  In a magnetic field bilayer graphene supports, at the lowest Landau level,
eight characteristic zero-energy levels with an extra twofold degeneracy in
Landau orbitals $n=\{0,1\}$. They, under general one-body and many-body
interactions, evolve into pseudo-zero-mode (PZM) levels. A close look is made
into the detailed structure and characteristics of such PZM levels and
cyclotron resonance they host, with full account taken of spin splitting,
interlayer bias, weak electron-hole asymmetry and Coulomb interactions. It is
pointed out that the PZM levels generally undergo orbital level mixing (in one
valley or both valleys) as they are gradually filled with electrons and that an
observation of interband cyclotron resonance over a finite range of filling
provides a direct and sensitive probe for exploring many-body effects and
orbital mixing in bilayer graphene.
