  A two-body interaction or force between quantum particles is ubiquitous in
nature, and the microscopic description in terms of the bare two-body
interaction is the basis for quantitatively describing interacting few- and
many-body systems. Alternatively, the effective description in terms of an
effective two-body interaction successfully captures the essence of the
systems. However, for several important observations, the explanation in terms
of an effective two-body interaction is not satisfactory, and the effective
three-body interaction has played an essential role in understanding the
systems. In this study, we investigate a few-body system comprising of
ultracold bosons tightly confined in a deep optical lattice site, which is
effectively described as zero-dimensional bosons. By combining an
occupancy-resolving high-resolution laser spectroscopy with an inter-orbital
Feshbach resonance controlling the bare two-body interaction over a wide range,
we obtain a clear evidence of an effective four-body force, which has never
been observed in any few-body quantum system so far. This will open the door
for the study of multi-body forces in various few-body systems.
