Thermal dynamics on the lattice with exponentially improved accuracy

Abstract: We present a novel simulation prescription for thermal quantum fields on a lattice that operates directly in imaginary frequency space. By distinguishing initial conditions from quantum dynamics it provides access to correlation functions also outside of the conventional Matsubara frequencies $\omega_n=2\pi n T$. In particular it resolves their frequency dependence between $\omega=0$ and $\omega_1=2\pi T$, where the thermal physics $\omega\sim T$ of e.g.~transport phenomena is dominantly encoded. Real-time spectral functions are related to these correlators via an integral transform with rational kernel, so their unfolding is exponentially improved compared to Euclidean simulations. We demonstrate this improvement within a $0+1$-dimensional scalar field theory and show that spectral features inaccessible in standard Euclidean simulations are quantitatively captured.