Leptonic three-body mass correction
= Leptonic three-body mass correction
{title2=$f(z)=1-8z+8z^3-z^4-12z^2\log z$}
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>.