  We present a combined study of the angle-resolved-photoemission spectroscopy
(ARPES) and quantum Monte Carlo simulations to propose a novel polaronic
metallic state in underdoped cuprates. An approximation scheme is proposed to
represent underdoped cuprates away from 1/2 filling, replacing the many-body
Hamiltonian by that of a single polaron with effective electron-phonon
interaction (EPI), that successfully explains many puzzles such as a large
momentum-dependent dichotomy between nodal and anti-nodal directions, and an
unconventional doping dependence of ARPES in the underdoped region.
