Capital cursive E in math mode?

To assemble the suggestions in the comments, you can do the following:

Code

\documentclass{article}

\usepackage{mathrsfs} % https://www.ctan.org/pkg/mathrsfs

\begin{document}

\begin{equation}
  \mathscr{E} % uses `mathrsfs`
  \mathcal{E}
  \varepsilon
\end{equation}

\end{document}

Output

As void-pointer points out, the fonts Cursif, ALS Script, and Ecolier come even closer to the desired output.

I'd also recommend Detexify! It allows you to draw a symbol and it shows you possible matches and the TeX required to draw them. It works very well, I use it all the time!

There are several fonts that have cursive Es close to that in the sample.

Kunstlerscreibsch

http://www.font-db.com/font/kunstlerschreibschjoidbol/38809

Ecolier Court

(SIL license) http://www.fontspace.com/jean-douteau/ecolier