Effect of the relativistic spin rotation on two--particle spin composition

The effect of the relativistic spin rotation on two--particle spin states, conditioned by the setting of the spins of the particles in their rest frames and by the noncommutativity of the Lorentz transformations along noncolinear directions, is discussed. Particularly, the transition from the c.m.s. of two spin-1/2 particles to the laboratory is considered. When the vectors of the c.m.s. particle velocities are not colinear with the velocity vector of the c.m.s., the angles of the relativistic spin rotation for the two particles are different. As a result, the relative fractions of the singlet and triplet states in the relativistic system of two spin-1/2 particles depend on the concrete frame in which the two-particle system is analyzed.