Global persistence exponent for nonequilibrium critical dynamics

A ''persistence exponent'' theta is defined for nonequilibrium critical phenomena. It describes the probability, p(t) similar to t(-theta), that the global order parameter has not changed sign in the time interval t following a quench to the critical point from a disordered state. This exponent is calculated in mean-field theory, in the n = infinity limit of the O(n) model, to first order in epsilon = 4 - d, and for the 1D Ising model. Numerical results are obtained for the 2D Ising model. We argue that theta is a new independent exponent.