Access Type

Open Access Dissertation

Date of Award

January 2022

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Joern H. Putschke

Abstract

Observing a modification of high-energy probes in the hot nuclear environment created by ultrarelativistic heavy-ion collisions is critical for understanding the microscopic structure of the QCD medium. However, before a modification can be observed, the probes must be calibrated. For this reason, measurements of jet observables in vacuum (pp collisions) and in the presence of cold nuclear matter (pAu collisions) are presented, which will allow for future comparison of jet substructure measurements in AuAu to these baseline measurements. \parData from the STAR detector on the Relativistic Heavy Ion Collider at Brookhaven National Laboratory are used for both analyses. The pp measurement uses data taken in 2012 with protons colliding with center-of-mass energy of $\sqrt{s} = \unit[200]{GeV}$, while the pAu measurement uses data taken in 2015 at the same energy per nucleon pair. Data are fully corrected for detector effects using a two-dimensional Bayesian unfolding procedure, including a reweighting of the detector response matrix in the pAu analysis to account for the additional underlying event in the simulated event embedding. \par The observables presented in the pp analysis are the jet mass, defined as the magnitude of the four-momentum sum of jet constituents, and the SoftDrop groomed jet mass. The SoftDrop grooming algorithm is known to reduce the effect of non-perturbative radiation on jet substructure. The pp results are presented differentially in jet $p_{\mathrm{T}}$ and anti-$k_{\mathrm{T}}$ jet resolution parameter, $R$, to investigate the dependence of jet mass on the momentum and angular scales of the jet shower. Results are compared to a number of leading-order Monte Carlo simulations as well as an NLL calculation by Lee et al. It is found in this first measurement of inclusive jet mass at RHIC that the effect of hadronization is large in this regime and can indeed be reduced significantly with SoftDrop. Tension between some models and the data helped to motivate a now-completed simulation tuning effort at STAR, and it is hoped that the disagreement between the data and the calculation can be used to obtain a non-perturbative shape function for convolution of parton-level calculations. \par In the pAu analysis, a suite of SoftDrop observables are presented, including the SoftDrop subjet shared momentum fraction, $z_{\mathrm{g}}$, groomed jet radius, $R_{\mathrm{g}}$, and groomed jet mass, $M_{\mathrm{g}}$, as well as the inclusive jet mass, $M$. Comparison to the published pp data shows that regardless of event activity, and across a range in $p_{\mathrm{T}}$, the jet substructure is consistent between these two collision systems in this kinematic regime. Additional comparisons to a heavy-ion model with no collective effects are inconclusive due to the need to tune the model to RHIC kinematics. Finally, by comparing low- and high-event-activity classes in the data, it is shown that there are no significant activity-dependent final state effects, which can be seen as evidence against jet quenching in a hot nuclear environment. In general, no cold or hot nuclear matter effects on the jet substructure are observed.

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