在 ln latex 中自我循环

Question

\documentclass[12pt,a4paper]{article}
\usepackage{tikz}
\tikzset{mynode/.style={inner sep=2pt,fill,outer sep=0,circle}}

\begin{document}


 \begin{tikzpicture}[scale=.60,
       thick,
       acteur/.style={
         circle,
         fill=black,
         thick,
         inner sep=1.2pt,
         minimum size=0.01cm
       }
     ] 
   % \tikzstyle{every loop}=[]
           
           
          
           
           \node (a1) at (0,0) [acteur,label=left:\scriptsize{$1$}]{};
           \node (a2) at (1.5,0)[acteur,label=left:\scriptsize{$2$}]{}; 
           \node (a3) at (0,-1.5) [acteur,label=above left:\scriptsize{$3$}]{}; 
           \node (a4) at (1.5,-1.5) [acteur,label=above right:\scriptsize{$4$}]{}; 
          
          
           \draw  (a1) -- (a2); 
           \draw  (a2) -- (a4); 
           \draw  (a3) -- (a4);
           \draw  (a1) -- (a3);
           \draw  (a2) -- (a3);
          
           
        \draw  (a1) -- (a1);
        
        
%\node at (2.25,-1) {\textit{$U^{+}$}};
\end{tikzpicture}



\end{document}

这里我尝试在顶点 1 和 2 处绘制一个自循环。如何绘制？请帮忙。

我需要这种没有箭头的循环