Renormalization Group Flow in 2D
Wilson's RG: integrating out short-wavelength fluctuations generates flow in coupling space. Fixed points govern universality classes; relevant operators drive flow away, irrelevant ones decay.
The Wilson-Fisher fixed point (ε expansion) controls the Ising universality class. Relevant perturbations (mass term r) drive the system toward ordered or disordered phases. The u=0 line is a line of Gaussian fixed points. Flow lines show RG trajectories under repeated coarse-graining.