Novel critical phenomena in compressible polar active fluids: A functional renormalization group approach

Renormalization group (RG) methods provide physicists a systematic way to categorize macroscopic behaviour of a dynamical system, be it equilibrium or nonequilibrium, into distinct universality classes (UCs). For instance, dynamic RG analyses together with the perturbative epsilon-expansion method have uncovered diverse novel nonequilibrium UCs in dry polar active fluids (PAF), albeit only in the incompressible and infinitely compressible (or Malthusian) limits. Indeed, the study of compressible PAF has been hampered by technical difficulties in the applications of perturbative RG methods. Here, we use functional RG methods to bypass these difficulties and unveil for the first time novel critical behavior in compressible PAF. Specifically, we focus on the multicritical point that emerges when the critical order-disorder transition coincides with critical motility-induced phase separation, and uncover three novel UCs and their associated scaling behavior near the upper critical dimension d_c = 6.