  The visible world is founded on the proton, the only composite building block
of matter that is stable in nature. Consequently, understanding the formation
of matter relies on explaining the dynamics and the properties of the proton's
bound state.A fundamental property of the proton involves the response of the
system to an external electromagnetic field. It is characterized by the
electromagnetic polarizabilities that describe how easily the charge and
magnetization distributions inside the system are distorted by the
electromagnetic field. Moreover, the generalized polarizabilities map out the
resulting deformation of the densities in a proton subject to an
electromagnetic field. They disclose essential information about the underlying
system dynamics and provide a key for decoding the proton structure in terms of
the theory of the strong interaction that binds its elementary quark and gluon
constituents. Of particular interest is a puzzle in the electric generalized
polarizability of the proton that remains unresolved for two decades. Here we
report measurements of the proton's electromagnetic generalized
polarizabilities at low four-momentum transfer squared. We show evidence of an
anomaly to the behaviour of the proton's electric generalized polarizability
that contradicts the predictions of nuclear theory and derive its signature in
the spatial distribution of the induced polarization in the proton. The
reported measurements suggest the presence of a new, not-yet-understood
dynamical mechanism in the proton and present notable challenges to the nuclear
theory.
