title: Critical assessment of the Boltzmann approach to active systems.
abstract: Generic models of propelled particle systems posit that the emergence of polar order is driven by the competition between local alignment and noise. Although this notion has been confirmed employing the Boltzmann equation, the range of applicability of this equation remains elusive. We introduce a broad class of mesoscopic collision rules and analyze the prerequisites for the emergence of polar order in the framework of kinetic theory. Our findings suggest that a Boltzmann approach is appropriate for weakly aligning systems but is incompatible with experiments on cluster forming systems.
authors.affiliation: Department of Physics, Arnold Sommerfeld Center for Theoretical Physics and Center for NanoScience, Ludwig-Maximilians-Universität München, Theresienstrasse 37, D-80333 Munich, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Thüroff
authors.forename: Christoph A
authors.initials: CA
authors.lastname: Weber
authors.forename: Erwin
authors.initials: E
authors.lastname: Frey