Abstract
Recent results from jet shower-shape and substructure analyses done by the ALICE collaboration in central Pb–Pb collisions at √sNN = 2.76 TeV and in pp collisions at √s = 7 TeV are reviewed. The presented jet shower- shape observables are angularity g and transverse momentum dispersion pTD, which were studied for a small resolution parameter R = 0.2, track-based jets with a minimum constituent transverse momentum (pT) cut-off of 0.15 GeV/c. Jet substructure is explored for track-based anti-kT jets with R = 0.4 by means of iterative declustering and grooming techniques, which were used to measure the absolutely-normalized leading subjet momentum fraction zg and the number of hard splittings in the reclustered jet shower, nSD. These observables provide complementary information on the jet fragmentation and help to discriminate between different scenarios for medium-induced modifications of the parton shower in heavy-ion collisions due to jet quenching.
Highlights
Collisions of ultra-relativistic heavy ions are used to probe strongly interacting matter in the regime of high-energy densities and temperatures
The jet-medium interaction leads to jet substructure modifications which can be quantified by different jet-shape observables or identified as changes in shower evolution when unwinding jet clusterization history
80–120 GeV/c track-based jets in Pb–Pb collisions at measured data are compared to PYTHIA prediction which was smeared by instrumental effects and embedded into central Pb–Pb collisions and smeared predictions of JEWEL [10] and the Hybrid model [11]
Summary
Collisions of ultra-relativistic heavy ions are used to probe strongly interacting matter in the regime of high-energy densities and temperatures. The jet-medium interaction leads to jet substructure modifications which can be quantified by different jet-shape observables or identified as changes in shower evolution when unwinding jet clusterization history. Jets with small angularity are more collimated and in the limit of pencil-like jets, the angularity tends to zero Another jet-shape observable, called transverse momentum dispersion, quantifies the hardness of the jet fragmentation and it is related to dispersion of the constituent momenta [4], pTD =. In heavy-ion collisions, jets are usually reconstructed using the anti-kT jet algorithm [5] This algorithm has reduced susceptibility with respect to the soft underlying event but at the same time, it does not reflect important characteristics of a partonic shower, e.g., the angular ordering. The second observable counts the number of splittings that fulfill z > 0.1 when we follow the hardest branch and it will be denoted nSD
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