Exclusive production of the isoscalar vector mesons $\omega$ and $\phi$ is measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable $x_{ F}$ of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{ F}$ and on the invariant mass $M_{ p\mathrm{ V}}$ of the system formed by fast proton $p_\mathrm{ fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_\mathrm{ fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $\rho_{ 00}$ of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on $x_{ F}$ and on $M_{ p\mathrm{ V}}$ are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to $\omega$ production which are absent in the case of the $\phi$ meson. Removing the low-mass $M_{ p\mathrm{ V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_\mathrm{ F}$.
Categories: hep-ex nucl-ex