Medium modification of the shape of small-radius jets in central Pb-Pb collisions at √sN-N-=2.76 TeV

Abstract

We present the measurement of a new set of jet shape observables for trackbased jets in central Pb-Pb collisions at √sNN = 2.76 TeV. The set of jet shapes includesthe first radial moment or angularity, g; the momentum dispersion, pTD; and the difference between the leading and sub-leading constituent track transverse momentum, LeSub.These observables provide complementary information on the jet fragmentation and canconstrain different aspects of the theoretical description of jet-medium interactions. The jetshapes were measured for a small resolution parameter R = 0.2 and were fully corrected toparticle level. The observed jet shape modifications indicate that in-medium fragmentationis harder and more collimated than vacuum fragmentation as obtained by PYTHIA calculations, which were validated with the measurements of the jet shapes in proton-protoncollisions at √s = 7 TeV. The comparison of the measured distributions to templates forquark and gluon-initiated jets indicates that in-medium fragmentation resembles that ofquark jets in vacuum. We further argue that the observed modifications are not consistentwith a totally coherent energy loss picture where the jet loses energy as a single colourcharge, suggesting that the medium resolves the jet structure at the angular scales probedby our measurements (R = 0.2). Furthermore, we observe that small-R jets can help toisolate purely energy loss effects from other effects that contribute to the modifications ofthe jet shower in medium such as the correlated background or medium response.