Nonequilibrium Fluctuations and Enhanced Diffusion of a Driven Particle in a Dense Environment

We study the diffusion of a tracer particle driven out of equilibrium by an external force and traveling ina dense environment of arbitrary density. The system evolves on a discrete lattice and its stochasticdynamics is described by a master equation. Relying on a decoupling approximation that goes beyondthe naive mean-field treatment of the problem, we calculate the fluctuations of the position of the traceraround its mean value on a lattice of arbitrary dimension, and with different boundary conditions. We revealintrinsically nonequilibrium effects, such as enhanced diffusivity of the tracer induced by both thecrowding interactions and the external driving. We finally consider the high-density and low-density limitsof the model and show that our approximation scheme becomes exact in these limits.