Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Caines, Helen

Abstract

In high-energy particle collisions, when incoming quarks and gluons (partons) exchange a large momentum during scattering, the outgoing partons undergo parton showers by primarily radiating off gluons successively. Hadrons are formed when the parton energies are sufficiently low. The collimated sprays of particles created through this process are called jets. Studies of the internal structure of jets, known as jet substructure, can provide insight into Quantum Chromodynamics (QCD), the theory that governs the interaction between partons and their formation into hadrons. In this thesis, I present two sets of measurements of jet substructure observables that are sensitive to the theoretically challenging non-perturbative effects of jet evolution. The deconfined state of QCD matter, known as the quark-gluon plasma (QGP), can be created in high-energy nuclear collisions. Measurements of jets created in these collisions may shed light on the properties of the QGP and the emergent phenomena of QCD in general. I also present an analysis with jets created in heavy-ion collisions to probe for potential modification of the jet evolution due to interaction with the QGP. The charge correlator ratio observable, $r_c$, which aims to probe hadronization through the charge correlation between the leading and subleading charged particles in jets, is measured in $\sqrt{s}=200$ GeV \pp\ collisions at STAR. The result demonstrates that opposite-sign pairs are more preferentially produced than same-sign pairs, and that $r_c$ is constant with the jet transverse momentum, $p_{\mathrm{T}}$. Predictions from event generators over-estimate such a correlation in the data. Further studies show that resonance decays also affect the values of $r_c$, although hadronization effects still dominate. A background subtraction technique is demonstrated for the ongoing measurement of $r_c$ in $\sqrt{s_{\mathrm{NN}}}=200$ GeV Ru+Ru and Zr+Zr collisions at STAR. The CollinearDrop groomed jet mass, which examines the soft and wide-angle radiation in the parton shower, is measured together with six other substructure observables in \pp\ collisions. The observables are then corrected for detector effects using a novel machine learning driven technique called MultiFold, which preserves the multidimensional correlation among the observables. The MultiFold technique is validated with closure tests and by showing consistency to previous one-dimensional results, such as the jet mass, that are obtained with traditional correction techniques. The CollinearDrop groomed jet mass is then studied as a function of the SoftDrop groomed radius and momentum imbalance, revealing a strong correlation between the amount of early-stage radiation and the later-stage parton shower kinematics. The measurement also demonstrates the unique versatility of MultiFold, which allows for a fully corrected measurement of an observable with selections in several others.

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