Universal scaling behavior of directed percolation and the pair contact   process in an external field
  We consider the scaling behavior of directed percolation and of the pair contact process with a conjugated field. In particular we determine numerically the equation of state and show that both models are characterized by the same universal scaling function. Furthermore we derive the equation of state for the pair contact process within a mean-field approach which again agrees with the mean-field equation of state of the directed percolation universality class. 
S. Lubeck, R. D. Willmann