Periodic Instanton Bifurcations and Thermal Transition Rate

It is shown that periodic instanton solutions may have bifurcations which qualitatively change the behaviour of the finite temperature transition rate. These bifurcations are studied numerically in a quantum mechanical model and in the massive two-dimensional sigma model with the Skyrme term. General rules which determine the behaviour of periodic instantons near bifurcations are derived and applied to the Electroweak Theory.