CLEO measurement of B⃗π⁺π^- and determination of weak phase α

The CLEO Collaboration has recently reported a (first) measurement of BR(B⃗π⁺π^-)=(4.7_-1.5^+1.8±0.6)×10^-6. We study, using recent results on QCD improved factorization, the implications of this measurement for the determination of the Cabibbo-Kobayashi-Maskawa (CKM) phase α and also for the rate for B⃗π⁰π⁰. If the B⃗π^- form factor is large (≳0.3), then we find that the CLEO measurement favors small |V_ub/V_cb| so that the expected error (due to neglecting the QCD penguin amplitude) in the measurement of α using only the time-dependent analysis of the decay B⃗π⁺π^- is large ∼15°. However, if |V_ub/V_cb| is known, then it is possible to determine the correct value of sin2α.