The photoproduction of $\omega$ and $\eta^\prime$ mesons off carbon and
niobium nuclei has been measured as a function of the meson momentum for
incident photon energies of 1.2-2.9 GeV at the ...electron accelerator ELSA. The
mesons have been identified via the $\omega \rightarrow \pi^0 \gamma
\rightarrow 3 \gamma$ and $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6
\gamma$ 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 $\omega$ and $\eta^\prime$ 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 $\omega$ width and $\eta^\prime$-N absorption cross sections. The
energy dependence of the imaginary part of the $\omega$- and
$\eta^\prime$-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 $\eta^\prime$ nucleus potential is found to be about three times
smaller than recently determined values of the real part of the
$\eta^\prime$-nucleus potential, which makes the $\eta^\prime$ meson a suitable
candidate for the search for meson-nucleus bound states. For the $\omega$
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 $\omega$ mesic states
very difficult experimentally.
The excitation function and momentum distribution of {\eta}' mesons have been measured in photoproduction off 93^Nb in the energy range of 1.2-2.9 GeV. The experiment has been performed with the ...combined Crystal Barrel and MiniTAPS detector system, using tagged photon beams from the ELSA electron accelerator. Information on the sign and magnitude of the real part of the {\eta}'-Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of -(41 \pm 10(stat) \pm 15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of -(37 \pm 10(stat) \pm 10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow {\eta}'-nucleus potential will make the search for {\eta}'-nucleus bound states more difficult.
This paper reports on a measurement of the double-polarization observable G in \(\pi^0\) 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 properly oriented diamond radiator. 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), J\"ulich-Bonn (J\"uBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.
Phys. Lett. B 748, 212-220 (2015) Data on the polarization observables T, P, and H for the reaction $\gamma
p\to p\pi^0$ are reported. Compared to earlier data from other experiments, our
data are ...more precise and extend the covered range in energy and angle
substantially. The results were extracted from azimuthal asymmetries measured
using a transversely polarized target and linearly polarized photons. The data
were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS
detector. Within the Bonn-Gatchina partial wave analysis, the new polarization
data lead to a significant narrowing of the error band for the multipoles for
neutral-pion photoproduction.
A measurement of the double-polarization observable \(E\) for the reaction \(\gamma p\to \pi^0 p\) is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using ...the Bonn frozen-spin butanol (C\(_4\)H\(_9\)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 \(E_\gamma =600\)~MeV to \(E_\gamma =2310\)~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.
The first measurements of the beam-target-helicity-asymmetries $E$ and $G$ in
the photoproduction of $\omega$-mesons off protons at the CBELSA/TAPS
experiment are reported. $E$ ($G$) was measured ...using circularly (linearly)
polarised photons and a longitudinally polarised target. $E$ was measured over
the photon energy range from close to threshold ($E_\gamma = 1108$~MeV) to
$E_\gamma = 2300$~MeV and $G$ at a single energy interval of $1108 < E_\gamma
<1300$~MeV. Both measurements cover the full solid angle. The observables $E$
and $G$ are highly sensitive to the contribution of baryon resonances, with $E$
acting as a helicity filter in the $s$-channel. The new results indicate
significant $s$-channel resonance contributions together with contributions
from $t$-channel exchange processes. A partial wave analysis reveals strong
contributions from the partial waves with spin-parity $J^P=3/2^+, 5/2^+$, and
$3/2^-$.
The polarization observables \(T, E, P, H\), and \(G\) in photoproduction of \(\eta\) mesons off protons are measured for photon energies from threshold to \(W=2400\,\)MeV (\(T\)), 2280 MeV (\(E\)), ...1620 MeV (\(P, H\)), or 1820 MeV (\(G\)), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, J\"uBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the \(N\eta\) decay of nucleon resonances. A \(N\eta\)-branching ratio of \(0.33\pm 0.04\) for \(N(1650)1/2^-\) is found, which reduces the large and controversially discussed \(N\eta\)-branching ratio difference of the two lowest mass \(J^P=1/2^-\)-resonances significantly.
In a search for $\omega$ mesic states, the production of $\omega$-mesons in
coincidence with forward going protons has been studied in photon induced
reactions on $^{12}$C for incident photon ...energies of 1250 - 3100 MeV. The
$\pi^0 \gamma$ pairs from decays of bound or quasi-free $\omega$-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 $\pi^0 \gamma$ pairs, which would indicate the population
and decay of bound $\omega~^{11}$B states, are not observed. The $\pi^0 \gamma$
cross section of 0.3 nb/MeV/sr 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 $\omega$-meson. A
comparison of the measured total energy distribution with calculations suggests
the real part $V_0$ of the $\omega~^{11}$B potential to be small and only
weakly attractive with $V_0(\rho=\rho_0) = -15\pm$ 35(stat) $\pm$20(syst) MeV
in contrast to some theoretical predictions of attractive potentials with a
depth of 100 - 150 MeV.
New data on the polarization observables T, P, and H for the reaction \(\gamma p \to p\pi^0\) are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol ...target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.
M. Gottschall et al., Phys. Rev. Lett. 112, 012003 (2014) 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.
