Critical behavior of active Brownian particles

We study active Brownian particles as a paradigm for a genuine nonequilibrium phase transition requiring steady driving. Access to the critical point in computer simulations is obstructed by the fact that the density is conserved. We propose a method based on arguments from finite-size scaling to determine critical points and successfully test it for the two-dimensional (2D) Ising model. Using this method allows us to accurately determine the critical point of two-dimensional active Brownian particles at Pe(cr) = 40(2),phi(cr) = 0.597(3). Based on this estimate, we study the corresponding critical exponents beta, gamma/nu, and nu. Our results are incompatible with the 2D Ising exponents, thus raising the question whether there exists a corresponding nonequilibrium universality class.