Leptonic decay

A leptonic decay has only leptons in its non-radiative final state. For example, a charged pion can decay as ${\pi }^{+}\to {\mu }^{+}{\nu }_{\mu }$. A semileptonic decay instead retains at least one hadron alongside the leptons. This classification concerns the final particles, not whether the initial particle is a hadron or a lepton.

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.