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1.
Eur Phys J A Hadron Nucl ; 59(10): 232, 2023.
Artigo em Inglês | MEDLINE | ID: mdl-37860634

RESUMO

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive π0 and η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the γp→π0p and γp→ηp reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the ηn system at W=1.68GeV. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the S11(1535) and S11(1650) resonances within the S11-partial wave.

2.
Phys Rev Lett ; 125(15): 152002, 2020 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-33095637

RESUMO

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 [E_{γ}=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.

3.
Phys Rev Lett ; 113(6): 062001, 2014 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-25148317

RESUMO

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.

4.
Phys Rev Lett ; 109(10): 102001, 2012 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-23005280

RESUMO

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.

6.
Phys Rev Lett ; 98(22): 225504, 2007 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-17677858

RESUMO

We report a generally applicable method to pattern organic molecules on mesoscopic scales. In our method, organic molecular beam deposition was conducted on substrate surfaces prepatterned with materials to which the organic molecules have larger binding energies in comparison to the substrate. Fully uniform nucleation control at these predefined locations can be achieved by an appropriate selection of the growth parameters including temperature and deposition rate. The physical mechanisms involved are studied by Monte Carlo simulations and stand in good agreement with the experimental findings.

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