PauliXi[I] represents a two-component Pauli spinor ξ\xiξ, while PauliXi[-I] stands for ξ†\xi^{\dagger }ξ†.
PauliXi[I]
PauliXi[-I]
Overview, PauliEta.
ξ\xiξ
ξ†\xi ^{\dagger }ξ†
PauliXi[-I] . SIS[p] . PauliEta[I] % // ComplexConjugate
ξ†.(σˉ⋅p‾).η\xi ^{\dagger }.\left(\bar{\sigma }\cdot \overline{p}\right).\etaξ†.(σˉ⋅p).η
η†.(σˉ⋅p‾).ξ\eta ^{\dagger }.\left(\bar{\sigma }\cdot \overline{p}\right).\xiη†.(σˉ⋅p).ξ