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We present the measurement of a new set of jet shape observables fortrack-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 leadingand sub-leading constituent track transverse momentum, $LeSub$. Theseobservables provide complementary information on the jet fragmentation and canconstrain different aspects of the theoretical description of jet-mediuminteractions. The jet shapes were measured for a small resolution parameter $R= 0.2$ and were fully corrected to particle level. The observed jet shapemodifications indicate that in-medium fragmentation is harder and morecollimated than vacuum fragmentation as obtained by PYTHIA calculations, whichwere validated with the measurements of the jet shapes in proton-protoncollisions at $\sqrt{s} = 7$ TeV. The comparison of the measured distributionsto templates for quark and gluon-initiated jets indicates that in-mediumfragmentation resembles that of quark jets in vacuum. We further argue that theobserved modifications are not consistent with a totally coherent energy losspicture where the jet loses energy as a single colour charge, suggesting thatthe medium resolves the jet structure at the angular scales probed by ourmeasurements ($R=0.2$). Furthermore, we observe that small-$R$ jets can help toisolate purely energy loss effects from other effects that contribute to themodifications of the jet shower in medium such as the correlated background ormedium response.

Medium modification of the shape of small-radius jets in central Pb-Pb collisions at $\sqrt{s_{\mathrm {NN}}} = 2.76\,\rm{TeV}$