Measurements of time-dependent CP asymmetries in B → D * ∓π± decays using a partial reconstruction technique

We report results on time-dependent CP asymmetries in B -> D*(-/+)pi(+/-) decays based on a data sample containing 657 x 10(6) B (B) over bar pairs collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider at the Y(4S) resonance. We use a partial reconstruction technique, wherein signal B -> D*(-/+)pi(+/-) events are identified using information only from the fast pion from the B decay and the slow pion from the subsequent decay of the D*(-/+), where the former (latter) corresponds to D*(+)(D*(-)) final states. We obtain CP violation parameters S+ = +0.061 +/- 0.018 (stat) +/- 0.012 (syst) and S- = +0.031 +/- 0.019 (stat) +/- 0.015 (syst).