Medium modification of the shape of small-radius jets in central Pb-Pb collisions at root s(NN)=2:76 TeV

We present the measurement of a new set of jet shape observables for trackbased jets in central Pb-Pb collisions at p sNN = 2 : 76TeV. The set of jet shapes includes the fi rst radial moment or angularity, g; the momentum dispersion, pTD; and the di ff erence between the leading and sub-leading constituent track transverse momentum, LeSub. These observables provide complementary information on the jet fragmentation and can constrain di ff erent 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 modi fi cations 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 p s = 7TeV. 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 modi fi cations 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 e ff ects from other e ff ects that contribute to the modi fi cations of the jet shower in medium such as the correlated background or medium response.