= Transverse massless leptonic current
{title2=$q^\mu\ell_\mu=0$}
For two outgoing massless leptons, $\ell_\mu=\bar u(q_1)\gamma_\mu(1-\gamma^5)v(q_2)$ is transverse to $q=q_1+q_2$. This follows from the <massless Dirac equation> and anticommutation of the <chirality matrix> with the <gamma matrices>. Thus the $q^\mu f_-(q^2)$ term in a <pseudoscalar-to-pseudoscalar form factor> does not contribute to this <scattering amplitude>. A nonzero charged-lepton mass spoils exact transversality and restores a mass-suppressed scalar-form-factor contribution.
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