  Organic polaritons resulting from the strong hybridisation between photons
and matter excitations have arisen as a suitable platform to device
light-matter technological interfaces at room temperature. Despite their
inherent complexity, organic polaritons are commonly regarded as coherent
light-matter excitations that can be described in terms of Landau's
quasiparticle approach. Here, we experimentally unveil the role of incoherent
matter excitations on the polaritons by exploring the relevant energy-momentum
parameter space. We demonstrate the emergence of a well-defined lower polariton
branch and an intriguing fading of the upper polariton at its entrance to a
continuum of matter excitations. This marks the breakdown of the simplistic
quasiparticle picture for this branch and the formation of a more complex
quantum state. Our experimental results are sustained by a general theoretical
framework that allows understanding the light-matter hybridisation beyond the
quasiparticle approach. Our work expands the understanding of organic
polaritons in all their complexity and increases their technological
significance.
