Seminarium Fizyki Materii Skondensowanej

I will present some exact results on the behavior of Branching andAnnihilating Random Walks, which are some simple out of equilibriumclassical systems which can suffer continuous phase transitions.Contrary to usual perturbation theory, we perform an expansion in thebranching rate around the non trivial Pure Annihilation model, whosecorrelation and response function we have computed exactly. With this,the non-universal threshold value for having a phase transition in thesimplest such system (belonging to the Directed Percolationuniversality class) is found to coincide with previous NonPerturbative Renormalization Group approximate results. We also showthat the Parity Conserving universality class has an unexpected RG fixed point structure, with a PA fixed point which is unstable inall dimensions of physical interest.