Understanding the strong nuclear force and its implications for the behavior of matter is one of the key questions currently under investigation in nuclear physics. Our current understanding is based on the theoretical framework of Quantum Chromodynamics (QCD), whose fundamental particles are the quarks and gluons that together build up protons and neutrons. One of the problems at the frontier of nuclear science is to understand how protons and neutrons melt into their constituent quarks and gluons, forming a state called the Quark-Gluon Plasma (QGP). This is the goal of relativistic heavy-ion collisions. Tantalizing evidence for the formation of QGP has been produced by the highest energy heavy-ion experiments done at the Relativistic Heavy-Ion Collider and the Large Hadron Collider. We are moving from a discovery phase towards a quantitative understanding of the physical characteristics of the QGP, and this project focuses on the experimental measurement of three distinct properties of this novel state of nuclear matter. The intellectual merit for this award can be summarized by the following three goals. The first is to measure the temperature produced in the hottest stages of the collision via the study of Upsilon mesons. In the early stage of the collision even heavy-quark bound states, like the Upsilon, will melt in the plasma if it is hot enough. We will study Upsilons via their decay into dielectrons and will participate in a detector upgrade to study the di-muon decay channel. The second goal is to study the thermodynamic phase structure of the strong force, testing for the existence of the QCD critical point. This is done by varying the collision energy and thereby exploring the transition between normal nuclear matter and QGP matter. The third goal is to measure the neutral electroweak boson, the Z0, for the first time in a relativistic heavy ion collision. This particle promises to be the cleanest reference of the initial quark content of the projectile nuclei in heavy-ion collisions because, when studying it in the di-muon decay channel, it will not be affected by the QGP. It can therefore act as the best standard candle against which to gauge other measurements of the medium. In particular, a joint study of Z0 + jet (the latter a bundle of strongly interacting particles) production gives us a research channel with an experimental tag that is undisturbed - the Z0 - together with an observable that can probe the density of the medium - the accompanying jet.
The broader impacts of this work are aimed at underrepresented minorities and women. The first is to communicate the excitement of high-energy nuclear physics to Latino students at the Society for the Advancement of Chicanos and Native Americans in Science (SACNAS), through mentoring activities, presentation selection and judging, and providing positive feedback. The second component is promotion of science and physics to elementary and high-school students through visits to elementary schools with large Latino populations, and via participation in the Adopt-A-Physicist program for high schools. Finally, the PIs in the project will mentor undergraduates, including NSF REU students, every year. A key part of these goals is the mentoring of our own graduate student to become leaders in these activities, since many are women and underrepresented minorities, and are therefore excellent role models.
