Diffusion of finite-sized hard-core interacting particles in a one-dimensional box: Tagged particle dynamics

We solve a nonequilibrium statistical-mechanics problem exactly, namely, the single-file dynamics of N hard-core interacting particles (the particles cannot pass each other) of size Delta diffusing in a one-dimensional system of finite length L with reflecting boundaries at the ends. We obtain an exact expression for the conditional probability density function rho T(yT,t vertical bar yT,0) that a tagged particle T (T=1,...,N) is at position yT at time t given that it at time t=0 was at position yT,0. Using a Bethe ansatz we obtain the N-particle probability density function and, by integrating out the coordinates (and averaging over initial positions) of all particles but particle T, we arrive at an exact expression for rho T(yT,t vertical bar yT,0) in terms of Jacobi polynomials or hypergeometric functions. Going beyond previous studies, we consider the asymptotic limit of large N, maintaining L finite, using a nonstandard asymptotic technique. We derive an exact expression for rho T(yT,t vertical bar yT,0) for a tagged particle located roughly in the middle of the system, from which we find that there are three time regimes of interest for finite-sized systems: (A) for times much smaller than the collision time t <tau(coll)=1/(rho D-2), where rho=N/L is the particle concentration and D is the diffusion constant for each particle, the tagged particle undergoes a normal diffusion; (B) for times much larger than the collision time t tau(coll) but times smaller than the equilibrium time t <tau(eq)=L-2/D, we find a single-file regime where rho T(yT,t vertical bar yT,0) is a Gaussian with a mean-square displacement scaling as t(1/2); and (C) for times longer than the equilibrium time t tau(eq), rho T(yT,t vertical bar yT,0) approaches a polynomial-type equilibrium probability density function. Notably, only regimes (A) and (B) are found in the previously considered infinite systems.