Theses Doctoral

Probing the Substructure of Large-Radius Jets in Lead-Lead Collisions at the LHC

Zou, Wenkai

Measurements of the jet substructure in Pb+Pb collisions provide information about the mechanism of jet quenching in the hot and dense QCD medium created in these collisions, over a wide range of energy scales. Recent measurements of the nuclear modification factor of large-radius jets in 5.02 TeV Pb+Pb collisions with ATLAS are presented. 2017 pp data of 257 inverse picobarns and 2018 Pb+Pb data of 1.72 inverse nanobarns are used in these measurements. The R=1.0 large-radius jets are reconstructed with the anti-kt algorithm from R=0.2 anti-kt jets. In each large-radius jet, the associated R=0.2 jets, as constituents, are re-clustered using the kt algorithm to obtain the earliest splitting substructure observables, including the splitting scale sqrt(d12) and the angular separation dR.

The nuclear modification factor RAA, which is the jet yield suppression in Pb+Pb compared to pp, is measured as a function of the jet transverse momentum pt and jet substructure observables sqrt(d12) and dR. A significant RAA decrease for jets with one single sub-jet is observed, compared to jets with more complex substructure. These results support the hypothesis that jets with hard internal splittings losemore energy propagating through the medium. Measurements of RAA with dependence on the jet substructure characterized using charged particle fragments are also presented.

Utilizing jets of a consistent definition, the substructure observables, specifically the splitting scale sqrt(d12) and the angular separation dR, are extracted from the kt re-clusterings of charged particles associated to each large-radius jet. The soft-drop grooming algorithm is applied with the kt re-clusterings of the charged particles to mitigate the strong underlying event effects. RAA is found to have a significant decrease with increasing sqrt(d12) and dR at their lower values, suggesting possible color coherence effects influencing the jet energy loss in the medium.


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More About This Work

Academic Units
Thesis Advisors
Cole, Brian A.
Ph.D., Columbia University
Published Here
June 12, 2024