Leptonic three-body mass correction ($f(z)=1-8z+8z^3-z^4-12z^2\log z$)

At tree level in a local Fermi interaction with negligible neutrino masses, the charged daughter mass modifies the integrated three-body V-A interaction rate by $f(m_a^2/m_l^2)$. The function tends to one at zero daughter mass and to zero at threshold. It includes both the phase space restriction and the momentum dependence of the matrix element.