Central limit theorem for random walks in divergence-free random drift field: "H-minus-one" suffices

We prove central limit theorem under diffusive scaling for the displacement of a random walk on ${\mathbb Z}^d$ in stationary divergence-free random drift field, under the ${\mathcal H}_{-1}$-condition imposed on the drift field. The condition is equivalent to assuming that the stream tensor be stationary and square integrable. This improves the best existing result of Komorowski, Landim and Olla (2012), where it is assumed that the stream tensor be in ${\mathcal L}^{\max\{2+\delta,d\}}$, with $\delta>0$. Our proof relies on the relaxed sector condition of Horv\'ath, T\'oth and Vet\H{o} (2012), and is technically rather simpler than existing earlier proofs of similar results by Oelschl\"ager (1988) and Komorowski, Landim, Olla (2012).