xetex beamer math accent

Question

The math accent looks ugly if I compile my beamer slides with xelatex.

\hat x in xelatex	\hat x in pdflatex

Here is the MWE:

\documentclass{beamer}
\usepackage{amsfonts,amsmath,amssymb,amsthm}
\begin{document}
\begin{frame}[plain]
  $\hat x$
\end{frame}
\end{document}