\documentclass[a4paper,10pt]{scrreprt}
\usepackage{pstricks,pst-text}
\DeclareFixedFont{\SF}{T1}{phv}{b}{ol}{5mm}
\newcommand{\p}{\partial}
\newcommand{\vs}{\vspace{.1in}}
\begin{document}







\begin{pspicture}(-4,-4)(4,4)


  \pstextpath[c]{\psarc[linestyle=none](0,0){2}{180}{360}}
{${\mathcal L}=R-\bar{\psi}_{\mu}\gamma^{\mu\rho\sigma}D_{\rho}\psi_{\sigma}$}

\end{pspicture}


\end{document}