RUI: Jet Tomography of Compressed Nuclear Matter with the ALICE Experiment at the LHC

大爆炸后不久，宇宙由一种叫做夸克和胶子的自由粒子等离子体组成。随着宇宙的膨胀和冷却，夸克和胶子凝聚成质子和中子，它们现在构成了所有原子的核心。这种从高温致密等离子体到正常物质气体的相变类似于蒸汽凝结成液态水的相变。科学家们利用大型粒子加速器，在实验室中以非常高的能量碰撞金或铅原子，并研究这种不寻常物质状态的有趣特性，从而重现了这种亚原子汤。相对论重离子对撞机（RHIC）实验中最重要的发现之一是，致密物质对碰撞中产生的高能夸克和胶子是不透明的。这些“粒子”在离开碰撞区时遭受了巨大的能量损失，在介质中沉积了很大一部分初始能量。结果，这些粒子逃逸成高能粒子射流的粒子比预期的要少得多，而那些在加速粒子后面的粒子则获得了一些额外的能量。碰撞产生的粒子能量分布的细微变化可以通过类似于x射线图像显示人体骨骼和器官密度变化的方式进行检测和研究。由于这个原因，这项技术被称为喷射断层扫描。从目前收集到的证据中得出了一个令人惊讶的结论：RHIC产生的物质，最初被认为是气体，实际上表现出更像液体的特性。位于瑞士的欧洲核子研究中心大型强子对撞机（LHC）将以比RHIC高30倍的能量碰撞原子核，产生更热、密度更大、寿命更长的介质。这个RUI项目将利用ALICE实验利用喷射层析成像来探索这种物质的性质。参与本实验的本科生将接受高能核物理学最前沿的最先进的数据收集、还原和分析技术的培训。他们将能够将这些技能应用于各种各样的职业，无论他们是否选择攻读基础科学的高级学位。他们的贡献还将帮助我们对宇宙基本组成部分之间的基本相互作用产生新的见解，从而更深入地了解我们周围的世界。

Shortly after the Big Bang, the Universe was composed of a plasma of free particles called quarks and gluons. As the Universe expanded and cooled, the quarks and gluons condensed into the protons and neutrons which now make up the cores of all atoms. This phase transition from a hot, dense plasma to a gas of normal matter is similar to that of steam condensing into liquid water. Using large particle accelerators, scientists are recreating this subatomic soup in the laboratory by colliding gold or lead atoms at very high energies and studying the intriguing properties of this unusual state of matter. One of the most important discoveries to come from experiments at the Relativistic Heavy Ion Collider (RHIC) is that the dense matter is opaque to very high energy quarks and gluons produced in the collisions. These `partons' suffer tremendous energy loss as they make their way out of the collision zone, depositing a large fraction of their initial energy in the medium. As a result, far fewer of these partons than expected escape to fragment into jets of high energy particles while those in the wake of the speeding parton pick up some additional energy. The subtle changes in the distribution of energy to particles coming from the collision can be detected and studied in a manner analogous to how X-ray images show variations in the density of bones and organs within the human body. For this reason, the technique is known as jet tomography.A surprising conclusion from evidence collected so far is that the matter produced at RHIC, which was initially expected to behave like a gas, actually exhibits properties more like a liquid. The CERN Large Hadron Collider (LHC) in Switzerland will collide nuclei at energies 30 times higher than at RHIC, producing an even hotter, denser, longer-lived medium. This RUI project will utilize the ALICE experiment to explore the properties of this matter using jet tomography. Undergraduate students working on this experiment will be trained in the most advanced data collection, reduction and analysis techniques at the cutting edge of high energy nuclear physics. They will be able to apply these skills in a wide variety of careers, whether they choose to pursue an advanced degree in basic science or not. Their contributions will also help us develop new insights into the fundamental interactions among the basic building blocks of the universe, leading to a deeper understanding of the world around us.