We report a measurement of the CP-violation parameter sin2$phi_1$ at the $Upsilon(5S)$ resonance using a new tagging method, called $B$-$pi$ tagging. In $Upsilon(5S)$ decays containing a neutral $B$ meson, a charged $B$, and a charged pion, the neutr
al $B$ 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 $Upsilon(5S)$ 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 $pm$ 0.58(stat) $pm$ 0.06(syst). The results are based on 121 fb$^{-1}$ of data recorded by the Belle detector at the KEKB $e^+ e^-$ collider.
