There are four fundamental forces that describe all known interactions in the universe: gravity; electromagnetism; the weak interaction; and the strong interaction, which is the topic of our research. The strong force binds quarks into hadrons such as protons and neutrons, which in turn form the nuclei of atoms, making up more than 99% of all the known matter in the universe.

Normally, the strong interaction binds quarks so tightly that they never escape from within hadrons. However, at temperatures greater than the deconfining temperature, T_C (which is several trillion Celsius!) it undergoes a substantial change in character, becoming considerably weaker, and quarks become “free”. This new phase of matter is called the “quark-gluon plasma” (QGP) and is presumed to have occurred in the first few microseconds after the Big Bang. Despite the QGP being crucial to the development of the Universe (it was born in this state!) it is poorly understood.

Physicists re-create a mini-version of the QGP by colliding large nuclei (such as gold or lead) in particle accelerators at virtually the speed of light. These experiments have been performed in the Large Hadron Collider at CERN and at the Brookhaven National Laboratory in the USA. The region of QGP created in these experiments is incredibly small – just the size of the colliding nuclei. The QGP “fireball” formed expands and cools very rapidly, quickly returning to normal matter where quarks are tightly bound inside hadrons.

Most hadrons melt in the QGP meaning that the quarks that were inside them become free, like satellites escaping the earth’s gravity. However it was proposed 30 years ago that some, heavy “mesons” may remain bound up to quite high temperatures. Our collaboration has calculated the spectral functions of these bottomonium mesons (comprised of two bottom quarks) and we show the results above. Each pane has the results from two nearby temperatures, with the coldest pair in the top-left, and the hottest bottom-right. The strong peak at around 9.5 GeV shows a bound state which gets weaker as the temperatures increase, but remains in existence up to around 2T_C. This confirms the decades-old hypothesis that these mesons do not melt as soon as the QGP is formed, and that measuring their features is an effective way of determining the system’s temperature, i.e. they can be used as a “thermometer” of the QGP.