feynlag.pheno.offshell.scalar_vv_s12_integral

feynlag.pheno.offshell.scalar_vv_s12_integral(m_S, m_V)

The inner Dalitz integral ∫ |M|² ds₁₂ done analytically.

|M|² is polynomial in s₁₂, so the inner integral over the Dalitz limits is a closed form (a Källén square root times a polynomial in s₂₃) — SymPy does it in a fraction of a second, where the full 2-D integral with the propagator does not. Returns a symbolic expression in s₂₃ (with m_S, m_V symbolic), the integrand of the remaining 1-D s₂₃ integral.