Sato, Y (reprint author), Tohoku Univ, Sendai, Miyagi 980, Japan.
We report a measurement of the CP-violation parameter sin2 phi(1) at the Y(5S) resonance using a new tagging method, called "B-pi tagging.'' In Y(5S) decays containing a neutral B meson, a charged B, and a charged pion, the neutral B is reconstructed in the J / psi K-S(0) CP-eigenstate decay channel. The initial flavor of the neutral B meson at the moment of the Y(5S) decay is opposite to that of the charged B and may thus be inferred from the charge of the pion without reconstructing the charged B. From the asymmetry between B-pi(+) and B-pi(-) tagged J / psi K-S(0) yields, we determine sin2 phi(1) 0.57 +/- 0.58(stat) +/- 0.06(syst). The results are based on 121 fb(-1) of data recorded by the Belle detector at the KEKB e(+)e(-) collider.