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、稠密的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。利用大型粒子加速器，科学家们正在实验室中重现这种亚原子汤，方法是以非常高的能量对撞金或铅原子，并研究这种不寻常的物质状态的有趣性质。相对论重离子对撞机(RHIC)实验中最重要的发现之一是，致密物质对碰撞产生的非常高能的夸克和胶子是不透明的。这些‘部分子’在离开碰撞区域时遭受巨大的能量损失，将其初始能量的很大一部分储存在介质中。结果，这些逃逸到高能粒子喷流中的部分子比预期的要少得多，而那些在超高速部分子之后获得一些额外能量的部分子则获得了一些额外的能量。可以检测和研究碰撞产生的粒子能量分布的细微变化，就像X射线图像显示人体内骨骼和器官密度的变化一样。出于这个原因，这项技术被称为喷射断层扫描。从迄今收集的证据中得出的一个令人惊讶的结论是，RHIC产生的物质最初被认为是气体，但实际上表现出更像液体的性质。位于瑞士的CERN大型强子对撞机(LHC)将以比RHIC高30倍的能量对撞原子核，从而产生更热、更密集、寿命更长的介质。这个RUI项目将利用爱丽丝实验，利用喷流断层扫描来探索这一物质的性质。从事这项实验的本科生将接受高能核物理前沿最先进的数据收集、简化和分析技术方面的培训。他们将能够在各种各样的职业中应用这些技能，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，从而加深对我们周围世界的理解。

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.