  One-dimensional (1D) magnetic insulators have attracted significant interest
as a platform for studying emergent phenomena such as quasiparticle
fractionalization and quantum criticality. The antiferromagnetic Heisenberg
chain of spins-1/2 is an important reference system; its elementary excitations
are spin-1/2 quasiparticles called spinons that are always created in pairs.
However, while inelastic neutron scattering (INS) experiments routinely observe
the excitation continuum associated with two-spinon states, the presence of
more complex dynamics associated with four-spinon states has only been inferred
from comparison with theory. Here, we show that resonant inelastic x-ray
scattering (RIXS) is capable of accessing the four-spinon excitations directly,
in a spectroscopic region separated from the two-spinon continuum. Our results
provide the first direct measurement of four-spinon excitations, which is made
possible by the fundamentally different correlation functions probed by RIXS
compared to INS. This advance holds great promise as a tool in the search for
novel quantum states, in particular quantum spin liquids.
