  We reveal the full energy-momentum structure of the pseudogap of underdoped
high-Tc cuprate superconductors. Our combined theoretical and experimental
analysis explains the spectral-weight suppression observed in the B2g Raman
response at finite energies in terms of a pseudogap appearing in the
single-electron excitation spectra above the Fermi level in the nodal direction
of momentum space. This result suggests an s-wave pseudogap (which never closes
in the energy-momentum space), distinct from the d-wave superconducting gap.
Recent tunneling and photoemission experiments on underdoped cuprates also find
a natural explanation within the s-wave-pseudogap scenario.
