Date of Award

Spring 2022

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Caines, Helen


At sufficient energy densities, ultra-relativistic heavy ioncollisions produce a quark gluon plasma (QGP), in which quarks and gluons are deconfined into an extended medium. High energy partons in the collision scatter at short time scales, may afterward interact with the QGP media, and ultimately hadronize into a collimated spray of particles. Experimentally, these particles are algorithmically clustered into jets, which are used as proxies for the initiating partons and therefore as probes of the QGP's properties. This thesis presents jet measurements from $\sqrt{s_\mathrm{NN}}$=200~GeV $p$+Au collisionsrecorded by STAR at RHIC in 2015. These are the first reported semi-inclusive jet results for small system collisions (\mbox{$p$/$d$/He+A} or ``\mbox{$s$+A}'') at RHIC kinematics and are particularly timely because of an ongoing revolution in the field's perception of, and use for, small system collisions. Originally \mbox{$s$+A} collisions were of principal interest to serve as a QGP-free benchmark of cold nuclear effects, which was used to compare\mbox{A+A} collisions with \pp collisions to quantify actual QGP effects. That paradigm began to shift with the discovery that most signals attributed to QGP formation are present, to some degree, in \mbox{$s$+A} collisions; however, as an exception, no jet quenching has been observed to date. In 2015, the ATLAS and PHENIX collaborations reported event activity (EA)dependent modification of jet spectra in \mbox{$p$+Au} and \mbox{$p$+Pb} collisions, a possible jet quenching signal. Intriguingly, the ATLAS jet modification, in the $p$ going direction, appears to scale with the ``Bjorken-$x_\mathrm{p}$'' of the jets ($\approx 2 E_\mathrm{jet} \sqrt{s}$) for $x_\mathrm{p}\gtrsim0.1$; at the same time, measurements of lower $x_\mathrm{p}$ jets from collisions at the same energy by the ALICE experiment found no EA jet spectra modification. Jets at kinematics up to $x_\mathrm{p}\approx0.5$ are reported in this thesis throughtheir $p_\mathrm{T}$ spectra per trigger ($S$), azimuthal distribution per trigger ($A$), and dijet $p_\mathrm{T}$ balance ($A_\mathrm{J}$). There is significant EA dependent modification of $S$, and some of $A$. This modification is demonstrated by this thesis to result primarily, perhaps completely, from observed anti-correlations between EA and the energy of the trigger particle ($E_\mathrm{T}^\mathrm{trig}$). These EA-to-$E_\mathrm{T}^\mathrm{trig}$ anti-correlations are also presented. The $A_\mathrm{J}$ distribution is independent of EA, further supporting the conclusion that the EA dependence of the jet $p_\mathrm{T}$ spectra is a constraint imposed by the initial conditions of the $x_\mathrm{p}$ hard scattering and not a result of subsequent jet-QGP interactions. As a result, EA-to-$E_\mathrm{T}^\mathrm{trig}$ correlations are presented as an opportunity to further probe the initial stage conditions of high energy ion collisions.