Code: Select all
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\begin{document}
The equation below is the one I want to split by breaking in between the big square brackets.
\begin{align}
&= \int D_n k \left\{ \left[ \frac{2}{M_\rho ^2D_1} + \frac{2}{M_\rho ^2D_3} - \frac{4p^2}{M_\rho ^2D_1D_3} - \frac{4p^2}{D_1D_2D_3} \right]k_2^\nu + \right. \nonumber\\
& \left. + \left[ - \frac{1}{M_\rho ^2 D_3} + \frac{1}{2D_1 D_2} - \frac{3}{2D_2 D_3} + \left(2+\frac{p^2}{M_\rho ^2} \right)\frac{1}{D_1D_3} + \frac{D_2}{2M_\rho ^4D_3}- \frac{D_2}{2M_\rho ^4D_1} + \left(\frac{p^2}{2M_\rho ^4} - \frac{1}{M_\rho ^2} \right)\frac{D_2}{D_1D_3} + \right. \left. + \left(4m_\pi ^2 - M_\rho ^2 + \frac{1}{2}p^2 \right)\frac{1}{D_1 D_2 D_3} \right] p^\nu +\right. \nonumber\\ & \left.+ k^\nu \left[ \frac{1}{M_\rho ^2D_1} + \frac{1}{M_\rho ^2D_3} - \frac{1}{D_1 D_2} - \frac{1}{D_2D_3} + \left( 4 - \frac{2p^2}{M_\rho ^2} \right)\frac{1}{D_1D_3} + \left( \frac{p^2}{M_\rho ^4} - \frac{2}{M_\rho ^2} \right)\frac{D_2}{D_1D_3} + \left( 8m_\pi ^2 - 2M_\rho ^2 - 3p^2 \right)\frac{1}{D_1D_2D_3} \right] \right\}
\end{align}
Below is me breaking between the square brackets
\begin{align}
&\left[ - \frac{1}{M_\rho ^2 D_3} + \frac{1}{2D_1 D_2} - \frac{3}{2D_2 D_3} + \left(2+\frac{p^2}{M_\rho ^2} \right)\frac{1}{D_1D_3} + \frac{D_2}{2M_\rho ^4D_3}- \frac{D_2}{2M_\rho ^4D_1} + \left(\frac{p^2}{2M_\rho ^4} - \frac{1}{M_\rho ^2} \right)\frac{D_2}{D_1D_3} + \right. \nonumber\\ &\left. + \left(4m_\pi ^2 - M_\rho ^2 + \frac{1}{2}p^2 \right)\frac{1}{D_1 D_2 D_3} \right] p^\nu
\end{align}
\end{document}
