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 $M_{pmathrm{V}}$ 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 $M_{pmathrm{V}}$ 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 $M_{pmathrm{V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_mathrm{F}$.
Download