  Understanding the nucleon spin structure in the regime where the strong
interaction becomes truly strong poses a challenge to both experiment and
theory. At energy scales below the nucleon mass of about 1 GeV, the intense
interaction among the quarks and gluons inside the nucleon makes them highly
correlated. Their coherent behaviour causes the emergence of effective degrees
of freedom, requiring the application of non-perturbative techniques, such as
chiral effective field theory. Here, we present measurements of the neutron's
generalized spin-polarizabilities that quantify the neutron's spin precession
under electromagnetic fields at very low energy-momentum transfer squared down
to 0.035 GeV$^2$. In this regime, chiral effective field theory calculations
are expected to be applicable. Our data, however, show a strong discrepancy
with these predictions, presenting a challenge to the current description of
the neutron's spin properties.
