\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
We calculate the disk level S-matrix element of one Ramond-Ramond, two gauge fields and one tachyon vertex operators in the world volume of non-BPS branes. We then find the momentum expansion of this amplitude and show that the infinite tachyon/massless poles and the contact terms of this amplitude can be reproduced by the tachyon DBI and the WZ actions, and by their higher derivative corrections.
\end{document}