Time evolution of effective central charge and signatures of RG irreversibility after a quantum quench

At thermal equilibrium, the concept of effective central charge for massive deformations of two-dimensional conformal field theories (CFT) is well understood, and can be defined by comparing the partition function of the massive model to that of a CFT. This temperature-dependent effective charge interpolates monotonically between the central charge values corresponding to the IR and UV fixed points at low and high temperatures, respectively. We propose a non-equilibrium, time-dependent generalization of the effective central charge for integrable models after a quantum quench, c_{\rm eff}(t), obtained by comparing the return amplitude to that of a CFT quench. We study this proposal for a large mass quench of a free boson, where the effective charge is seen to interpolate between c_{\rm eff}=0 at t=0t=0, and c_{\rm eff}\sim 1 at t\to\inftyt→∞, as is expected. We use our effective charge to define an “Ising to Tricritical Ising" quench protocol, where the charge evolves from c_{\rm eff}=1/2 at t=0t=0, to c_{\rm eff}=7/10 at t\to\inftyt→∞, the corresponding values of the first two unitary minimal CFT models. We then argue that the inverse “Tricritical Ising to Ising" quench is impossible with our methods. These conclusions can be generalized for quenches between any two adjacent unitary minimal CFT models. We finally study a large mass quench into the “staircase model" (sinh-Gordon with a particular complex coupling). At short times after the quench, the effective central charge increases in a discrete “staircase" structure, where the values of the charge at the steps can be computed in terms of the central charges of unitary minimal CFT models. When the initial state is a pure state, one always finds that c_{\rm eff}(t\to\infty)\geq c_{\rm eff}(t=0), though c_{\rm eff}(t), generally oscillates at finite times. We explore how this constraint may be related to RG flow irreversibility.