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16 2.1. Kinematics and observablesstraightforward rotation, they readC x = C x ′ cos θ ∗ K + C z ′ sin θ ∗ K ,C z = −C x ′ sin θ ∗ K + C z ′ cos θ ∗ K .(2.24)2.1.5 Electroproduction observablesStarting from the general expression (D.1), one finds the N(e, e ′ K)Y differential cross section can bewritten in the following factorised form 1d 5 σdE ′ edΩ ′ edΩ ∗ K= Γ d2 σ ∗dΩ ∗ , (2.25)KwhereΓ =α2π 2 E ′ eE eK HQ 2 11 − ɛ , (2.26)is the virtual photon flux factor andK H = E γ −Q22m N, (2.27)the equivalent-real-photon laboratory energy. The most general form for the virtual-photon crosssection reads [105]d 2 σ ∗dΩ ∗ K= P N α P Y βwith h the electron-beam helicity and|⃗pK ∗ | [( )|⃗p γ ∗ R βαT|+ ɛRβα L+ ɛ cR βαT T cos 2φ + s R βαT T sin 2φ+ √ ( )ɛ(1 + ɛ) cR βαLT cos φ + s R βαLT sin φ+ h √ 1 − ɛ 2 R βαT ′ + h √ ɛ(1 − ɛ)ɛ =(1 + 2|⃗p γ| 2Q 2(cR βαLTcos φ + s R βα′ LTsin φ) ] ,′(2.28)tan 2 θ ) −1e, (2.29)2the transverse linear polarisation of the virtual photon. In the expression for the virtual-photon crosssection, a summation over the Greek indices (α, β = 0, 1, 2, 3) is implied. The operatorsP i µ ≡ (1, ⃗ P i ) , with i = N, Y , (2.30)are given in terms of the polarisation vectors P ⃗ N and P ⃗ Y of the nucleon and hyperon respectively.The polarisations of these baryons can be evaluated in any of the frames presented in Figure 2.2.The dynamics of the EM kaon-production reaction is contained in the response functions, which are1 A detailed account of the factorisation of the electroproduction cross section for EM kaon production from thedeuteron is given in Section 5.4. The derivation of the results given in Eqs. (2.25)–(2.31) proceeds along similar lines.