Why ismumdso much smaller thanmcmsandmtmb?

We propose that the smallness of $\frac{m_u}{m_d}$ is due to the "wrong" Yukawa coupling of the up quark in a two-Higgs-doublet model. The model has natural flavor conservation only among charge $-\frac{1}{3}$ quarks and leptons. We estimate that $D^0-{\overline{D}}^0$ mixing is much larger than the value of the standard-model box diagrams, and could be larger than or comparable to the long-distance contributions.