String scattering diagrams with TikZ or other package in LaTeX

Question

I am working on a paper and I need in the introduction to put a diagram. The diagram is the following (or something similar) and I was wondering how can I reproduce it in LaTeX code.

Answer 1

Using Andrew Stacey's purpose-built tqft library:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{tqft}
\begin{document}
\begin{tikzpicture}[tqft/cobordism/.style={draw},tqft/every boundary component/.style={draw}]
\pic [tqft/pair of pants];
\end{tikzpicture}

There are many customization options as well as other shapes and connection methods, if you need a substantial number of these diagrams. You may refer to the manual for complete details.

Answer 2

A TikZ solution:

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \draw (0,0) ellipse (.5 and .25);
  \draw (-1,-2) ellipse (.5 and .25);
  \draw (1,-2) ellipse (.5 and .25);
  \draw (-.5,0) to[out=-90,in=90] (-1.5,-2);
  \draw (.5,0) to[out=-90,in=90] (1.5,-2);
  \draw (-.5,-2) to[out=90,in=90] (.5,-2);
\end{tikzpicture}
\end{document}

With some pseudo-3D

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}[x={(1cm,0cm)},y={(-.2cm,-.2cm)},z={(0cm,1cm)}]
  \draw (0,0,0) circle (.5);
  \draw (-1,0,-2) circle (.5);
  \draw (1,0,-2) circle (.5);
  \draw (-.5,0,0) to[out=-90,in=90] (-1.5,0,-2);
  \draw (.5,0,0) to[out=-90,in=90] (1.5,0,-2);
  \draw (-.5,0,-2) to[out=90,in=90] (.5,0,-2);
\end{tikzpicture}
\end{document}