D0-D¯0mixing in the presence of isosinglet quarks

We analyze ΔC=2 transitions in the framework of a minimal extension of the standard model where either a Q=2/3 or a Q=-1/3 isosinglet quark is added to the standard quark spectrum. In the case of a Q=2/3 isosinglet quark, it is shown that there is a significant region of parameter space where ${\mathit{D}}^0$-D${\mathrm{¯}}^0$ mixing is sufficiently enhanced to be observed at the next round of experiments. On the contrary, in the case of a Q=-1/3 isosinglet quark, it is pointed out that obtaining a substantial enhancement of ${\mathit{D}}^0$-D${\mathrm{¯}}^0$ mixing, while complying with the experimental constraints on rare kaon decays, requires a contrived choice of parameters.