Photoemission experiments in high $T_c$ cuprates indicate that quasiparticles are heavily
'dressed' in the normal state, particularly in the low doping regime. Furthermore these
experiments show that a gradual undressing occurs both in the normal state as the system is
doped and the carrier concentration increases, as well as at fixed carrier concentration as the
temperature is lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be simply understood
with the single assumption that the quasiparticle dressing is a function of the local carrier
concentration. Microscopic Hamiltonians describing this physics are discussed. The undressing
process manifests itself in both the one-particle and two-particle Green's functions, hence leads
to observable consequences in photoemission and optical experiments respectively. An essential
consequence of this phenomenology is that the microscopic Hamiltonians describing it break
electron-hole symmetry: these Hamiltonians predict that superconductivity will only occur for
carriers with hole-like character, as proposed in the theory of hole superconductivity.
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