Nonlinear susceptibilities and the measurement of a cooperative length

We derive the exact beyond-linear fluctuation-dissipation relation, connecting the response of a generic observable to the appropriate correlation functions, for Markov systems. The relation, which takes a similar form for systems governed by a master equation or by a Langevin equation, can be derived to every order in large generality with respect to the considered model in equilibrium and out of equilibrium, as well. On the basis of the fluctuation-dissipation relation, we propose a particular response function, namely the second-order susceptibility of the two-particle correlation function, as an effective quantity to detect and quantify cooperative effects in glasses and disordered systems. We test this idea by numerical simulations of the Edwards-Anderson model in one and two dimensions