Survival probability of a diffusing particle in the presence of Poisson-distributed mobile traps.

The problem of a diffusing particle moving among diffusing traps is analyzed in general space dimension d. We consider the case where the traps are initially randomly distributed in space, with uniform density rho, and derive upper and lower bounds for the probability Q(t) (averaged over all particle and trap trajectories) that the particle survives up to time t. We show that, for 1< or =d< or =2, the bounds converge asymptotically to give Q(t) approximately exp(-lambda(d)t(d/2)) for 1< or =d<2, where lambda(d)=(2/pid)sin(pid/2)(4piD)(d/2)rho and D is the diffusion constant of the traps, and that Q(t) approximately exp(-4pirhoDt/ln t) for d=2. For d>2 bounds can still be derived, but they no longer converge for large t. For 1< or =d< or =2, these asymptotic forms are independent of the diffusion constant of the particle. The results are compared with simulation results obtained using a new algorithm [V. Mehra and P. Grassberger, Phys. Rev. E 65, 050101 (2002)] which is described in detail. Deviations from the predicted asymptotic forms are found to be large even for very small values of Q(t), indicating slowly decaying corrections whose form is consistent with the bounds. We also present results in d=1 for the case where the trap densities on either side of the particle are different. For this case we can still obtain exact bounds but they no longer converge.