  The effective interaction between two probe particles in a one-dimensional
driven system is studied. The analysis is carried out using an asymmetric
simple exclusion process with nearest-neighbor interactions. It is found that
the driven fluid mediates an effective long-range attraction between the two
probes, with a force that decays at large distances x as -b/x, where b is a
function of the interaction parameters. Depending on the amplitude b the two
probes may form one of three states: (a) an unbound state, where the distance
grows diffusively with time; (b) a weakly bound state, in which the distance
grows sub-diffusively; and (c) a strongly bound state, where the average
distance stays finite in the long time limit. Similar results are found for the
behavior of any finite number of probes.
