  Jet substructure observables have significantly extended the search program
for physics beyond the Standard Model at the Large Hadron Collider. The
state-of-the-art tools have been motivated by theoretical calculations, but
there has never been a direct comparison between data and calculations of jet
substructure observables that are accurate beyond leading-logarithm
approximation. Such observables are significant not only for probing the
collinear regime of QCD that is largely unexplored at a hadron collider, but
also for improving the understanding of jet substructure properties that are
used in many studies at the Large Hadron Collider. This Letter documents a
measurement of the first jet substructure quantity at a hadron collider to be
calculated at next-to-next-to-leading-logarithm accuracy. The normalized,
differential cross-section is measured as a function of log$_{10}\rho^2$, where
$\rho$ is the ratio of the soft-drop mass to the ungroomed jet transverse
momentum. This quantity is measured in dijet events from 32.9 fb$^{-1}$ of
$\sqrt{s} = 13$ TeV proton-proton collisions recorded by the ATLAS detector.
The data are unfolded to correct for detector effects and compared to precise
QCD calculations and leading-logarithm particle-level Monte Carlo simulations.
