Spin alignment and violation of the OZI rule in exclusive $\omega$ and $\phi$ production in pp collisions

TL;DR

This study measures exclusive $$ and $$ meson production in proton-proton collisions, revealing significant violations of the OZI rule and analyzing the reaction mechanisms through spin alignment and resonance contributions.

Contribution

It provides new measurements of cross section ratios and spin density matrix elements, demonstrating the kinematic dependence of OZI violation and the role of baryon resonances in $$ and $$ production.

Findings

Significant violation of the OZI rule, up to a factor of eight.

Dependence of meson spin alignment on $x_F$ and $M_{pV}$.

Resonance contributions explain differences between $$ and $$ production.

Abstract

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 $x_\mathrm{F}$.