Spontaneous Emission from Excited Quantum Kinks

Many kink solutions enjoy internal excitations, called shape modes. In some 1+1d scalar models, such as the $\phi^4$ double-well model, when a kink's shape mode is excited twice it may decay to a ground state kink plus a meson. We analytically calculate the decay rates of both the twice-excited shape mode and also a coherent state corresponding to the classically excited shape mode. In the case of the $\phi^4$ model, we find that the latter agrees with the classical result of Manton and Merabet.