Spacetime evolution of jets with the STAR experiment

Most of the stable matter in the universe is made of protons and neutrons, bound in atomic nuclei. Protons and neutrons, or hadrons, are themselves composite objects made of quarks and gluons, or partons, which are, as far as we know, the fundamental particles of the strong nuclear force. Quantum Chromodynamics (QCD), the field theory of the strong force, confines partons inside hadrons under ``normal'' circumstances. However, when matter is heated to extremely high temperatures or compressed to extremely high densities, QCD predicts a transition from hadronic matter into a state of deconfined quarks and gluons, the Quark-Gluon Plasma (QGP). The QGP is created in the laboratory by colliding nuclei at nearly the speed of light. In the earliest stage of these collisions, before the QGP forms, partons scatter with large momentum transfers. In vacuum, the scattered partons radiate and fragment into collimated sprays of hadrons called jets. In heavy-ion collisions, they must first traverse the QGP and lose energy through collisional and radiative interactions with it, which modifies the resulting jets compared to proton-proton ($p+p$) collisions. This phenomenon, jet quenching, is a primary probe of the QGP. This thesis presents two jet measurements with the STAR experiment at RHIC. The first measures the distributions of charged hadrons around jets in 20--50% central Au+Au collisions at sqrtsₘathrmNN = 200~GeV, separately for jets oriented in, between and out of the event plane, to test whether jet quenching depends on the path length through the QGP. Within the experimental uncertainties, the associated yields and the widths of the jet peaks show no dependence on the event plane. The second characterizes the angular spread of energy inside jets with generalized angularities, corrected for detector effects with unbinned, machine-learning based unfolding. In Au+Au collisions, the angularities of jets with hard constituents are consistent between central and peripheral collisions, in the first heavy-ion application of this unfolding technique. In $p+p$ collisions at sqrts = 200~GeV, the inclusive and SoftDrop-groomed angularities of inclusive jets are measured as a baseline for future heavy-ion measurements. The jets become narrower with increasing jet momentum and with grooming, and the event generators PYTHIA--8 and HERWIG--7 predict narrower jets than measured. Measuring the mean angularities as functions of the groomed jet radius, which the unbinned unfolding makes possible, shows that these generators produce too little radiation at a given splitting angle.