  A first-principles approach for active chiral hard disks is presented, that
explicitly accounts for steric interactions on the two-body level. We derive an
effective one-body equation for the joint probability distribution of positions
and angles of the particles. By projecting on the angular modes, we write a
hierarchy for the lowest hydrodynamic modes, i.e. particle density,
polarization, and nematic tensor. By undimensionalising the equations, we
highlight the assumptions, which - though inherent - are often included
implicit in closing the hierarchy for finally arriving at an effective
field-theoretical equation for the particle density. By considering different
regimes of the P{\'e}clet number, the well-known models in active matter can be
obtained through our conisderation. Explicitly, we derive the phenomenological
Model B and by going to higher orders in the closure scheme, we show that this
first-principles approach results in the recently introduced Active Model B +,
a natural extension of the Model B for active processes. Remarkably, here we
find that chirality can change the sign of the phenomonological activity
parameters.

Statistical Mechanics