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We suggest that certain nonlinear field theories possess a particle spectrum, richer than has been heretofore discussed. In addition to states associated with quantization of the free-field modes of oscillation---these are the conventional particles of the theory---there also appear heavy particles, which carry a new quantum number and are stable. An approximation scheme is developed in which the signal for these new particles is the existence of stable static solutions with finite energy to the classical equations of motion. We give a systematic expansion for the theory, with special emphasis on the translational motion.
Goldstone et al. (1975) studied this question.