Ultra-relativistic heavy ion collisions - Application for bridging support

宇宙中大部分可见的物质都是以质子和中子的形式存在的，它们构成了我们在周围所看到的元素中所发现的日常原子核。这可能表明质子和中子是物质的基本组成部分，但事实上，质子和中子本身是由更基本的粒子夸克组成的。夸克相互之间有很强的吸引力，而且永远不会孤立地出现。它们的吸引力来自胶子，胶子是将夸克粘在一起的力粒子。这种吸引力与其他类型的力的不同之处在于，胶子也相互吸引。正是这种力粒子之间的相互作用使得核力如此强大。它还有一个相当令人惊讶的效果：相互作用的强度随着距离的增加而减弱。这表明，在足够高的密度下，夸克和胶子的行为就好像它们是自由粒子。在这种情况下，质子和中子根本不存在。相反，物质将由夸克和胶子等离子体组成。这就是大爆炸后最初几分之一秒的物质状态。现在，人们正在尝试在实验室中重现大爆炸的条件，尽管规模要小得多，方法是以非常高的能量碰撞重原子核。在两个原子核之间的正面碰撞中，大量的动能转化为新的粒子，产生了密度极高、温度极高的物质。我们需要的是一个实验探测器，它可以告诉我们它的密度和温度。其中一个方法就是研究喷射流。当夸克和胶子正面碰撞并向侧面散射时，就会产生喷流。由于没有观测到自由夸克和胶子，它们会形成一股强子喷流。这是一个罕见的过程，但产生了足够的数量，使其成为一个强大的诊断工具。它们有用的关键在于它们可以被热的稠密介质（夸克-胶子等离子体）吸收，这使它们成为研究这种新物质状态的理想工具。该提案旨在联合伯明翰大学的两个实验小组，在位于瑞士欧洲核研究中心（CERN）的大型强子对撞机（LHC）的重离子碰撞中研究射流等可观测量。大型强子对撞机将在2007年底开始质子对撞，第一批重离子束预计将在2008年底产生。与之前在纽约附近的布鲁克海文国家实验室的相对论重离子对撞机（RHIC）的研究相比，LHC将实现比以前高30倍的碰撞能量。预计初始温度将比观察到夸克退禁闭物质转变所需的临界温度高4-5倍。伯明翰的一个小组已经积极参与了ALICE实验中首次碰撞的准备工作。另一组参与了RHIC设施的一项名为星星的实验，并带来了使用类似探测器系统进行数据分析的经验。希望这两个小组将共同对国际探索大爆炸后几分之一秒的物质产生重大影响。

Most of the visible mass of the universe is in the form of protons and neutrons that make up the everday nuclei found within the elements we see around us. This might suggest that protons and neutrons are the fundamental building blocks of matter, but, in fact, protons and neutrons are themselves composed of more fundamental particles known as quarks. Quarks are strongly attracted to each other and are never seen in isolation. They owe their attraction to gluons, which are force particles that stick quarks together. What makes this attraction different from other types of forces is that the gluons attract each other too. It is this interaction amongst the force particles that makes the nuclear force so strong. It also has a rather surprising effect: the strength of the interaction decreases with distance. This suggests that at high enough densities quarks and gluons behave as if they are free particles. In this case protons and neutrons would not exist at all. Instead matter would be comprised of a plasma of quarks and gluons. This would have been what matter was like during the first fraction of a second after the Big Bang. Attempts are now underway to recreate the conditions of the Big Bang in the laboratory, albeit on a much smaller scale, by colliding heavy nuclei at very high energies. In a head-on collision between two nuclei a significant amount of kinetic energy is converted into new particles, producing matter which is both extremely dense and extremely hot. What is needed is an experimental probe that can tell us exactly how dense and how hot it really is. One way to do this is to study jets. Jets occur when quarks and gluons collide head-on and are scattered sideways. As free quarks and gluons are not observed, they shower into a jet of hadrons. This is a rare process, but sufficient numbers are produced to make them a powerful diagnostic tool. The key to their usefulness lies in the fact that they can be absorbed in the hot dense medium that is the quark-gluon plasma, making them an ideal tool for studying the properties of this new state of matter. This proposal seeks to unite two experimental groups at Birmingham University to study jets, amongst other observables, in heavy-ion collisions at the Large Hadron Collider (LHC), which is situtated at the European Centre for Nuclear Research (CERN) in Switzerland. The LHC will start colliding protons in late 2007 and the first heavy-ion beams are expected at the end of 2008. Compared to previous studies at the Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC), near New York, the LHC will achieve collision energies 30 times higher than seen before. It is expected that the initial temperature will be 4-5 times higher than the critical temperature required to observe a transition to quark deconfined matter. One of the groups at Birmingham is already heavily involved in preparations for the first collisions to be seen in the ALICE experiment. The other group has been involved in an experiment called STAR at the RHIC facility and bring with them experience of data analysis using a similar detector system. Together, it is hoped that the two groups will make a major impact on an international quest to discover what matter was like a fraction of a second after the Big Bang.