Off-shellωproduction in proton-proton collisions near threshold

Abstract

The $\ensuremath{\omega}$ production in nucleon-nucleon collisions is described through decays of intermediate nucleon resonances. The near-threshold $p\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{pp}\ensuremath{\omega}$ cross section is found to be dominated by the off-shell production of $\ensuremath{\omega}$ mesons with masses far below the physical $\ensuremath{\omega}$ mass. The ${N}^{*}(1535)$ resonance plays thereby a crucial role. As a result of a strong $N\ensuremath{\omega}$ coupling, this resonance leads to an off-shell contribution which is about one order of magnitude larger than the experimentally measured contribution from the $\ensuremath{\omega}$ peak. After a subtraction of the theoretical ``background'' from the off-shell $\ensuremath{\omega}$ production, the available data are accurately reproduced over the entire energy range from 5 MeV up to several GeV above the threshold. The scenario of a weaker ${N}^{*}(1535)\ensuremath{\rightarrow}N\ensuremath{\omega}$ decay mode which is still consistent with electro- and photoproduction data is discussed as well. In the latter case, the off-shell contribution to the $p\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{pp}\ensuremath{\omega}$ cross section is substantially reduced but the description of the experimental cross section is poor above threshold.