For a > 0, let W a 1 (t) and W a 2 (t) be the a-neighbourhoods of two independent standard Brownian motions in R d starting at 0 and observed until time t. We prove that, for d ≥ 3 and c > 0,and derive a variational representation for the rate constant I κa d (c). Here, κ a is the Newtonian capacity of the ball with radius a. We show that the optimal strategy to realise the above large deviation is for W a 1 (ct) and W a 2 (ct) to "form a Swiss cheese": the two Wiener sausages cover part of the space, leaving random holes whose sizes are of order 1 and whose density varies on scale t 1/d according to a certain optimal profile.We study in detail the function c → I We also derive the analogous result for d = 2, namely,
742M. VAN DEN BERG, E. BOLTHAUSEN, AND F. DEN HOLLANDER where the rate constant has the same variational representation as in d ≥ 3 after κ a is replaced by 2π. In this case I 2π 2 (c) = Θ 2 (2πc)/2π with Θ 2 (u) < ∞ if and only if u ∈ (u , ∞) and Θ 2 is strictly decreasing on (u , ∞) with a zero limit.Acknowledgment.