\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\exp[-(S_\text{new}-S_\text{old})]\frac{T(\text{new}->\text{old})}{T(\text{old}->\text{new})}
\end{equation*}
\end{document}