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28 2.4. The RPR model: the good, the bad and the uglyO z’O x’1.51.00.50.00.50.0W KY1.5= 1649 MeV1.00.50.0­0.5­0.5­1.0­1.0­1.5­1.5­1.0 ­0.5 0.0 W = 1649 0.5 MeV1.01.0KY1.00.50.0­0.5­0.5Regge­1.0­1.0Regge + resonances­1.5­1.5­1.0 ­0.5 0.0 0.5 1.0W KY1.5= 1781 MeV1.00.50.0­0.5­1.0­1.5­0.5 0.0 W = 1781 0.5 MeV1.0­0.5 0.0 0.5 1.0KYW = 1906 MeV1.0KY0.50.0­0.5­1.0­0.5 0.0 0.5­1.51.0*cosθ K= 1906 MeVW KY­0.5 0.0 0.5 1.0Figure 2.11 – The angular dependence of the beam-recoil polarisation asymmetries O z ′ (upper panels)and O x ′ (lower panels) for p(γ, K + )Λ at three values of W KY . The solid and dashed curves are calculationswith the RPR and Regge models respectively. Data from Ref. [141].an off-shell extension to the traditional kaon form factor and fit the parameters corresponding tothe degree of off-shellness to the data of Refs [143, 145]. This procedure significantly improves thedescription of the data. Our results indicate that the inclusion of resonance-exchange diagrams inthe s-channel provides an alternative explanation. The coupling constants of these contributions arefixed in the real-photon point, and their Q 2 dependence is dictated by EM form factors computed inthe Bonn CQM. The RPR-model results describe the W KY dependence of the cross section in bothQ 2 bins without having to fit additional parameters to the data.Finally, we compare RPR-model predictions to measurements of the transferred polarisation over alarge range of photon virtuality in Figure 2.13. This observable can be considered the electroproductionextension of the beam-recoil polarisation asymmetries C x and C z that have been presentedin Figure 2.10. Again, a satisfactory correspondence is attained. The Regge model is able to accountfor the data, and the addition of resonances introduces only a slight modulation as function of Q 2 .In this chapter, EM strangeness production from the proton has been described within the RPRframework. The RPR-2007 models published in Refs. [39, 40] have been introduced. The values fortheir free parameters can be found in Appendix I. These parameters have been optimised to theavailable data in the p(γ, K + )Λ and p(γ, K + )Σ 0 reaction channels, where experimental results areabundant. A successful description of strangeness production from the deuteron, however, requires areaction model which accounts for all six kaon-production final states, including those where data isscarce or lacking. This issue constitutes the topic of the next chapter.