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 ${\text{Pe}}_{\text{cr}}=40(2)$, ${\varphi }_{\text{cr}}=0.597(3)$. Based on this estimate, we study the corresponding critical exponents $\beta$, $\gamma \mathrm{/}\nu$, and $\nu$. Our results are incompatible with the 2D-Ising exponents, thus raising the question whether there exists a corresponding non-equilibrium universality class.