  Higgs and Goldstone modes are possible collective modes of an order parameter
upon spontaneously breaking a continuous symmetry. Whereas the low-energy
Goldstone (phase) mode is always stable, additional symmetries are required to
prevent the Higgs (amplitude) mode from rapidly decaying into low-energy
excitations. In high-energy physics, where the Higgs boson has been found after
a decades-long search, the stability is ensured by Lorentz invariance. In the
realm of condensed--matter physics, particle--hole symmetry can play this role
and a Higgs mode has been observed in weakly-interacting superconductors.
However, whether the Higgs mode is also stable for strongly-correlated
superconductors in which particle--hole symmetry is not precisely fulfilled or
whether this mode becomes overdamped has been subject of numerous discussions.
Experimental evidence is still lacking, in particular owing to the difficulty
to excite the Higgs mode directly. Here, we observe the Higgs mode in a
strongly-interacting superfluid Fermi gas. By inducing a periodic modulation of
the amplitude of the superconducting order parameter $\Delta$, we observe an
excitation resonance at frequency $2\Delta/h$. For strong coupling, the peak
width broadens and eventually the mode disappears when the Cooper pairs turn
into tightly bound dimers signalling the eventual instability of the Higgs
mode.
