Evidence for $D^0 - \bar{D}^0$ Mixing

We present evidence for D{sup 0}-{bar D}{sup 0} mixing in D{sup 0} {yields} K{sup +}{pi}{sup -} decays from 384 fb{sup -1} of e{sup +}e{sup -} colliding-beam data recorded near {radical}s = 10.6GeV with the BABAR detector at the PEP-II storage rings at SLAC. We find the mixing parameters x{prime}{sup 2} = [-0.22 {+-} 0.30 (stat.) {+-} 0.21 (syst.)] x 10{sup -3} and y{prime} = [9.7 {+-} 4.4(stat.) {+-} 3.1(syst.)] x 10{sup -3}, and a correlation between them of -0.94. This result is inconsistent with the no-mixing hypothesis with a significance of 3.9 standard deviations. We measure R{sub D}, the ratio of doubly Cabibbo-suppressed to Cabibbo-favored decay rates, to be [0.303 {+-} 0.016 (stat.) {+-} 0.010 (syst.)]%. We find no evidence for CP violation.