Long-time tails in the SYK chain from the effective field theory with a large number of derivatives

We study nonlinear energy diffusion in the SYK chain within the framework of Schwinger-Keldysh effective field theory. We analytically construct the corresponding effective action up to 40th order in the derivative expansion. According to this effective action, we calculate the first order loop correction of the energy density response function, whose pole is the dispersion relation of energy diffusion. As expected, the standard derivative expansion of the classical dispersion relation breaks down due to the long-time tails. However, we find that the nonlinear contributions are so that one can still derive the dispersion relation in the power series. In fact, due to the long-time tails, the classical dispersion relation is split into two series distinct from the derivative expansion, and we show they are convergent. The radius of convergence is proportional to the ratio of thermal conductivity to diffusion constant.