Singular behavior of fluctuations in a relaxation process

Carrying out explicitly the computation in a paradigmatic model of non-interacting systems, the Gaussian model, we show the existence of a singular point in the probability distribution P(M) of an extensive variable M. Interpreting P(M) as a thermodynamic potential of a dual system obtained from the original one by applying a constraint, we discuss how the non-analytical point of P(M) is the counterpart of a phase-transition in the companion system. We show the generality of such mechanism by considering both the system in equilibrium or in non-equilibrium state following a temperature quench.