Abstract
The differential cross sections and unpolarized spin-density matrix elements for the reaction γp→pω were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the ω meson, ω→π0γ. The cross sections cover the full angular range and show the full extent of the t-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements.These data were included in the Bonn–Gatchina partial wave analysis (PWA). The dominant contribution to ω photoproduction near threshold was found to be the 3/2+ partial wave, which is primarily due to the sub-threshold N(1720)3/2+ resonance. At higher energies, pomeron-exchange was found to dominate whereas π-exchange remained small. These t-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the 1/2−, 3/2−, and 5/2+ partial waves were also found to contribute significantly.
Highlights
The spectrum of excited states has historically given essential information on the nature of any composite quantum system
The CBELSA/TAPS experiment was conducted at the electron stretcher accelerator (ELSA) facility [4] located at the University of Bonn in Germany
The differential cross sections, dσ /d, for γ p → pω from this analysis are shown in Fig. 2 for a few selected angle bins
Summary
The spectrum of excited states has historically given essential information on the nature of any composite quantum system. CBELSA/TAPS Collaboration / Physics Letters B 749 (2015) 407–413 careful mapping of the excited states of baryons shines light on the nature of the nonperturbative regime of quantum chromodynamics (QCD). This spectrum depends on the effective degrees of freedom and the forces confining the quarks. The predicted baryon states for masses above 1.8 GeV/c2 greatly outnumber those which have been found experimentally. Most known light-flavor baryon resonances lie below 2 GeV/c2 and were discovered in elastic π N scattering experiments. For a recent review on baryon resonances, see [1,2]
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