Study of D0– mixing and D0 doubly Cabibbo-suppressed decays

Abstract Using a sample of four million hadronic Z events collected in ALEPH from 1991 to 1995, the decays D ∗+ → D 0 π s + , with D 0 decaying to K − π + or to K + π − , are studied. The relative branching ratio B (D 0 →K + π − )/ B (D 0 →K − π + ) is measured to be 1.84±0.59( stat. ) ±0.34( syst. ) %. The two possible contributions to the D 0 →K + π − decay, doubly Cabibbo-suppressed decays and D 0 – D 0 mixing, are disentangled by measuring the proper-time distribution of the reconstructed D 0 's. Assuming no interference between the two processes, the upper limit obtained on the mixing rate is 0.92% at 95% CL . The possible effect of interference between the two amplitudes is also assessed.