Neutral-meson-antimeson mixing for a heavy isosinglet quark constituent

If there exists a heavy isosinglet h quark that mixes with the d quark in the weak eigenstates, then the neutral meson $H^0$(hd¯) can mix maximally with its antiparticle H${¯}^0$(h¯d) through lowest-order flavor-changing neutral currents regardless of how small the h-d mixing angle may be. The $H^0$-H${¯}^0$ mixing parameter ε depends essentially on the $H^0$ mass; for $m_H$<25 GeV, ε can be as large as 0.4.