\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning} 
\tikzset{
    %Define standard arrow tip
    >=stealth',
    % Define arrow style
    pil/.style={
           ->,
           thick}
}

\begin{document}
\begin{preview}
\begin{tikzpicture}
    \draw[fill=green!30] (0,0) -- (90:.75cm) arc (90:27:.75cm);
    \draw[pil,color=black] (0,0) -- node[right=2pt] {$\vec a$} (27:2.2cm);
    \draw[pil,color=black] (0,0) -- node[near end, right=-3pt] {$\vec b$} (90:2cm);
    \draw(60:0.5cm) node {$\varphi$};
  \end{tikzpicture}
\end{preview}
\end{document}
