Studies of the Quark-Gluon Plasma with STAR and CMS at UC Davis

University Of California-Davis, Davis CA

Investigators

Abstract

The study of the strong nuclear force under extremely high temperature and energy density is one of the key areas of investigation in nuclear physics. The theoretical framework of the strong force is Quantum Chromodynamics (QCD), whose fundamental particles are the quarks and gluons. These in turn build up protons and neutrons, which are understood to be the low-energy phase of quarks, bound by the strong nuclear force. At the very high temperatures reached in relativistic heavy ion collisions, the aim of this project to study how protons and neutrons melt into constituent quarks and gluons, which are no longer bound. 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. Given this background, the project focuses on the experimental measurement of three distinct thermodynamic properties of this novel state of nuclear matter with the STAR detector at the Relativistic Heavy Ion Collider and the CMS experiment at the Large Hadron Collider at CERN. 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 proton-proton, proton-nucleus, and nucleus-nucleus 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, and thereby exploring the transition between normal nuclear matter and QGP matter. The third goal is to study the density of the medium using events in which a neutral electroweak boson, the Z0, is produced in coincidence with a quark jet. The Z0 is unaffected by the QGP, providing a clean estimate of the transverse momentum of the quark jet. As the partner jet traverses the dense QGP medium and loses energy, these Z0 + quark jet events are one of the best ways to study the energy loss of quarks. 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 our own graduate students to become leaders in these activities, since many are women and underrepresented minorities, and are therefore excellent role models.