Jets as Probes of Ultra-Relativistic Heavy-Ion Collisions

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For several microseconds after the big bang the entire universe passed through a phase called the Quark-Gluon Plasma (QGP). More than a million times hotter than the sun, the QGP is a form of matter governed by quantum chromodynamics (QCD) where the individual quarks and gluons localized within nucleons are expected to scatter out over distances and times many times larger than a femtometer (fm). Only in the last ten years has it become possible to create and study this form of matter in the laboratory using the fleetingly short-lived nuclear sized droplets of plasma created in the aftermath of a heavy-ion collision. The very short lifetime (10 fm/c, where c is the speed of light) and small size (10 fm) have made the study of this matter a theoretical and experimental challenge. The modification of hard QCD jets (bundles of strongly interacting particles) as they pass through this exotic form of matter presents the best tool to date to study the various properties of the QGP. The goal of the research supported by this grant is to continue to formulate both a systematic and sophisticated approach to the theoretical study of these modified jets. Considerable effort will be directed towards a deeper understanding of the factorization of the hard jet cascade from the soft rescattering experienced by the patrons in the jet, as they propagate through the QGP. Yet another goal will be to numerically simulate the partonic cascade process within the dense medium to better understand the event-by-event characteristics of such modified showers. A third goal will be a rigorous analysis of the data within the framework of perturbative QCD based jet modification to isolate the transport properties of the plasma and place stringent error bars on the extracted transport coefficients of jets in a QGP.This research, at the intersection of particle and nuclear physics, will greatly enhance our understanding of perturbative and non-perturbative QCD in the complicated environments of a dynamically expanding quark gluon plasma. It will introduce state of the art numerical tools to simulate such processes. It will lead to the training of graduate students in both the sophisticated analytical approaches used as well as the numerical simulation tools used in such efforts. The project also includes an outreach effort to convey the latest results from such analyses as well as from other new cutting edge fields to the general public.