SU(3)breaking andD0−D0mixing

The main challenge in the standard model calculation of the mass and width difference in the $D^0-D^0$ system is to estimate the size of $\mathrm{SU}\left(3\right)\mathrm{}$ breaking effects. We prove that D meson mixing occurs in the standard model only at second order in $\mathrm{SU}\left(3\right)\mathrm{}$ violation. We consider the possibility that phase space effects may be the dominant source of $\mathrm{SU}\left(3\right)\mathrm{}$ breaking. We find that $y=\Delta \Gamma /2\mathrm{}\Gamma$ of the order of one percent is natural in the standard model, potentially reducing the sensitivity to new physics of measurements of D meson mixing.