Diffusion of tagged particle in an exclusion process.

We study the diffusion of tagged hard-core interacting Brownian point particles under the influence of an external force field in one dimension. Using the Jepsen line we map this many-particle problem onto a single particle one. We obtain general equations for the distribution and the mean-square displacement <(xT)2> of the tagged center particle valid for rather general external force fields and initial conditions. The case of symmetric distribution of initial conditions around the initial position of the tagged particle on x=0 and symmetric potential fields V(x)=V(-x) yields zero drift =0 and is investigated in detail. We find <(xT)2>=R(1-R)/2Nr2 where 2N is the (large) number of particles in the system. R is a single particle reflection coefficient, i.e., the probability that a particle free of collisions starts on x0>0 and remains in x>0 while r is the probability density of noninteracting particles on the origin. We show that this equation is related to the mathematical theory of order statistics and it can be used to find <(xT)2> even when the motion between collision events is not Brownian (e.g., it might be ballistic or anomalous diffusion). As an example we derive the Percus relation for non-Gaussian diffusion. A wide range of physical behaviors emerge which are very different than the classical single file subdiffusion <(xT)2> approximately t1/2 found for uniformly distributed particles in an infinite space and in the absence of force fields.