$\rho_0$ decay angle for the rest frame of $\rho_0$

[bdrum]

During 4$\pi$ photoproduction in reaction of the $\rho'$ decay:

$$\rho'\rightarrow \pi^+\pi^-\pi^+\pi^-$$

I have a intermediate two-body decay state when $\rho_0\rightarrow \pi^+\pi^-$.

This allows us to check $\rho_0$ via determining of the decay angle for rest frame of $\rho_0$.

[bdrum]

Givens rotation the same also in Brandt A.4.1