Rosi Reed

Associate Professor

Lehigh University

Department of Physics

Host: Renee Fatemi

Title: Golden Age of Jet Tomography in Heavy Ion Collisions: The Evolution of Jets as Probes of the Quark Gluon Plasma

Abstract: The strong nuclear force, or quantum chromodynamics (QCD), remains one of the most enigmatic of the four fundamental forces of nature due to its rich structure and many emergent properties. The collision of heavy ions at ultra-relativistic speeds gives rise to a remarkable medium known as the quark gluon plasma (QGP). In this extreme state of QCD matter, protons and neutrons dissolve into partons - quarks and gluons - creating a nearly perfect liquid.  A fundamental question is whether quasi-particles exist in this medium, given the incredibly low viscosity over entropy ratio.  Answering questions regarding the evolution of the QGP structure, and the connection to QCD, requires probes with extremely small spatial resolution. Enter the particle jets created in high momentum transfer parton-parton collisions. These jets, produced early in the heavy-ion collision evolution, traverse the newly formed QGP and lose energy in a process called jet quenching, which modifies the final state properties of the resultant jets.  Understanding the mechanisms of jet quenching will allow the determination of QGP transport coefficients and thus lead to a better understanding of QCD.  The path-length dependence of the jet energy loss is one way to better understand the underlying mechanisms, though it has proven challenging to derive explicit values for the path-length dependence from experimental data.  We can gain critical insights into the nature of the QGP at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), to determine the temperature dependence, and system-size dependence of many jet observables.  In this presentation, I will discuss the latest Solenoidal Tracker at RHIC (STAR) experimental jet results, including a new measurement of the azimuthal asymmetry of jets in Ruthenium and Zirconium collisions, and compare these results with similar results from the LHC experiments, as well as the potential outlook for measurements with the newly built sPHENIX detector.  I will compare these results to recent advances in theory and connect them to other heavy ion jet measurements such as photon jet correlations and measurements of the structure of the jets themselves.