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ReuseItems deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.PHYSICAL REVIEW C 94, 025205 (2016) Determination of the real part of the η ′ -Nb optical potential The excitation function and momentum distribution of η ′ 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 η ′ -Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of −(41 ± 10(stat) ± 15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of −(37 ± 10(stat) ± 10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow η ′ -nucleus potential will make the search for η ′ -nucleus bound states more difficult.

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 12 C for incident photon energies of 1250 -3100 MeV. The π 0 γ 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 π 0 γ pairs, which would indicate the population and decay of bound ω 11 B states, are not observed. The π 0 γ 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 ωmeson. A comparison of the measured total energy distribution with calculations suggests the real part V 0 of the ω 11 B potential to be small and only weakly attractive with V 0 (ρ = ρ 0 ) = −15± 35(stat) ±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 γp→pπ(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.

We report on the first Q^{2}-dependent measurement of the beam-normal single spin asymmetry A_{n} in the elastic scattering of 570 MeV vertically polarized electrons off ^{12}C. We cover the Q^{2} range between 0.02 and 0.05  GeV^{2}/c^{2} and determine A_{n} at four different Q^{2} values. The experimental results are compared to a theoretical calculation that relates A_{n} to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q^{2} behavior of A_{n} given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.

Abstract. The photoproduction of ω and η 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 ω → π 0 γ → 3γ and η → π 0 π 0 η → 6γ 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 ω and η 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 ω width and η -N absorption cross sections. The energy dependence of the imaginary part of the ω-and η -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 η nucleus potential is found to be about three times smaller than recently determined values of the real part of the η -nucleus potential, which makes the η meson a suitable candidate for the search for meson-nucleus bound states. For the ω 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 ω mesic states very difficult experimentally.

The first measurements of the beam-target-helicity-asymmetries E and G in the photoproduction of ω-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 γ = 1108 MeV) to E γ = 2300 MeV and G at a single energy interval of 1108 < E γ < 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 − .