How hot will it get? Heavy quarks in the quark-gluon plasma

夸克是物质的组成部分，当它们被置于高温的极端条件下时，会发生什么？夸克通过强大的核力相互作用，这是自然界的四种基本力量之一，由量子色动力学或QCD描述，并由胶子调节。而在正常情况下，夸克和胶子被限制为强子，如质子、中子和介子，当温度升高时，会发生到解禁闭相的相变，物质被组织成夸克和胶子的等离子体，而不是强子系统。高温相称为夸克-胶子等离子体(QGP)。发生转变的临界温度约为175 MeV，或2x10^(12)K，这在您的实验室中是不容易获得的！事实上，宇宙唯一一次处于如此高的温度是在很久以前，就在大爆炸之后。对我们来说幸运的是，在过去的十年里，在布鲁克海文国家实验室(美国纽约)的相对论重离子对撞机上，重离子(铅和金)在重离子碰撞中相互碰撞，重新创造了这样的高温，尽管时间很短。这导致了对转变温度以上强相互作用物质动力学的非凡洞察，这种物质被称为完美流体。这些实验目前正由瑞士日内瓦欧洲核子研究中心的大型强子对撞机(LHC)更上一层楼。事实上，去年11月，欧洲核子研究中心发生了第一次重离子碰撞，这是实验和理论物理学家热切期待的事件。如何估算夸克-胶子等离子体的温度？大约25年前，日本的松井哲夫和德国的赫尔穆特·萨茨两位理论物理学家提出夸克尼亚，即由一个夸克和一个反夸克组成的状态，可以为这一点提供见解。有人争辩说，不同的夸克态在不同的温度下熔化，或者换句话说，夸克态充当温度计！为了使这一点精确，有必要知道某一特定状态在什么温度下会熔化。这就是我们的研究发挥作用的地方。由于夸克和胶子相互作用很强，用纸和笔计算熔化温度并非易事。事实上，考虑所有相互作用的唯一方法是使用一种称为格子QCD的数值方法。在这里，夸克和胶子被放置在一个时空格子上，必须考虑夸克和胶子的不同排列才能找到更好的组态。结果表明，在较低温度下，当夸克距离较近时，即被禁闭，而在较高温度下，夸克可能相距较远，即解禁闭。通过扫描一定的温度范围，就可以推断出不同状态的熔化温度。在我们的工作中，我们提出了一种新的方法来计算这些熔化温度。依靠重夸克运动缓慢的事实，我们可以忽略爱因斯坦的夸克狭义相对论，而使用非相对论近似。事实证明，这大大简化了分析。由于大型强子对撞机几个月前才刚刚开始第一次重离子碰撞，现在是研究这一主题的特别令人兴奋的时刻。希望它将为我们提供对统治我们宇宙的基本力量之一的新见解。

What happens to quarks, the building blocks of matter, when they are put under the extreme conditions of high temperature? Quarks interact via the strong nuclear force, one of the four fundamental forces of Nature, described by Quantum Chromodynamics or QCD, and mediated by gluons. While under normal conditions quarks and gluons are confined into hadrons, such as protons, neutrons and pions, when the temperature is raised a phase transition to a deconfined phase occurs, and matter is organized as a plasma of quarks and gluons, rather than as a hadronic system. The high-temperature phase is known as the quark-gluon plasma (QGP). The critical temperature where the transition occurs is of the order of 175 MeV, or 2 x 10^(12) K, not readily available in your lab! In fact, the only time when the Universe was at such a high temperature, was a very long time ago, right after the Big Bang. Luckily for us, in the past decade such high temperatures have been recreated, albeit only for a very short time, by colliding heavy ions (lead and gold) together in heavy ion collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory (NY, USA). This has led to remarkable insight in the dynamics of strongly interacting matter just above the transition temperature, which has been dubbed the perfect fluid. These experiments are currently taken to the next level by the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. In fact, in November of last year, the first heavy ion collisions took place at CERN, an event eagerly awaited for by experimental and theoretical physicists. How can the temperature of the quark-gluon plasma be estimated? Almost 25 years ago, it was proposed by two theoretical physicists, Tetsuo Matsui from Japan and Helmut Satz from Germany, that quarkonia, states built from one quark and one anti-quark, can provide insight into this. It was argued that various quarkonium states melt at different temperatures, or in other words, quarkonium states act as a thermometer! In order to make this precise, it is necessary to know at which temperature a particular state will melt. And this is where our research comes in. Since quarks and gluons are strongly interacting, it is not easy to compute melting temperatures with pen and paper. In fact, the only way to take into account all interactions is by using a numerical approach, known as lattice QCD. Here quarks and gluons are placed on a spacetime lattice and different arrangements of quarks and gluons have to be considered to find the preferred configurations. It turns out that at low temperatures configurations are preferred where the quarks are close together, i.e. confined, whereas at high temperature, quarks can be far apart, i.e. deconfined. By scanning over a range of temperatures, the melting temperatures of different states can then be deduced. In our work we propose a new way to find these melting temperatures. By relying on the fact that heavy quarks move slowly, we can ignore Einstein's Theory of Special Relativity for the quarks and use a non-relativistic approximation. It turns out that this simplifies the analysis considerably. Since the LHC has just started its first heavy ion collisions a few months ago, it is a particularly exciting time to work on this topic at this moment. Hopefully it will provide new insight into one of the fundamental forces ruling our Universe.