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

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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.