  Ultrafast terahertz spectroscopy accesses the {\em dark} excitonic ground
state in resonantly-excited (6,5) SWNTs via internal, direct dipole-allowed
transitions between lowest lying dark-bright pair state $\sim$6 meV. An
analytical model reproduces the response which enables quantitative analysis of
transient densities of dark excitons and {\em e-h} plasma, oscillator strength,
transition energy renormalization and dynamics. %excitation-induced
renormalization. Non-equilibrium, yet stable, quasi-1D quantum states with dark
excitonic correlations rapidly emerge even with increasing off-resonance
photoexcitation and experience a unique crossover to complex phase-space
filling of %a complex distribution between both dark and bright pair states,
different from dense 2D/3D excitons influenced by the thermalization, cooling
and ionization to free carriers.
