  We show that an atomic system in a periodically modulated optical trap
displays an ideal mean-field symmetry-breaking transition. The symmetry is
broken with respect to time translation by the modulation period. The
transition is due to the interplay of the long-range interatomic interaction
and nonequilibrium fluctuations. The observed critical dynamics, including
anomalous fluctuations in the symmetry broken phase, are fully described by the
proposed microscopic theory.
