  We present a rigorous photonic mode model to describe the strong coupling
between a monolayer of $\rm MoSe_2$ and a single gold nanoparticle. The onset
of strong coupling is quantified by computing the three-dimensional hybrid
quasinormal modes of the combined structure, allowing one to accurately model
light-matter interactions without invoking the usual phenomenological theories
of strong coupling. We explore the hybrid quasinormal modes as a function of
gap size and temperature and find spectral splittings in the range of around
80-110 meV, with no fitting parameters for the material models. We also show
how the hybrid modes exhibit Fano-like resonances and quantify the complex
poles of the hybrid modes as well as the Purcell factor resonances from
embedded dipole emitters. These effects cannot be described with the usual
heuristic normal mode theories.
