Abstract

Exclusive production of the isoscalar vector mesons ω and ϕ 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 xF of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on xF and on the invariant mass MpV of the system formed by fast proton pfast and vector meson V is discussed in terms of diffractive production of pfastV resonances in competition with central production. The measurement of the spin density matrix element ρ00 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 xF and on MpV are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to ω production which are absent in the case of the ϕ meson. Removing the low-mass MpV resonant region, the OZI rule is found to be violated by a factor of eight, independently of xF.

Highlights

  • The Okubo–Zweig–Iizuka (OZI) rule [1] was formulated in the early days of the quark model, stating that all hadronic processes with disconnected quark lines are suppressed

  • Using the known deviation from the ideal mixing angle of the vector mesons ω and φ, δV = 3.7◦, the production cross section of φ with respect to that of ω should be suppressed according to σ (AB → Xφ)/σ (AB → Xω) = tan2 δV = 0.0042, where A, B and X are non-strange hadrons [2]

  • In the case of a vector meson decaying into two pseudoscalars, e.g. φ → K+K−, one chooses the momentum vector of either one

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Summary

Introduction

The Okubo–Zweig–Iizuka (OZI) rule [1] was formulated in the early days of the quark model, stating that all hadronic processes with disconnected quark lines are suppressed It qualitatively explains phenomena like suppression of φ meson decays into non-strange particles and suppression of exclusive φ production in non-strange hadron collisions. The yield of ω mesons is determined from a fit of a Breit–Wigner shape as explained above, but this time convoluted with two Gaussians to account for different resolutions of the two electromagnetic calorimeters. This fit includes a second-degree polynomial background

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Conclusion

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