Improved search for D^0 mixing using semileptonic decays at Belle

A search for mixing in the neutral D meson system has been performed using semileptonic D0 -> K(*)- e+ u and D0 -> K(*)- mu+ u decays. Neutral D mesons from D*+ -> D0 pi_s^+ decays are used and the flavor at production is tagged by the charge of the slow pion. The measurement is performed using 492 fb^-1 of data recorded by the Belle detector. From the yield of right-sign and wrong-sign decays arising from non-mixed and mixed events, respectively, we measure the ratio of the time-integrated mixing rate to the unmixed rate to be R_M = (1.3 +- 2.2 +- 2.0) x 10^-4. This corresponds to an upper limit of R_M < 6.1 x 10^-4 at the 90% C.L.