Experimental Studies of the Properties of the QGP and the QCD Phase Diagram

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Quantum Chromodynamics (QCD) is the theoretical framework that describes the strong nuclear force. The fundamental particles that interact via this force are the quarks and gluons that build up protons and neutrons, which in turn build up the atomic nuclei at the cores of all atoms. This project aims to study the nature of matter at the very high temperatures reached in relativistic heavy ion collisions. The group will study how protons and neutrons melt into constituent quarks and gluons, which are no longer bound under these conditions. This high-temperature phase of nuclear matter is called the Quark-Gluon Plasma (QGP). Heavy-ion experiments at the Relativistic Heavy-Ion Collider and the Large Hadron Collider have produced striking evidence for QGP formation. The field is moving towards a quantitative understanding of the thermodynamic properties of the QGP, and this work will contribute to that understanding. This project focuses on the experimental measurement of two distinct thermodynamic properties of this novel state of nuclear matter with the STAR and the CMS experiments. The first property to be measured is the temperature produced in the hottest stages of the collision. This is done via the study of Upsilon mesons, a bound state of heavy "bottom" quarks. In the hot, early stage of the collision even heavy-quark bound states, like the Upsilon, are expected to melt in the collision. The group will study the melting pattern of Upsilons and their excited states with STAR and with CMS in pp, pA, and AA collisions. This measurement will provide crucial information to test models of the deconfinement of quarks in the high-temperature QGP. The second goal is to study the thermodynamic phase structure of the strong force. In particular, testing the existence of a predicted QCD critical point is a key component of this goal. This study is done with STAR by varying the collision energy both in collider and fixed-target collisions, and thereby exploring the transition between normal nuclear matter and QGP matter. Adding the fixed target energy scan is important because the current evidence suggests that the critical point may lie at the lowest energy accessible to the collider, and it is important to scan both above and below this transition. 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 at the conference and support of the UC Davis SACNAS chapter. 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 graduate students to become leaders in these activities, since many are women and underrepresented minorities, and are therefore excellent role models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.