How to write a letter "a" in Tex

Question

This seems to be a fairly simple question. But I want to know how can I write the alphabet "a" in Latex as shown below: I cannot use the regular "a" in this case.

Thanks for any suggestion.

The working example is as follows:

\documentclass[11pt]{beamer} 

% Setup appearance:



%\setbeamertemplate{navigation symbols}{}
\setbeamersize{text margin left=4mm, text margin right=3mm} 


%Pacakges
\usepackage{graphics}
\usepackage{tikz}
\usepackage{amsmath}
\usepackage{verbatim}
\usetikzlibrary{arrows,shapes}
\usepackage{amsmath}
\RequirePackage{atbegshi}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools
\usetikzlibrary{arrows,shapes}
\usetikzlibrary{arrows,shapes}
\RequirePackage{atbegshi}
\graphicspath{./figures/} 
\usepackage{ragged2e}
\justifying
\usepackage{empheq}

\mode<presentation>{}

\begin{document}
\begin{frame}[shrink=10]

\begin{equation*}
a_i(t,x) =
        \frac{
        \tikz[baseline]{
            \node[fill=blue!20,anchor=base] (t1)
            {$ \int \limits_{0}^{\infty}{ \tau c_i(t,x,\tau )\partial \tau}$};
        }}{
        \tikz[baseline]{
            \node[fill=red!20, ellipse,anchor=base] (t2)
            {$ \int \limits_{0}^{\infty} c_i(t,x,\tau) \partial \tau$};
        }} 
\end{equation*}

% Now it's time to draw some edges between the global nodes. Note that we
% have to apply the 'overlay' style.


\end{frame}
\end{document}