Critical behavior of active Brownian particles

We study active Brownian particles as a paradigm for genuine non-equilibrium phase transitions. Access to the critical point in computer simulations is obstructed by the fact that the density is conserved. We propose a modification of sampling finite-size fluctuations and successfully test this method for the 2D Ising model. Using this model allows us to determine accurately the critical point of two dimensional active Brownian particles at Pe_cr=40(2), ϕ_cr=0.597(3). Based on this estimate, we study the corresponding critical exponents β, γ/ν, and ν. Our results are incompatible with the 2D-Ising exponents, thus raising the question whether there exists a corresponding non-equilibrium universality class.