2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18743We study the symmetry classes for the random Dirac fermions in 2 dimensions. We consider $N_f$ species of fermions, coupled by different types of disorder. We analyse the renormalisation group flow at the order of one loop. At $N_f$ large, the disorder distribution flows to an isotropic distribution and the effective action is a sigma model.12 pages, contribution to the proceedings of Advanced NATO Workshop on Statistical Field Theories, Como, June 18-23, 2001Mesoscale and Nanoscale PhysicsDisordered Systems and Neural Networkstext