  The question of whether or not passive sub-wavelength cavities can alter the
properties of quantum materials is currently attracting a great deal of
attention. In this Letter we show that the Fermi liquid parameters of a
two-dimensional metal are modified by cavity polariton modes, and that these
changes can be monitored by measuring a paradigmatic magneto-transport
phenomenon, Shubnikov-de Haas oscillations in a weak perpendicular magnetic
field. This effect is intrinsic, and totally unrelated to disorder. As an
illustrative example, we carry out explicit calculations of the Fermi liquid
parameters of graphene in a planar van der Waals cavity formed by natural
hyperbolic crystals and metal gates.
