Search for D0-D0bar Mixing

We report on a search for D0-D0bar mixing made by a study of the `wrong-sign' process D0 -> K+pi-. The data come from 9.0 fb^-1 of integrated luminosity of e+e- collisions at sqrt{s} approximately 10 GeV produced by CESR and accumulated with the CLEO II.V detector. We measure the time-integrated rate of the `wrong-sign' process D0 -> K+pi-, relative to that of the Cabibbo-favored D0bar -> K+pi-, to be R_ws=(0.34+-0.07+-0.06)%. By a study of the rate as a function of the decay time of the D0, we measure the rate of direct, doubly Cabibbo suppressed decay D0 -> K+pi- relative to D0bar -> K+pi-, to be R_D=(0.50+0.11-0.12+-0.08)%. The amplitudes that describe D0-D0bar mixing, x' and y' are consistent with zero. The one-dimensional limits, at the 95% C.L., that we determine are (1/2)x^'2<0.05%, and -5.9%<y'<0.3%. All results are preliminary.