Abstract
This article presents groomed jet substructure measurements in pp and Pb-Pb collisions at sqrt[s_{NN}]=5.02 TeV with the ALICE detector. The soft drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, z_{g}, and the (scaled) groomed jet radius, θ_{g}. Charged-particle jets are reconstructed at midrapidity using the anti-k_{T} algorithm with resolution parameters R=0.2 and R=0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the θ_{g} distribution in Pb-Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark-gluon plasma. No significant modification of the z_{g} distribution in Pb-Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark-gluon plasma.
Highlights
Introduction.—Ultrarelativistic heavy-ion collisions at the Large Hadron Collider (LHC) are used to study the high temperature deconfined phase of strongly interacting matter known as the quark-gluon plasma (QGP) [1–5]
Access to the hard splittings isolates substructures that are wellcontrolled in perturbative QCD (PQCD), which in heavy-ion collisions may help constrain various jet quenching effects such as energy loss, transverse-momentum broadening, and color coherence
The soft drop (SD) [19–21] grooming algorithm identifies a single splitting by first reconstructing a jet with the anti-kT algorithm and reclustering the constituents of the jet using the Cambridge-Aachen (CA) algorithm [25] in order to follow the angular ordering of the QCD parton shower
Summary
Introduction.—Ultrarelativistic heavy-ion collisions at the Large Hadron Collider (LHC) are used to study the high temperature deconfined phase of strongly interacting matter known as the quark-gluon plasma (QGP) [1–5]. Access to the hard splittings isolates substructures that are wellcontrolled in perturbative QCD (PQCD), which in heavy-ion collisions may help constrain various jet quenching effects such as energy loss, transverse-momentum broadening, and color coherence. The rate of prong mistagging from residual background effects was evaluated by embedding jets simulated with the PYTHIA8 event generator [57] into measured Pb-Pb data and following the procedure in Ref.
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