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22 2.3. Kaon production in the resonance regionproduction [117], as well as the production of η and η ′ mesons [63, 64].We describe the resonant contributions using standard tree-level Feynman diagrams. The exact formof the transition amplitudes for spin-1/2 and spin-3/2 nucleon-resonance exchange can be found inSection D.3. By substitutings KY − m 2 R → s KY − m 2 R + im R Γ R , (2.47)in the propagator’s denominator, we take into account the finite lifetime of resonances with massm R and width Γ R . To limit the number of fit parameters, we keep the resonances’ mass and widthfixed at the values given in the Particle Data Group’s Review of Particle Physics (RPP) [1]. Eachspin-1/2 resonance introduces one free parameterG N ∗ = κ N ∗ p × g K + Y N ∗ , (2.48)the product of the coupling constants at the electromagnetic and the strong-interaction vertex.Spin-3/2 resonances have an additional degree of freedom at the photon vertex and give rise to twofree parametersG (1)N ∗ = κ (1)N ∗ p × g K + Y N ∗ ,G (2)N ∗ = κ (2)N ∗ p × g K + Y N ∗ . (2.49)The most general interaction Lagrangian for spin-3/2 fields allows for an additional three degrees-offreedom,often called off-shell parameters, in the strong and EM vertices [118]. To ensure that theeffects of the resonant diagrams fade at higher energies, we introduce Gaussian form factors at thestrong interaction vertices [69](g K + Y N ∗ → g K + Y N[−∗ × exp sKY − m 2 2]R), (2.50)Λ 4 strongwith Λ strong a cutoff mass. A single cutoff mass is used for all resonance-exchange diagrams and it isconsidered a free parameter in the fitting procedure. For both Λ and Σ 0 production, the value ofΛ strong is about 1600 MeV.The dynamics of EM kaon production can be fairly involved, with several contributing nucleonand delta resonances that interfere with an eminent background. Disentangling these contributionsis challenging. In the RPR approach, we seek to determine the resonant and non-resonant termsseparately. The Regge model that was the subject of the previous section has been fitted againstthe available high-energy data. Its parameters are frozen and the Regge amplitude serves as aneffective parametrisation of the background. In the resonance region, a large body of data isavailable. Lists of published data sets for p(γ, K + )Λ and p(γ, K + )Σ 0 are presented in Tables J.1and J.2. In Refs. [39, 40], the database [119–122] published at that time was used to constrainthe resonance parameters of the RPR model, while keeping the background unaltered. In theK + Λ-production channel at forward angles, a substantial discrepancy exists between results from theCLAS [119, 121, 123] and SAPHIR [124, 125] collaborations [126]. This makes it impossible to findan optimal model that describes both data sets simultaneously. In the optimisation procedure, onlythe CLAS data are retained. Since the Regge model is derived in the limit of asymptotic CM energys KY and vanishing momentum transfer t, the RPR model analysis of Refs. [39, 40] was restrictedto forward-kaon production. Only differential cross sections for cos θK ∗ ≥ 0.35 and polarisationasymmetries satisfying cos θK ∗ ≥ 0 were considered in the fitting procedure.