Abstract
We study the process of thermal activation mediated by sphaleron transitions by analyzing the real-time dynamics of the decay out of equilibrium in a $1+1$ dimensional field theory with a metastable state. The situation considered is that of a rapid supercooling in which the system is trapped in a metastable state at a temperature larger than the mass of the quanta, but smaller than the energy to create a critical droplet. The initial density matrix is evolved in time and the nucleation rate (probability current at the saddle point) is computed. The nucleation rate is {\it time dependent}, vanishing at early times, reaching a maximum at a time $t \approx 1/m$ with $m$ the mass of quanta in the metastable state, and decreasing at long times as a consequence of unitarity. An estimate for the average number of particles of ``true vacuum'' produced as a function of time during the nucleation process is obtained.
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