Mixing in the $D^0$ system: Results from collider experiments

Mixing in the $D^0$ system may provide a sensitive probe for new physics beyond the Standard Model (SM) but has so far eluded experimental observation. The SM predictions are typically small ($< 10^{-3}$) for the mixing parameters $x$, $y$ which, in the absence of charge-parity ($CP$) symmetry violation, measure the mass ($x= Delta m/ Gamma$) and lifetime ($y= Delta Gamma / 2 Gamma$) difference of the $CP$ eigenstates in the $D^0$ system. The asymmetric $B$-factory experiments BABAR and Belle open up the opportunity of measuring $x$, $y$ with unprecedented statistical precision and sample purities. Results from BABAR and Belle, and from CLEO are reviewed.