CERN Accelerating science

Jet substructure observables are powerful tools to search for new physics and test theoretical descriptions of perturbative and non-perturbative processes in QCD. In heavy-ion collisions, jet substructure observables are used to elucidate the structure and dynamics of the quark-gluon plasma. One substructure observable is jet mass, which probes the virtuality of hard-scattered partons and their modified fragmentation. Additionally, generalized jet angularities allow differential measurements of the jet shower and its modification, as two parameters independently vary the weight of the jet constituents’ relative angle and transverse momentum. Previous measurements of the jet mass and jet angularities show conflicting deviations when compared with models. This thesis presents new measurements of the jet mass and jet angularities to resolve this conflict, using charged-particle tracks in pp and Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV and jet resolution parameters $R = 0.2$ and $0.4$. The results from this work are compared to ALICE measurements of heavy-flavor jets, which provide a high-powered probe of perturbative QCD at low transverse momentum. Jet angularities in jets containing a charm meson are compared to the inclusive measurements of this thesis, revealing a significant narrowing due to the QCD dead cone modifying jet fragmentation. Jet angularity results are also compared to QCD predictions using both folding and shape function corrections for nonperturbative effects. Jet grooming can be used to isolate specific splittings inside the jet fragmentation history, and ALICE measurements of the groomed-jet splitting angle and momentum fraction with soft drop and dynamical grooming are also compared to QCD predictions to test parton branching. The high-precision tracking system of ALICE enables these measurements over a broad range in transverse momentum, with a low-transverse momentum reach that is unique at the LHC.