  We present the measurement of a new set of jet shape observables for
track-based jets in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$
TeV. The set of jet shapes includes the first radial moment or angularity, $g$;
the momentum dispersion, $p_{\rm T}D$; and the difference between the leading
and sub-leading constituent track transverse momentum, $LeSub$. These
observables provide complementary information on the jet fragmentation and can
constrain different aspects of the theoretical description of jet-medium
interactions. The jet shapes were measured for a small resolution parameter $R
= 0.2$ and were fully corrected to particle level. The observed jet shape
modifications indicate that in-medium fragmentation is harder and more
collimated than vacuum fragmentation as obtained by PYTHIA calculations, which
were validated with the measurements of the jet shapes in proton-proton
collisions at $\sqrt{s} = 7$ TeV. The comparison of the measured distributions
to templates for quark and gluon-initiated jets indicates that in-medium
fragmentation resembles that of quark jets in vacuum. We further argue that the
observed modifications are not consistent with a totally coherent energy loss
picture where the jet loses energy as a single colour charge, suggesting that
the medium resolves the jet structure at the angular scales probed by our
measurements ($R=0.2$). Furthermore, we observe that small-$R$ jets can help to
isolate purely energy loss effects from other effects that contribute to the
modifications of the jet shower in medium such as the correlated background or
medium response.
