Extra problem for sheet_2 (or other)

[vanamalishastry]

This could be another problem:

Using the polarization vectors given in the problem above, evaluate the amplitude for the decay of $a_1$ meson ($J^P=1^+$) to $\rho\pi$ in the following frames of reference: \begin{enumerate} \item $a_1$ is at rest and the decay particles are aligned along the $z$-axis. \item $\rho$ is at rest and $a_1$ and $\pi$ are aligned along the $z$-axis. \end{enumerate}

[mmikhasenko]

Added to the sheet 5. Thanks @vanamalishastry