Suppose a matter A with mass, m1, and spin up. Suppose a second matter B with mass,m2 and spin down. If both particles have the same mass, m1=m2. Is this a particle-antiparticle reaction? That is:
for m1=m2=m, the reaction A Sup + B Sdown --> 0
or
A Sup + B Sdown --> (A+B) with S = 0