Nonuniversality in the pair contact process with diffusion

We study the static and dynamic behavior of the one dimensional pair contact process with diffusion. Several critical exponents are found to vary with the diffusion rate, while the order-parameter moment ratio m = (rho(2)) over bar/(rho) over bar (2) grows logarithmically with system size. The anomalous behavior of m is traced to a violation of scaling in the order parameter probability density, which in turn reflects the presence of two distinct sectors, one purely diffusive, the other reactive, within the active phase. Studies restricted to the reactive sector yield precise estimates for exponents beta and nu(perpendicular to), and confirm finite size scaling. We also determine the value m(c) = 1.334 for the parity-conserving universality class in one dimension.