Systematic study of the pp $\rightarrow$ pp$\omega$ reaction

Abstract

A systematic study of the production of $\omega$ -mesons in proton-proton collisions was carried out in a kinematically complete experiment at three excess energies ( $\varepsilon$ = 92, 128, 173 MeV). Both protons were detected using the large-acceptance COSY-TOF spectrometer at an external beam line at the Cooler Synchrotron COSY at Forschungszentrum Jülich. The total cross-section, angular distributions of both $\omega$ -mesons and protons were measured and presented in various reference frames such as the overall CMS, helicity and Jackson frame. In addition, the orientation of the $\omega$ -spin and invariant-mass spectra were determined. We observe $\omega$ -production to take place dominantly in Ss and Sp final states at $\varepsilon$ = 92, 128 MeV and, additionally, in Sd at $\varepsilon$ = 173 MeV. No obvious indication of resonant $\omega$ -production via N^*-resonances was found, as proton angular distributions are almost isotropic and invariant-mass spectra are compatible with phase space distributions. A dominant role of ³ P ₁ and ¹ S ₀ initial partial waves for $\omega$ -production was concluded from the orientation of the decay plane of the $\omega$ -meson. Although the Jackson angle distributions in the $\omega$ p-Jackson frame are anisotropic we argue that this is not an indication of a resonance but rather a kinematical effect reflecting the anisotropy of the $\omega$ angular distribution. The helicity angle distribution in the $\omega$ p-helicity frame shows an anisotropy which probably reflects effects of the $\omega$ angular momenta in the final state; this observable may be, in addition to the orientation of the $\omega$ decay plane, the most sensitive one to judge the validity of theoretical descriptions of the production process.