The decay of D0 → K− + π+ is accomplished by the exchange of W+ boson as illustrated by the Feynman diagram Fig. 10.23.
The quark composition (D0 = cu bar , K− = su bar , π+ = ud bar) is also indicated.
The favoured route for the decay is via c → *.
Hence the decay occurs via lowest order charge current weak interaction. However, for D+ → K0 + π+ the c-quark is required to decay to d-quark via W emission and the subsequent decay of W+ to π+. This would mean that the d quark in the D+ decay to an * quark in the K0 which is not possible as they both have the same charge. Furthermore the transition c → d is not favoured in the Cabibbo scheme.
