HalfNormal Distribution

This is a special case of the chi distribution with $L=a$ and $S=b$ and $\nu =1.$ This is also a special case of the folded normal with shape parameter $c=0$ and $S=S.$ If $Z$ is (standard) normally distributed then, $\left|Z\right|$ is half-normal. The standard form is

Implementation: scipy.stats.halfnorm