On October 10, 2019, graduate student Raymond Ehlers successfully defended his thesis, “Jet-Hadron Correlations Measured in Pb–Pb Collisions at √(sNN)=5.02 TeV with ALICE” (advisor Helen Caines).
Ehlers explained, “Our goal is better understand the properties of the strong force, one of the four fundamental interactions. This interaction describes how quarks and gluons, which make up protons and neutrons in nuclei, interact with each other. By colliding nuclei at very high energies, we create a new state of matter known as the Quark-Gluon Plasma (QGP) where the strongly interacting quarks and gluons act as if they are nearly free. The universe was last in this state approximately 10 microseconds after the Big Bang, and now we can produce and study it at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Many of the properties of the QGP remain unknown. My thesis aims to better measure (constrain) some of these unknown properties by utilizing the ALICE (A Large Ion Collider Experiment) detector at the LHC - specifically, measuring how much energy a quark or gluon loses as it takes different paths through the QGP.”
Ehlers will continue his research as a postdoctoral associate at Oak Ridge National Laboratory.
Thesis abstract: Quantum Chromodrynamics (QCD) describes the interactions of quarks and gluons. Due to asymptotic freedom, sufficiently high energy density can cause matter to transition to a deconfined state of matter known as the Quark-Gluon Plasma. As partons propagate through this QCD medium, which can be formed in ultra-relativistic heavy-ion collisions, they lose energy and the resulting jets are known to be modified in a phenomena known as jet quenching. This thesis investigates potential path length dependence of jet quenching via the measurement of azimuthal jet-hadron correlations with respect to the event plane orientation in Pb–Pb collisions at √(sNN)=5.02 TeV with the ALICE detector. Such studies also help constrain the large background underlying this measurement. The associated hadron yields and correlation widths associated with the trigger and recoil jets are compared as a function of event plane orientation. They are found to be predominately consistent between the different orientations within uncertainties, although there are suggestions of deviations at low associated particle transverse momentum. Indeed, theoretical predictions suggest that any deviations are expected to be small, which may be due to competing processes associated with jet quenching. I also discuss my contributions to ALICE Overwatch, a project to enable nearly real-time data quality monitoring and assurance using the capabilities of the High Level Trigger.