\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
p_{\rm trial}^{\rm arb}(\theta_{\rm g_a,f,p}) = 
{
{2}
\over
{
\left[
{\rm Erf}\left({{\pi-\mu}\over{\sigma \sqrt{2}}}\right)
- {\rm Erf}\left({{0-\mu}\over{\sigma \sqrt{2}}}\right)
\right]
\sigma \sqrt{2 \pi}
}
}
\exp\left(-{{(\theta_{\rm g_a,f,p} - \mu)^2}\over{2 \sigma^2}}\right)
\end{displaymath}
\end{document}