  One-dimensional electronic fluids are peculiar conducting systems, where the
fundamental role of interactions leads to exotic, emergent phenomena, such as
spin-charge (spinon-holon) separation. The distinct low-energy properties of
these 1D metals are successfully described within the theory of linear
Luttinger liquids, but the challenging task of describing their high-energy
nonlinear properties has long remained elusive. Recently, novel theoretical
approaches accounting for nonlinearity have been developed, yet the rich
phenomenology that they predict remains barely explored experimentally. Here,
we probe the nonlinear spectral characteristics of short GaAs quantum wires by
tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We
find evidence for the existence of an inverted (spinon) shadow band in the main
region of the particle sector, one of the central predictions of the new
nonlinear theories. A (holon) band with reduced effective mass is clearly
visible in the particle sector at high energies.
