  Electron-electron interaction is fundamental in condensed matter physics and
can lead to composite quasiparticles called plasmarons, which strongly
renormalize the dispersion and carry information of electron-electron coupling
strength as defined by the effective fine structure constant $\alpha_{ee}^*$.
Although h-BN with unique dielectric properties has been widely used as an
important substrate for graphene, so far there is no experimental report of
plasmarons in graphene/h-BN yet. Here, we report direct experimental
observation of plasmaron dispersion in graphene/h-BN heterostructures through
angle-resolved photoemission spectroscopy (ARPES) measurements upon {\it in
situ} electron doping. Characteristic diamond-shaped dispersion is observed
near the Dirac cone in both 0$^\circ$ (aligned) and 13.5$^\circ$ (twisted)
graphene/h-BN, and the electron-electron interaction strength $\alpha_{ee}^*$
is extracted to be $\alpha_{ee}^*\approx0.9\pm 0.1$, highlighting the important
role of electron-electron interaction. Our results suggest graphene/h-BN as an
ideal platform for investigating strong electron-electron interaction with weak
dielectric screening, and lays fundamental physics for gate-tunable
nano-electronics and nano-plasmonics.
