  We study the infrared cyclotron resonance of high mobility monolayer graphene
encapsulated in hexagonal boron nitride, and simultaneously observe several
narrow resonance lines due to interband Landau level transitions. By holding
the magnetic field strength, $B$, constant while tuning the carrier density,
$n$, we find the transition energies show a pronounced non-monotonic dependence
on the Landau level filling factor, $\nu\propto n/B$. This constitutes direct
evidence that electron-electron interactions contribute to the Landau level
transition energies in graphene, beyond the single-particle picture.
Additionally, a splitting occurs in transitions to or from the lowest Landau
level, which is interpreted as a Dirac mass arising from coupling of the
graphene and boron nitride lattices.
