Al Jaghoub, Mahmoud Ismail; (1995) Decay & final-state interaction effects in near-threshold ω production. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The ω production amplitude in the [pi]-p → ωn reaction seems to be sharply suppressed near threshold, despite remaining isotropic and consistent with S-wave production. Furthermore, the ratio of the [pi]-[pi]+[pi]0 and [pi]0[gamma] decay channels of the ω is constant. The threshold suppression is also seen in the pd → 3Heω process. Theoretical models are studied which might lead to such abrupt behaviour. The effect of final-state interactions between the ω decay products and the recoil neutron (nucleus) has been investigated in a simple semi-classical model and a more realistic quantum mechanical one. The results of the former reproduced well the near-threshold suppression, as well as the scale in energy of the effect. On the other hand, the quantum model, where only the radiative decay channel is considered, produced a 9% reduction, which is significantly smaller than the 70% observed experimentally. An attractive [eta]n interaction, which leads to the formation of the N*(1535) S-wave resonance, enhances the [eta] production in [pi]-p → [eta]n near threshold. A similar ω - n resonance might also be possible if the ω and the neutron interacted via an attractive potential, but such a system may be destroyed due to the short lifetime of the ω. This is an alternative threshold suppression mechanism which is independent of the w decay channel. This hypothesis was studied by deriving a dynamical ωn interaction amplitude which was fit using the scattering parameters of the elastic [eta]n channel, as the corresponding ωn ones are not known. The effect of a short lifetime was then investigated by assuming the n to have a width equals to that of the ω. Two different models, which could lead to an S-wave resonance production, were adopted. A two-term S-wave separable non-local potential was first considered. The Schrodinger equation was solved, and a dynamical ωn interaction amplitude extracted, but this could not describe the data. A plausible description is obtained when the resonance is trapped as a virtual state in a multi-channel system. Three channels were considered, [eta]n, [pi]-p and K[lambda], and an [eta]n interaction amplitude derived. Introducing an [eta] lifetime equal to that of the [omega] does indeed greatly reduce the enhancement near threshold, and the effect is most important at low energies. As an alternative approach, the amplitude of the overall pd → 3Heω reaction has been expressed in terms of pp → [pi]+d and [pi]+n → ωp intermediate amplitudes using a two-step quantum model. Such a model reproduces the rapid rise from threshold, showing that the reduction effect can propagate from the [pi]-p to the pd case. Furthermore, it is shown that the multiple scattering of the ω off the nucleons in 3He, which might lead to a threshold enhancement, is suppressed through the ω decaying before it is multiply scattered.

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