\right] not displaying ]

Question

I have the following code:

\begin{equation}
 \begin{aligned}
     R^{1}_{\mu\nu} = \int & \frac{d^4p}{(2\pi)^4} \tr \left[ \frac{1}{\slashed{p} - \slashed{k}_2-m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{q}-m} \gamma_{\mu} \\
    & - \frac{1}{\slashed{p} -m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{k}_1-m} \gamma_{\mu} \right] 
 \end{aligned}   
\end{equation}

Here's a capture of what I get from it

As you can see, I'm using a long equation which splits in two with \begin{aligned}, and in the first line I have a \left[, which is displaying, but in the second line I have the \right], which isn't displayed on my final equation. You know what could be wrong?

Answer 1

To be able to use \left[ and \right] inside an equation with linebreaks you have to use package breqn:

\documentclass{article}
\usepackage{amsmath}
\usepackage{slashed}
\usepackage{breqn}
\newcommand*{\tr}{\ensuremath{\mathrm{tr}}}
\begin{document}
\begin{dmath}
     R^{1}_{\mu\nu} = \int \frac{d^4p}{(2\pi)^4} \tr \left[ \frac{1}{\slashed{p} - \slashed{k}_2-m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{q}-m} \gamma_{\mu} \\
     - \frac{1}{\slashed{p} -m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{k}_1-m} \gamma_{\mu} \right] 
\end{dmath}
\end{document}

An alternative would be, to use either \right. and \left., but with the risk of having different size:

\documentclass{article}
\usepackage{amsmath}
\usepackage{slashed}
\newcommand*{\tr}{\ensuremath{\mathrm{tr}}}
\begin{document}
\begin{equation}
 \begin{aligned}
     R^{1}_{\mu\nu} = \int & \frac{d^4p}{(2\pi)^4} \tr \left[
                             \frac{1}{\slashed{p} - \slashed{k}_2-m}
                             \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} -
                             \slashed{q}-m} \gamma_{\mu} \right. \\
    & \left. - \frac{1}{\slashed{p} -m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{k}_1-m} \gamma_{\mu} \right] 
 \end{aligned}   
\end{equation}
\end{document}

or to explicitly use a size:

\documentclass{article}
\usepackage{amsmath}
\usepackage{slashed}
\newcommand*{\tr}{\ensuremath{\mathrm{tr}}}
\begin{document}
\begin{equation}
 \begin{aligned}
     R^{1}_{\mu\nu} = \int & \frac{d^4p}{(2\pi)^4} \tr \biggl[
                             \frac{1}{\slashed{p} - \slashed{k}_2-m}
                             \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} -
                             \slashed{q}-m} \gamma_{\mu} \\
    & - \frac{1}{\slashed{p} -m} \gamma_{5} \gamma_{\nu} \frac{1}{\slashed{p} - \slashed{k}_1-m} \gamma_{\mu} \biggr] 
 \end{aligned}   
\end{equation}
\end{document}