feynlag.pheno.loop.f_function

feynlag.pheno.loop.f_function(tau)

The scalar one-loop function $f(tau)$.

$f(tau)=arcsin^2sqrttau$ for $taule1$ (loop particle above threshold, the physical SM case), and the analytically-continued log form (with the $-ipi$) for $tau>1$ (a light loop particle — kept for BSM spectra).