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Berry's phase in the relativistic theory of spinning particles.

We show that the change of the phase of the wave function of a free relativistic spinning particle under the action of the Poincar\'e group is described by a certain universal connection in momentum space. This connection determines the parallel transport of the spin during the motion of the particle through external fields. In the special case of photons we reproduce the formula for Berry's phase calculated recently by Chiao and Wu.