feynlag.pheno.loop.A_one

feynlag.pheno.loop.A_one(tau)

Spin-1 ($W$-loop) form factor, $A_1(tau)=-[2tau^2+3tau+3(2tau-1)f(tau)]/tau^2$.

Heavy-loop limit $A_1to -7$ as $tauto0$. Its sign is opposite the fermion factor’s, so the $W$ and top interfere destructively in $htogammagamma$.