  We present a theory for the steady-state dynamics of a two-dimensional system
of spherically symmetric active Brownian particles. The derivation of the
theory consists of two steps. First, we integrate out the self-propulsions and
obtain a many-particle evolution equation for the probability distribution of
the particles' positions. Second, we use projection operator technique and a
mode-coupling-like factorization approximation to derive an equation of motion
for the density correlation function. The nonequilibrium character of the
active system manifests itself through the presence of a steady-state
correlation function that quantifies spatial correlations of microscopic
steady-state currents of the particles. This function determines the dependence
of the short-time dynamics on the activity. It also enters into the expression
for the memory matrix and thus influences the long-time glassy dynamics.
