The role of symmetry in the short-time critical dymamics

We show that the short-time critical exponent $\theta$ related to the critical initial slip in a stochastic model can be determined by the time correlation of the order parameter. In our procedure it suffices to start with an uncorrelated state with zero order parameter instead of departing, as usually done, from an initial state with a nonzero order parameter. The proof uses the group of symmetry operations related to the Markovian dynamics. Our scheme is extended to cover models with absorbing states.