are observed in the decay of N^{*} resonances and at most weakly in Δ^{*} decays. It is argued that these decay modes provide evidence for a 3-quark nature of N^{*} resonances rather than a quark-diquark structure.
Abstract
A measurement of the double-polarization observable
E
for the reaction
$$\gamma p\rightarrow \pi ^0 p$$
γ
p
→
π
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
$$_4$$
4
H
$$_9$$
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
$$E_\gamma =600$$
E
γ
=
600
to 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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