  Many-body physics describes phenomena which cannot be understood looking at a
systems' constituents alone. Striking manifestations are broken symmetry, phase
transitions, and collective excitations. Understanding how such collective
behaviour emerges when assembling a system from individual particles has been a
vision in atomic, nuclear, and solid-state physics for decades. Here, we
observe the few-body precursor of a quantum phase transition from a normal to a
superfluid phase. The transition is signalled by the softening of the mode
associated with amplitude vibrations of the order parameter, commonly referred
to as a Higgs mode. We achieve exquisite control over ultracold fermions
confined to two-dimensional harmonic potentials and prepare closed-shell
configurations of 2, 6 and 12 fermionic atoms in the ground state with high
fidelity. Spectroscopy is then performed on our mesoscopic system while tuning
the pair energy from zero to being larger than the shell spacing. Using full
atom counting statistics, we find the lowest resonance to consist of coherently
excited pairs only. The distinct non-monotonic interaction dependence of this
many-body excitation as well as comparison with numerical calculations allows
us to identify it as the precursor of the Higgs mode. Our atomic simulator
opens new pathways to systematically unravel the emergence of collective
phenomena and the thermodynamic limit particle by particle.
