A lot of the work has gone into setting up a computer simulation called ALPACA of the QGP based on a successful effective kinetic theory by Arnold, Moore and Yaffe. In kinetic theory the evolution of a system of particles interacting with each other is described by tracking the particles individually. It is a very powerful framework, but some caution is needed as it cannot be applied in all situations. In this case the particles are quarks and gluons, that make up the protons and neutrons in atomic nuclei. The interactions among them can be normal elastic scattering (like scattering of billard balls), but also processes where a quark (or a gluon) splits into a quark (or a gluon) and another gluon, or absorbs a gluon. An important problem was solved on the way: the scattering probability depends not only on the properties of the scattering particles, but also on the particles in the surroundings (even though they do not participate in the scattering). We managed to calculate the scattering probabilities knowing only the particles in the system and nothing else. This had not been achieved before. Simulating the absorption of a gluon also required developing a new technique. We then tested ALPACA to make sure that it reproduces known results in the cases where solutions existed before.
Next, we studied what happens in the effective kinetic theory when there is at most one scattering during the evolution of the system. We did this by directly solving the theory for this case and with the full simulation. In this set-up the system shows a behaviour characteristic for liquids, but it does not come close to being a QGP. We understand how this comes about in the theory, and this is important for understanding small collision systems like proton-proton. A surprising discovery was that under certain conditions a quantity used to characterise fluid-like behaviour can change sign in small systems. Measuring the sign is extremely challenging, but our work has encouraged some of our experimental colleagues to try to measure it. This also sparked a fruitful collaboration with another theoretical group.
The work with the kinetic theory was inspired by the traditional view on collisions of heavy nuclei and QGP formation, and we applied this picture to small systems where it is unclear whether a QGP can form. In an attempt to approach the small systems puzzle from the other side we also worked on extending a model for proton-proton collisions to collisions involving nuclei. We worked out how this can be done and also started writing the simulation software, but did not continue because the model was not working well enough already for proton-proton collisions.
The third building block in the project were energetic particles, which can serve as a complementary probe of the QGP. When such a particle travels through a QGP it interacts and loses some of its energy. We studied how such energetic particles can be used to detect earliest signs of processes that ultimately lead to QGP formation. Building on the experience with the kinetic theory we have a new picture the quarks and gluons produced in a collision of protons or small nuclei start to interact in a way similar to what happens in a large system. But before enough interactions have happened to form a QGP the systems flies apart. We have come up with suggestions for how even a single interaction of an energetic particle could perhaps me measured in experiments.
We have published our results in scientific journals and presented them at a number of international conferences. The software that we have written is freely available as soon as it is ready.
