Study of universality crossover in the contact process

We consider a generalization of the contact process stochastic model, including an additional autocatalitic process. The phase diagram of this model in the proper two-parameter space displays a line of transitions between an active and an absorbing phase which starts at the critical point of the contact process and ends at the transition point of the voter model. Thus, a crossover between the directed percolation and the compact percolation universality classes is observed at this latter point. We study this crossover by a variety of techniques. Using supercritical series expansions analyzed with partial differential approximants, we obtain precise estimates of the crossover behavior of the model. In particular, we find an estimate for the crossover exponent $\phi=2.00 \pm 0.02$. We also show arguments that support the conjecture $\phi=2$.