  Harnessing electronic excitations involving coherent coupling to bosonic
modes is essential for the design and control of emergent phenomena in quantum
materials [1]. In situations where charge carriers induce a lattice distortion
due to the electron-phonon interaction, the conducting states get "dressed".
This leads to the formation of polaronic quasiparticles that dramatically
impact charge transport, surface reactivity, thermoelectric and optical
properties, as observed in a variety of crystals and interfaces composed of
polar materials [2-6]. Similarly, when oscillations of the charge density
couple to conduction electrons the more elusive plasmon polaron emerges [7],
which has been detected in electron-doped semiconductors [8-10]. However, the
exploration of polaronic effects on low energy excitations is still in its
infancy in two-dimensional (2D) materials. Here, we present the discovery of an
interlayer plasmon polaron in heterostructures composed of graphene on top of
SL WS$_2$. By using micro-focused angle-resolved photoemission spectroscopy
(microARPES) during in situ doping of the top graphene layer, we observe a
strong quasiparticle peak accompanied by several carrier density-dependent
shake-off replicas around the SL WS$_2$ conduction band minimum (CBM). Our
results are explained by an effective many-body model in terms of a coupling
between SL WS$_2$ conduction electrons and graphene plasmon modes. It is
important to take into account the presence of such interlayer collective
modes, as they have profound consequences for the electronic and optical
properties of heterostructures that are routinely explored in many device
architectures involving 2D transition metal dichalcogenides (TMDs) [11-15].
