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The first measurement is reported of the double-polarization observable G in the photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand γp→pπ(0) as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N(1535) resonance with spin parity J(P)=1/2(-) and N(1520) with J(P)=3/2(-). In the third resonance region, where N(1680) with J(P)=5/2(+) production dominates, the new data are reasonably close to the predictions.

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.

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.

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.