Ex-14.8   
The reaction can be rewritten as follows U1+U--->S+S1. which implies that U and U1 annihiliate creating S and S1
 
 The pi+ has the quark composition Ud1.Since no quarks are present in the final state. One possible way to get rid of the quarks is to change U into d   
U--->d+W(+). Hence the remaining processes are d+d(+)--->energy and 
 W(+)--->u(+) and vu.