  Understanding the structure of matter or materials and interaction or
correlations among the constituent elementary particles are the central tasks
of all branches of science, from physics, chemistry, to biology. In physics,
this ultimate goal has spurred a constant search for high-order correlated
entities or composite particles for nearly all states and forms of matter, from
elementary particles, nuclei, cold atoms, to condensed matter. So far, such
composite particles involving two or three constituent particles have been
experimentally identified, such as the Cooper pairs, excitons, and trions in
condensed matter physics, or diquarks and mesons in quantum chromodynamics.
Although the four-body irreducible entities have long been predicted
theoretically in a variety of materials systems alternatively as quadruplons,
quadrons, or quartets, the closely related experimental observation so far
seems to be restricted to the field of elementary particles (e.g. the recent
tetraquark at CERN) only. In this article, we present the first experimental
evidence for the existence of a four-body irreducible entity, the quadruplon,
involving two electrons and two holes in a monolayer of Molybdenum Ditelluride.
Using the optical pump-probe technique, we discovered a series of new spectral
features that are distinct from those of trions and bi-excitons. By solving the
four-body Bethe-Salpeter equation in conjunction with the cluster expansion
approach, we are able to explain these spectral features in terms of the
four-body irreducible cluster or the quadruplons. In contrast to a bi-exciton
which consists of two weakly bound excitons, a quadruplon consists of two
electrons and two holes without the presence of an exciton.
