The Smallest Drops of the Hottest Matter: Exploring the Small Size Limit of the Quark Gluon Plasma

The hottest matter that currently exists in the universe is briefly created in the collisions of large nuclei which are smashed into each other while traveling nearly the speed of light in particle accelerators. Such collisions are called heavy ion collisions and, aside from their extreme temperature, provide a unique opportunity to study the strong force. There are four fundamental forces in nature: gravity, electromagnetic, strong and weak. All interactions that we know of can be described through one of these interactions. In everyday life, we are most familiar with the first two of these forces. Gravity constrains massive objects near the earth and the electromagnetic force governs the electricity and chemistry that we use everyday. The strong and weak forces are nuclear forces and as such are somewhat more remote in daily experience, however are central to the existence of the universe as we know it. Atoms consist of a central core, the nucleus, surrounded by electrons orbiting it in nearly empty space. The nucleus itself is composed of protons and neutrons (collectively termed nucleons bound tightly together and contains the vast majority of the mass of atoms (and thus visible matter) in a radius approximately 105 times smaller than the atomic radius. Protons have positive charge and neutrons are charge neutral, so the electromagnetic force cannot keep the nucleus together. A separate short range force, the strong force holds the nucleus together. The details of this force determine the properties of nuclei. Protons and neutrons, collectively nucleons, themselves are not fundamental particles, but composed of quarks and gluons which are likewise held together by the strong force. Nominally, nucleons are composed of three quarks, held together by gluons. These quarks determine the quantum numbers of the nucleon. However, interactions inside the nucleon give rise to a sea of gluons and quark-antiquark pairs. This structure is hidden by confinement, the property of the strong force that stable matter carries no net color charge (color charge is the strong force equivalent of the more familiar electromagnetic charge).