Improving the measurement of the CKM phase $\phi_2 = \alpha$ in $B \to   \pi \pi$ and $B \to \rho \rho$ decays
  CP-violating asymmetries in $B \to \pi \pi$ and $B \to \rho \rho$ decays can help specify the weak phase $\phi_2 = \alpha$ of the Cabibbo-Kobayashi-% Maskawa (CKM) matrix. We discuss the impact of improved measurements of these processes such as will be available in the near future, finding special value in better measurement of the time-dependent CP violation parameter $S_{00}$ in $B^0 \to \pi^0 \pi^0$ and $B^0 \to \rho^0 \rho^0$. Reducing the errors on $B \to \rho \rho$ measurements by a factor of two can potentially lead to an error in $\phi_2 = \alpha$ just above $2^\circ$, at which level isospin-breaking corrections must be considered. 
Michael Gronau and Jonathan L. Rosner
hep-ph hep-ex