Formation and relaxation of quasistationary states in particle systems with power-law interactions

We explore the formation and relaxation of the so-called quasistationary states (QSS) for particle distributions in three dimensions interacting via an attractive radial pair potential V (r -> infinity) similar to 1/r(gamma) with gamma > 0, and either a soft core or hard core regularization at small r. In the first part of the paper, we generalize, for any spatial dimension d >= 2, Chandrasekhar's approach for the case of gravity to obtain analytic estimates of the rate of collisional relaxation due to two-body collisions. The resultant relaxation rates indicate an essential qualitative difference depending on the integrability of the pair force at large distances: for gamma > d - 1, the rate diverges in the large particle number N (mean-field) limit, unless a sufficiently large soft core is present; for gamma < d - 1, on the other hand, the rate vanishes in the same limit even in the absence of any regularization. In the second part of the paper we compare our analytical predictions with the results of extensive parallel numerical simulations in d = 3 performed with an appropriate modification of the GADGET code, for a range of different exponents gamma and soft cores leading to the formation of QSS. We find, just as for the previously well studied case of gravity (which we also revisit), excellent agreement between the parametric dependence of the observed relaxation times and our analytic predictions. Further, as in the case of gravity, we find that the results indicate that, when large impact factors dominate, the appropriate cutoff is the size of the system (rather than, for example, the mean interparticle distance). Our results provide strong evidence that the existence of QSS is robust only for long-range interactions with a large distance behavior gamma < d - 1; for gamma >= d - 1, the existence of such states will be conditioned strongly on the short-range properties of the interaction.