Field Theory of Branching and Annihilating Random Walks

We develop a systematic analytic approach to the problem of branching and annihilating random walks, equivalent to the diffusion-limited reaction processes 2A->0 and A->(m+1)A, where m>=1. Starting from the master equation, a field-theoretic representation of the problem is derived, and fluctuation effects are taken into account via diagrammatic and renormalization group methods. For d>2, the mean-field rate equation, which predicts an active phase as soon as the branching process is switched on, applies qualitatively for both even and odd m, but the behavior in lower dimensions is shown to be quite different for these two cases. For even m, and d~2, the active phase still appears immediately, but with non-trivial crossover exponents which we compute in an expansion in eps=2-d, and with logarithmic corrections in d=2. However, there exists a second critical dimension d_c'~4/3 below which a non-trivial inactive phase emerges, with asymptotic behavior characteristic of the pure annihilation process. This is confirmed by an exact calculation in d=1. The subsequent transition to the active phase, which represents a new non-trivial dynamic universality class, is then investigated within a truncated loop expansion. For odd m, we show that the fluctuations of the annihilation process are strong enough to create a non-trivial inactive phase for all d<=2. In this case, the transition to the active phase is in the directed percolation universality class.Comment: 39 pages, LaTex, 10 figures included as eps-files; submitted to J. Stat. Phys; slightly revised versio

Multicritical behavior in coupled directed percolation processes

We study a hierarchy of directed percolation (DP) processes for particle species A, B, ..., unidirectionally coupled via the reactions A -> B, ... When the DP critical points at all levels coincide, multicritical behavior emerges, with density exponents \beta^{(k)} which are markedly reduced at each hierarchy level k >= 2. We compute the fluctuation corrections to \beta^{(2)} to O(\epsilon = 4-d) using field-theoretic renormalization group techniques. Monte Carlo simulations are employed to determine the new exponents in dimensions d <= 3.Comment: 5 pages, RevTex, no figures; final version, to appear in Phys. Rev. Lett. (1998