  The strong interaction is not well understood at low energy, or for
interactions with low momentum transfer $Q^2$, but one of the clearest insights
we have comes from Chiral Perturbation Theory ($\chi$PT). This effective
treatment gives testable predictions for the nucleonic generalized
polarizabilities -- fundamental quantities describing the nucleon's response to
an external field. We have measured the proton's generalized spin
polarizabilities in the region where $\chi$PT is expected to be valid. Our
results include the first ever data for the transverse-longitudinal spin
polarizability $\delta_{LT}$, and also extend the coverage of the
polarizability $\bar{d_2}$ to very low $Q^2$ for the first time. These results
were extracted from moments of the structure function $g_2$, a quantity which
characterizes the internal spin structure of the proton. Our experiment ran at
Jefferson Lab using a polarized electron beam and a polarized solid ammonia
(NH$_3$) target. The $\delta_{LT}$ polarizability has remained a challenging
quantity for $\chi$PT to reproduce, despite its reduced sensitivity to higher
resonance contributions; recent competing calculations still disagree with each
other and also diverge from the measured neutron data at very low $Q^2$. Our
proton results provide discriminating power between existing calculations, and
will help provide a better understanding of this strong QCD regime.
