Theoretical value of sin(2\beta)

[Magister]

I am looking for the theoretical value of $sin(2\beta)$ but I just keep finding the experimental one. I supose there is a theoretical value because I have read that they choose in the BaBar experience the channel $B^0\rightarrow J/\psi K^0_S$ because the theoretical error for $sin(2\beta)$ was very small...

Thanks for your replies.

 

[Barmecides]

I am looking for the theoretical value of $sin(2\beta)$ but I just keep finding the experimental one. I supose there is a theoretical value because I have read that they choose in the BaBar experience the channel $B^0\rightarrow J/\psi K^0_S$ because the theoretical error for $sin(2\beta)$ was very small...

Ahhh... never heard about this argument...
I thought cp phase is an input of SM. I guess it can be infered from existing measurements like K mixing via lattice QCD but that's very hard task.