Kardar-Parisi-Zhang universality class — nonlinear stochastic growth with exact exponents χ=1/2, β=1/3, z=3/2
The KPZ equation ∂h/∂t = ν∇²h + (λ/2)(∇h)² + η describes interface growth. The nonlinear term breaks up-down symmetry and places the system in the KPZ universality class with exact exponents β=1/3, χ=1/2, z=3/2 in 1+1D.