feynlag.pheno.loop.A_half

feynlag.pheno.loop.A_half(tau)

Spin-$tfrac12$ (fermion-loop) form factor, $A_{1/2}(tau)=2[tau+(tau-1)f(tau)]/tau^2$.

$tau=m_h^2/4m^2$. Heavy-loop-particle limit $A_{1/2}to 4/3$ as $tauto0$.