A Search for D0 - anti-D0 mixing and doubly Cabibbo suppressed decays of the D0 in hadronic final states

We present results of a search for ${D}^{0}\ensuremath{-}{D}^{0}$ mixing and doubly-Cabibbo-suppressed decays of the ${D}^{0}$ in Fermilab experiment E791, a fixed-target charm hadroproduction experiment. We look for evidence of mixing in the decay chain ${D}^{*}\ensuremath{\rightarrow}\ensuremath{\pi}\stackrel{\ensuremath{\rightarrow}}{D}\ensuremath{\pi}(K\ensuremath{\pi}$ or $K\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\pi})$. If the charge of the pion from the ${D}^{*}$ decay is the same as the charge of the kaon from the $D$ decay (a ``wrong-sign'' event), mixing may have occurred. Mixing can be distinguished from other sources of wrong-sign events (such as doubly-Cabibbo-suppressed decays) by analyzing the distribution of decay times. We see no evidence of mixing. Allowing for $\mathrm{CP}$ violation in the interference between DCS and mixing amplitudes our fitted ratio for mixed to unmixed decay rates is ${r}_{\mathrm{mix}}{=(0.39}_{\ensuremath{-}0.32}^{+0.36}\ifmmode\pm\else\textpm\fi{}0.16)%$. This corresponds to a 90% C.L. upper limit of ${r}_{\mathrm{mix}}l0.85%$. The sensitivity of this result is comparable to that of previous measurements, but the assumptions made in fitting the data are notably more general. We present results from many fits to our data under various assumptions. If we assume ${r}_{\mathrm{mix}}=0$, we find a two-sigma wrong-sign enhancement in the $K\ensuremath{\pi}$ mode which we ascribe to doubly Cabibbo-suppressed decays. The ratios of doubly Cabibbo-suppressed decays to Cabibbo-favored decays are ${r}_{\mathrm{dcs}}(K\ensuremath{\pi}{)=(0.68}_{\ensuremath{-}0.33}^{+0.34}\ifmmode\pm\else\textpm\fi{}0.07)%$ and ${r}_{\mathrm{dcs}}(K\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\pi}{)=(0.25}_{\ensuremath{-}0.34}^{+0.36}\ifmmode\pm\else\textpm\fi{}0.03)%$.