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

极高温度和能量密度下的强核力研究是核物理学研究的重点领域之一。强作用力的理论框架是量子色动力学（QCD），其基本粒子是夸克和胶子。质子和中子被认为是夸克的低能相，受到强核力的束缚。在相对论重离子碰撞达到的极高温度下，这个项目的目的是研究质子和中子如何融化成不再结合的夸克和胶子。这种核物质的高温阶段被称为夸克-胶子等离子体（QGP）。在相对论重离子对撞机和大型强子对撞机上进行的重离子实验已经为QGP的形成提供了惊人的证据。该领域正朝着定量理解QGP的热力学性质的方向发展。在此背景下，本项目将重点利用相对论重离子对撞机上的STAR探测器和欧洲核子研究中心大型强子对撞机上的CMS实验，对这种新型核物质的三种不同的热力学性质进行实验测量。要测量的第一个性质是碰撞最热阶段产生的温度。这是通过研究厄普西隆介子来完成的，厄普西隆介子是一种重“底”夸克的束缚态。在碰撞的早期高温阶段，即使像厄普西隆这样的重夸克束缚态也会在碰撞中融化。该小组将用STAR和CMS在质子-质子、质子-核和核-核碰撞中研究Upsilons的熔化模式及其激发态。这一测量将为测试高温QGP中夸克的定义模型提供关键信息。第二个目标是研究强作用力的热力学相结构。特别是，测试预测的QCD临界点的存在性是该目标的关键组成部分。本研究通过改变碰撞能量来进行STAR，从而探索正常核物质与QGP物质之间的跃迁。第三个目标是利用中性电弱玻色子Z0与夸克喷流同时产生的事件来研究介质的密度。Z0不受QGP的影响，提供了夸克喷流横向动量的清晰估计。当伴星喷流穿过密集的QGP介质并损失能量时，这些Z0 +夸克喷流事件是研究夸克能量损失的最佳方法之一。这项工作的更广泛影响是针对代表性不足的少数民族和妇女。第一个是通过会议上的指导活动和加州大学戴维斯分校SACNAS分会的支持，向奇卡诺人和美洲原住民科学进步协会（SACNAS）的拉丁裔学生传达高能核物理的兴奋。第二个组成部分是通过参观拉丁裔人口较多的小学和参加高中收养物理学家项目，向小学和高中学生推广科学和物理。最后，该项目的pi每年将指导本科生，包括NSF REU学生。这些目标的一个关键部分是指导我们自己的研究生成为这些活动的领导者，因为许多是女性和代表性不足的少数民族，因此是优秀的榜样。

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.