  Understanding the nature of charge carriers in doped Mott insulators holds
the key to unravelling puzzling properties of strongly correlated electron
systems, including cuprate superconductors. Several theoretical models
suggested that dopants can be understood as bound states of partons, the
analogues of quarks in high-energy physics. However, direct signatures of
spinon-chargon bound states are lacking, both in experiment and theory. Here we
numerically identify long-lived rotational resonances at low doping, which
directly reveal the microscopic structure of spinon-chargon bound states.
Similar to Regge trajectories reflecting the quark structure of mesons, we
establish a linear dependence of the rotational energy on the super-exchange
coupling. Rotational excitations are strongly suppressed in standard
angle-resolved photo-emission (ARPES) spectra, but we propose a multi-photon
rotational extension of ARPES where they have strong spectral weight. Our
findings suggest that multi-photon spectroscopy experiments should provide new
insights into emergent universal features of strongly correlated electron
systems.
